YT YouTube2Text

YouTube2Text → Transcripts → A Look at Neuroanesthesia

A Look at Neuroanesthesia — Transcript

by Sterling Anesthesia Education Solutions, Inc. · 14,198 words · 2,541 segments · language en · Watch on YouTube

Open in transcriber Summarize with AI Download SRT/VTT

Full transcript

0:38the objectives for this lecture are as
0:40follows review anatomy and physiology of
0:43the neurological system discuss
0:46anesthetic techniques and considerations
0:47used during neurosurgery and discuss
0:50common medications and monitoring used
0:52during neurosurgery
0:56a basic knowledge of neuroanatomy is
0:59essential for all anesthetist
1:01particularly those caring for patients
1:02with diseases of the CNS although the
1:05brain and spinal cord which make up the
1:07central nervous system are fragile
1:08organs the bony structures that surround
1:10them provide protection yet by virtue of
1:13their protective nature these structures
1:14are nonde sensible the intracranial
1:17volume is fixed thereby providing little
1:19room for anything other than the brain
1:20CSF and blood contained in the cerebral
1:23vascular Chur even the space in the
1:25spinal column although not as
1:27restrictive as the cranium is quickly
1:28exhausted by an expanding hematoma or
1:30abscess it is in the context of this
1:33restrictive nature of the space in which
1:35the CNS is housed that all interventions
1:37must be considered the blood supply to
1:39the brain is also unique the carotid
1:42artery in the neck bifurcates into the
1:44external and internal carotid arteries
1:45sending the internal branch through the
1:47base of the skull profusing the IV of
1:49the ophthalmic artery and ultimately
1:51bifurcating into the anterior and middle
1:53cerebral arteries
1:54these vessels define the anterior
1:57cerebral circulation
2:01the posterior circulation results from
2:03the vertebral arteries which chastened
2:05in the posterior aspect of the neck
2:07through the foramen a' in the cervical
2:08vertebral bodies before exiting coursing
2:11around the brainstem and joining the
2:12contralateral vessel to form the basilar
2:14artery the basilar artery asons along
2:17the brainstem before dividing into the
2:18posterior cerebral arteries the anterior
2:21and posterior circulation anastomosis
2:23through the posterior communicating
2:24artery is to provide collateral flow
2:27collateral circulation can also occur
2:29through the anterior communicating
2:30artery connecting the bilateral anterior
2:32cerebral arteries the spinal column is a
2:37bony structure made up of the seventh
2:39cervical vertebra 12 thoracic vertebra
2:41five lumbar vertebra as well as the
2:43sacrum the spinal cord exits a skull
2:45through the foramen magnum and enters
2:47the canal formed by the vertebral bodies
2:48in the adult the core typically ends at
2:51the lower aspect of the first lumbar
2:53vertebral body blood supply to the
2:55entire cord is provided by several
2:57sources the anterior spinal artery which
3:00arises from the vertebral arteries
3:01supplies the anterior two-thirds of the
3:03spinal cord the posterior third of the
3:06cord is supplied by two posterior spinal
3:08arteries which arise from the vertebral
3:10bodies and also receive contribution
3:11from the radicular arteries the cerebral
3:16metabolic rate is directly related to
3:18the number and frequency of neuron
3:19depolarizations CSF is produced by the
3:22choroid plexus and the ventricles the
3:25CSF occupies the subarachnoid space
3:27providing a protective layer of fluid
3:29between the brain and the tissue that
3:30surrounds it absorption into the dural
3:33venous sinuses occurs to the arachnoid
3:35granulations
3:36although CSF volume is approximately 150
3:39mils more than three times this amount
3:41is produced in a 24-hour period
3:43this continuous flow of CSF from source
3:45to sink allows it to participate in many
3:47functions in addition to cushioning the
3:49brain intracranial pressure is low
3:52except in pathologic States the Monroe
3:55Kelly doctrine states that in the
3:56setting of a non distensible cranial
3:58vault the volume of blood CSF and brain
4:00tissue must be in equilibrium an
4:02increase in one of these three elements
4:04or the addition of a space occupying
4:06lesion can be accommodated initially
4:08through the displacement of CSF into the
4:10thecal sac but only to a small extent
4:12further increase
4:13as was significant cerebral edema or the
4:16accumulation of an extra dural hematoma
4:18will quickly lead to a market increase
4:20in intracranial pressure due to the low
4:21intracranial compliance
4:24as mentioned earlier blood flow to the
4:27brain is tightly coupled to cerebral
4:29metabolism as such many factors affects
4:32cerebral blood flow because of their
4:33effect on metabolism stimulation arousal
4:37nociception and mild hyperthermia
4:39elevate metabolism and flow while
4:41sedative hypnotic agents and hypothermia
4:43decreased both metabolism and low a
4:45number of other factors governs through
4:47table flow directly without changing
4:49metabolism as CBF changes so does
4:52cerebral blood volume which is why
4:54hyperventilation can be used for short
4:56periods of time to relax a brain or
4:58decrease intracranial pressure
4:59this effect is thought to be short-lived
5:02however CSF pH normalizes over time and
5:05vessel caliber returns to baseline the
5:08exact duration of hypocapnia
5:10vasoconstriction is uncertain a period
5:13of minutes to hours has been found in
5:14different patient populations because
5:17the decrease in cerebral blood flow
5:18occurs without a change in cerebral
5:20metabolic rate the risk of ischemia is a
5:22theoretical concern there is no evidence
5:25of harm of moderate hyperventilation to
5:27the normal brain under general
5:28anesthesia but early hyperventilation
5:30and traumatic brain injury is often
5:32associated with a poor outcome
5:41you
6:55anesthetic agents have a variable
6:57influence on the cerebral blood flow
6:59cerebral metabolic rate co2 reactivity
7:02and autoregulation inhalation
7:05anesthetics tend to cause vasodilation
7:06in a dose related manner but do not per
7:09se uncouple flow and metabolism
7:11thus the basal dilatory influence is
7:14opposed by metabolism mediated decreases
7:16in flow the result in effect is that
7:18during low doses of inhalational agent
7:20cerebral blood flow is either unchanged
7:22or slightly increased sevoflurane has
7:25been shown to actually result in a
7:27decrease in CBF in positron emission
7:29tomography studies higher doses result
7:32in dominance of the base of dilatory
7:34effect and an increase in cerebral blood
7:36flow intravenous agents including
7:39thiopental and propofol cause
7:40vasoconstriction coupled with the
7:42reduction in the metabolism ketamine on
7:44the other hand increases flow and
7:46metabolism co2 reactivity is a robust
7:49mechanism and is preserved under all in
7:51aesthetic conditions cerebral auto
7:54regulation on the other hand is
7:55abolished by inhalational agents in a
7:57dose related manner but preserved during
7:59propofol anesthesia the homeostatic
8:03mechanisms that ensure protection of the
8:05brain and spinal cord the removal of
8:07waste and the delivery of adequate
8:09oxygen and substrate to the tissue can
8:11be interrupted through a multitude of
8:12mechanisms traumatic insults may result
8:15in contusion with subsequent edema
8:17formation direct injury from depressed
8:19skull fractures or spine fractures
8:20diffuse injury to neurons from rapid
8:22deceleration and disruption of the
8:24vasculature resulting in ischemia or
8:27hemorrhage all of these insults may
8:29ultimately compromise CNS perfusion mass
8:32lesions such as tumors may compress
8:34adjacent structures raise the ICP and
8:37obstruct normal flow of CSF hemorrhage
8:41may be spontaneous or traumatic
8:43depending on its location they may cause
8:45mass effect impaired CSF circulation or
8:48in the case of subarachnoid blood
8:50breakdown up the blood may lead to
8:51further ischemic injury by causing
8:53cerebral vasospasm hydrocephalus is
8:56caused by an imbalance between CSF
8:58production and removal it frequently
9:00results in an elevation of ICP
9:03hydrocephalus is commonly divided into
9:05two categories communicating
9:07hydrocephalus and obstructive
9:08hydrocephalus
9:09the former is characterized by a failure
9:11to absorb CSF typically because of
9:14dysfunctional arachnoid granulations the
9:17latter may be caused by any direct
9:18obstruction or extrinsic compression of
9:20a passageway through which CSF must pass
9:22such as a cerebral aqueduct this
9:25obstruction for example may result from
9:27a clot within the space or from a tumor
9:28adjacent to it depending on the
9:31circumstances hydrocephalus may have a
9:33subtle or dramatic presentation for
9:35example acute hydrocephalus following an
9:38intraventricular hemorrhage may result
9:39in rapidly progressive optin Dacian that
9:41dramatically improves with external
9:43ventricular drain age in contrast normal
9:46pressure hydrocephalus may evolve over
9:48years resulting in barely perceptible
9:50changes in cognition and gait
9:56anesthesia for neurosurgery and spine
9:58surgery requires a standard a si
10:00monitors for physiologic parameters
10:02however the risk imposed to the CNS by
10:05these surgical procedures warrants more
10:07extensive monitoring for many procedures
10:10adequate oxygenation ventilation and
10:12systemic blood pressure did not ensure
10:14the well-being of the brain and spinal
10:15cord instead the integrity of the CNS
10:18needs to be evaluated intraoperatively
10:20with monitors that specifically detect
10:22CNS function profusion or metabolism at
10:26times the monitoring modalities can be
10:28combined to provide greater information
10:30regarding the well-being of the central
10:31nervous system this type of monitoring
10:34warrants a whole lecture on its own but
10:36we will briefly touch on several common
10:37modes of monitoring the
10:41electroencephalogram or EEG is a
10:43quintessential cerebral function monitor
10:45the depolarization of cortical neurons
10:48provides a pattern of electrical
10:49activity that can be measured on the
10:50scalp typically the activity is measured
10:53between two points on the scalp since
10:55there is no electrically neutral place
10:56for which to reference the signal other
10:59sources of electrical activity such as
11:01from the heart and muscles must be
11:02filtered from the signal otherwise they
11:04would overwhelm the small voltages
11:05generated by the cortical activity
11:07common mode rejection that is rejection
11:11of signal common to both electrodes
11:12allows interference from cardiac and
11:14muscle activity to be minimized EEG
11:17monitoring is commonly used during
11:19anesthesia for procedures like carotid
11:20or cerebral vascular surgery
11:25although EEG is a cerebral function
11:28monitor that detects spontaneous
11:29activity evokes potential modalities
11:31detects signals that are the result of
11:33specific stimuli applied to the patient
11:36these include somatosensory evoked
11:38potential brain stem auditory about
11:40potential visual evoked potential and
11:42motor evoke potential we will briefly
11:45take a look at each one
11:50somatosensory evoked potential or SSCP
11:53is a signal that is detectable on EEG
11:56that is generated in a time lock fashion
11:58in response to a specific applied
11:59sensory input typically a cutaneous
12:01electrical stimulation as a result an
12:05intact neural pathway from the periphery
12:07to the cerebral sensory cortex is
12:08essential for a signal to be generated
12:10this monitoring modality has application
12:13in any surgical procedure that may
12:15jeopardize this pathway specifically
12:18spine surgery in which the dorsal column
12:20of the spinal cord may be placed at risk
12:22is a particularly appropriate
12:23application but it may also be used
12:25during other procedures such as a
12:27craniotomy and carotid surgery where any
12:29part of the pathway may be subjected to
12:31ischemia or surgical retraction because
12:34of the presence of spontaneous EEG
12:36activity a single peripheral stimulus
12:38which generates cortical activity of
12:40relatively low amplitude would not be
12:42detectable admits the background noise
12:45summation followed by signal averaging a
12:47repetitive stimuli is therefore
12:49necessary in order to extract meaningful
12:50signals
12:54stimulation is typically done in the
12:56regions of the median nerve on Learner
12:58v' and posterior tibial nerve to
13:00generate predictable and reliable
13:01signals in theory however any sensory
13:05nerve could be used to generate an SS ep
13:07the SS EP is described by its polarity
13:10which is the direction of the wave
13:11deflection and its latency which is the
13:14time required for a signal to be
13:15detected after the stimulus has been
13:17applied this is quantified by both the
13:19amplitude of that signal and its latency
13:21the disruption of the neural pathway at
13:24any point will result in a complete loss
13:26of SSEP more commonly ischemia not
13:30mechanical disruption is the
13:31intraoperative insult as a result of
13:34ischemia the amplitude of the signal
13:35decreases and the latency increases a
13:3850% decrease in signal amplitude is
13:41generally accepted as clinically
13:42significant as is a 10% increase in
13:45latency
13:48the brainstem auditory evoked potential
13:51or BAE P is a specialized type of
13:54sensory evoke potential instead of an
13:57electrical stimulus applied to a somato
13:58sensory nerve a standardized sound or
14:01click is applied to the eighth cranial
14:03nerve via the auditory apparatus a
14:05recognized series of Peaks are generated
14:08with this technique where the latency of
14:10each peak has significance with respect
14:12to the integrity of the various parts of
14:14the auditory pathway although this
14:16monitoring modality is specific to
14:18cranial nerve 8 and is particularly
14:20useful in acoustic neuroma surgery it
14:22may be used during any surgical
14:24procedure around the brainstem to infer
14:26its integrity although such use is
14:28associated with both low sensitivity and
14:30specificity
14:33visual evoke potential or vep signals
14:36are generated via light stimulation of
14:38the retina typically goggles that emit
14:41LED lights are worn although this
14:44modality is particularly appealing to
14:46monitor the integrity of the optic nerve
14:47and settings in which visual loss is a
14:49concern the signals are not robust they
14:52are difficult to record in a consistent
14:54fashion during anesthesia research is
14:57ongoing with respect to its
14:58intraoperative use especially with
15:00regard to its interpretation
15:05motor evoke potential or MEP monitoring
15:08is different from the other evoke
15:10potential modalities describe thus far
15:12whereas SSEP BAE P and V EP provide
15:17information about a sending sensory
15:18neural pathways like from the periphery
15:20to the cerebral cortex MEP evaluates
15:23descending motor pathways from the
15:25cerebral cortex passenger a muscular
15:27Junction to the peripheral muscular
15:29groups this difference allows MEP to
15:32complement SSCP particularly in the
15:34setting of spine surgery in which the
15:36two modalities provide information about
15:38the integrity of anatomically different
15:40areas of the spinal cord with MEP the
15:43stimulus is applied in a transcranial
15:45fashion over the motor cortex the
15:47deflection essentially an
15:49electromyographic signal is then
15:51detected by electrodes embedded in the
15:53muscle belly although theoretically the
15:55stimulus can be delivered with either a
15:57magnetic or electrical source
15:58transcranial magnetic stimulation is
16:01obliterated under anesthesia
16:05the transcranial electrical signal is
16:07usually delivered as a rapid train of
16:09four or more stimuli the voltage of
16:11which is adjusted to achieve adequate
16:12signals in both the upper and lower
16:14extremities transcranial electrical MEP
16:17is of substantially greater magnitude
16:19compared with SSEP and signal averaging
16:21with repetitive stimuli is therefore not
16:23required however it is very sensitive to
16:26anesthetic agents particularly the
16:28inhalational agents it's amplitude can
16:31be augmented by increasing the
16:32transcranial voltage or the number of
16:34stimuli in the Train the stimulus can
16:37cause patient movement so MEP signals
16:39are typically obtained intermittently at
16:40points during the surgery when slight
16:42patient movements are not problematic a
16:44bite block is mandatory to prevent
16:46injury to the tongue during transcranial
16:48stimulation
16:51with MEP the latency of the signal is
16:54somewhat unreliable and not typically
16:56used to make clinical decisions
16:58decision-making is based on amplitude
17:00alone where a 50% decrease is considered
17:02significant although MEP s can be used
17:05during any spine or intracranial
17:07surgical procedure it is becoming
17:09increasingly used during cervical spine
17:11surgery MEP signals are much more
17:13sensitive to volatile anesthesia than SS
17:16EPS although there are some evidence
17:18that MEP signals are adequate during DES
17:20fluorine anesthesia more research on the
17:22efficacy of this technique is required
17:24and total intravenous anesthesia is a
17:26preferred technique when MEP monitoring
17:28is required some centers use a partial
17:31neuromuscular blockade but most centers
17:33avoid muscle relaxants all together with
17:35MEP in order to avoid compromise of the
17:37signal
17:40the spontaneous electromyography or EMG
17:44is different from other voc potentials
17:46and that a signal is not intentionally
17:47generated through stimulation at some
17:49point in a known neural pathway instead
17:52it is a continuous recording of EMG
17:54activity and the muscle of Regents
17:56innervated by nerve roots around which
17:58the surgeons are working its purpose is
18:01to detect injury to those nerve roots by
18:03the surgical procedure impingement on a
18:05nerve root by an instrument will cause
18:07immediate motor activity that is easily
18:09detectable which may allow the surgeon
18:10to modify his or her technique although
18:13spontaneous EMG is a robust signal that
18:16is tolerant of various anesthetic
18:17techniques muscle relaxant must be
18:19avoided
18:20spontaneous EMG is frequently used
18:22during cervical and lumbar spine surgery
18:24where the brachial plexus and
18:26lumbosacral plexus are encountered
18:31surgery in the posterior cranial fossa
18:33and adjacent to the brainstem places a
18:35surgeon in close proximity to the
18:37cranial nerves
18:37although cranial nerve 8 can be
18:40monitored with BAE P as discussed
18:42earlier several other cranial nerves can
18:44be monitored as well generally only the
18:46integrity of nerves with motor
18:47components can be detected either
18:49through spontaneous EMG or through EMG
18:51evoke by local electrical stimulation
18:54these include cranial nerves 5 7 9 11
18:58and 12
19:01as mentioned previously anesthetic
19:04agents can have profound influence on
19:05the amplitude and latency of evoke
19:07potentials for instance the quality of
19:10signals obtained with SSEP monitoring
19:13depends on the anesthetic agents used
19:15signals are obtainable under volatile
19:17anesthesia but the anesthetic is
19:19typically kept at sub Mak levels this
19:22avoids degradation and quality like an
19:24increase in latency and a decrease in
19:25amplitude as amplitude of SSEP signals
19:28are depressed by volatile agents and a
19:30dose related manner that is they are
19:31recordable during low dose and
19:33obliterated with high doses potent
19:36volatile anesthetics should not be
19:37combined with nitrous oxide as this
19:38technique will further compromise
19:40quality the signals are unaffected by
19:42opioids and opioid infusions are
19:44frequently used to facilitate low dose
19:46volatile anesthesia signal quality is
19:49also excellent under intravenous
19:51anesthesia with propofol
19:55these slides summarize the influence of
19:57anesthetic agents on a big potential
19:59monitoring general statements can be
20:02made inhalation agents including nitrous
20:04oxide generally have more depressant
20:06effects on evoke potential monitoring
20:08than intravenous agents cortical evoked
20:10potentials with long latency involving
20:12multiple synapses are exquisitely
20:14sensitive to influence of anesthetic
20:16while short latency brain stem and
20:18spinal components are resistant to
20:19anesthetic influence thus BAE P can be
20:23recorded under any anesthetic technique
20:24whereas vep and SSEP are very sensitive
20:28monitoring of mep and cranial nerve EMG
20:31in general preclude the use of muscle
20:33relaxants although the use of short
20:35acting neuromuscular blocking agents for
20:36the purpose of tracheal intubation is
20:38not contraindicated as its effects
20:40usually wear off before monitoring and
20:42surgery begin
20:45MEP is highly sensitive to the
20:48depressant effects of inhalation
20:49anesthetics including nitrous oxide
20:51although it can be recorded with low
20:53dose agents the signals are so severely
20:55attenuated that this practice is
20:57generally not advisable total
20:59intravenous anesthesia without nitrous
21:01is the ideal anesthetic technique for
21:03monitoring of MEP opioids and
21:05benzodiazepines have negligible effects
21:07on recording of evoke potentials
21:09propofol and thiopental attenuate the
21:12amplitude of virtually all modalities of
21:14evoke potential but do not obliterate
21:16them SSCP and MEP can be monitored even
21:19during birth suppression induced by
21:21these agents BAE P can be recorded with
21:24any anesthetic technique
21:28during crucial events in which part of
21:30the central neural pathway is
21:32specifically placed at rest by surgical
21:33manipulation as in the placement of a
21:35temporary clip during aneurysm surgery a
21:37change in anesthetic depth should be
21:39minimized to avoid misinterpretation of
21:41the changes in evoke potential
21:43recordings ketamine and etomidate have
21:45been reported to enhance the quality of
21:47signals in patients with weak baseline
21:49signals although the clinical
21:50significance and interpretation of
21:52signals obtained under these
21:53circumstances remain unclear
21:58although adequate cerebral blood flow
22:01does not guarantee the well-being of the
22:02central nervous system it is one factor
22:04that is essential to its integrity
22:06measuring CBF is therefore an attractive
22:09method of monitoring the central nervous
22:10system currently available techniques
22:13for quantitative measurement of CBF are
22:15not practical as an intraoperative
22:17monitor other methods for looking at
22:20relative changes in cerebral blood flow
22:21do lend themselves to use in the
22:23operating room transcranial Doppler
22:25ultrasonography or tcd and laser doppler
22:28flow matauri are examples as adequate
22:31cerebral blood flow depends on an
22:32appropriate CPP measuring intracranial
22:35pressure may be useful in certain
22:36patients to ensure conditions are
22:38adequate for sufficient cerebral blood
22:40flow
22:40finally numerous other modalities that
22:43evaluate cerebral blood flow and that
22:45may not be practical in the operating
22:46room are used commonly in the
22:48perioperative setting
22:51although monitoring ICP does not provide
22:54direct information about cerebral blood
22:56flow it allows one to calculate the CPP
22:59which must be in an appropriate range in
23:01order for CBF to be adequate again CPP
23:04is defined as a difference between Mapp
23:06and ICP in other words it is a net
23:09pressure acting to move blood through
23:11the cerebral vascular Chur assuming that
23:13the ICP is greater than the right atrial
23:15pressure CPP and CBF are not expected to
23:18be proportional as there are other
23:20factors determining cerebral blood flow
23:22which we will get to in fact within a
23:25physiologic range of CPP CBF should
23:28remain approximately constant a CPP that
23:31is too low will result in cerebral
23:33ischemia and the CPP that is too high
23:35will cause hyperemia
23:39cerebral ischemia and or hypoxia leads
23:42to neuronal death in multiple settings
23:44for example ischemic stroke and cerebral
23:47vasospasm following subarachnoid
23:48hemorrhage all effects cellular injury
23:50through ischemia efforts to avert
23:53neurologic insult using medications or
23:55through the manipulation of physiologic
23:57parameters have met with meager results
23:58in the setting of a seeming stroke for
24:01example thrombolysis may restore
24:03profusion and decrease infarct size but
24:05it may also lead to an expansion of the
24:07infarct some edema and even hemorrhage
24:09as a result of ischemia reperfusion
24:10injury in general a protective strategy
24:14that is effective an experimental
24:15cerebral ischemia has not been found to
24:17be useful in the clinical setting of
24:19recent advances that are intriguing and
24:21controversial none matches that
24:23generated by the concept of cerebral
24:25protection by mild or moderate
24:27hypothermia persons suffering out of
24:29hospital cardiac arrests have been shown
24:31to have improved neurologic outcome if
24:33they are made mildly hypothermic
24:34following resuscitation therefore it
24:37would seem that mild hypothermia is
24:39protective against global ischemia and
24:41hypoxia at least in the setting of
24:43cardiac arrest one problem with most
24:45settings in which cerebral ischemia is
24:47encountered is that the therapeutic
24:48intervention can be applied only after
24:50the insult has occurred that is during
24:52the reperfusion phase little opportunity
24:55exists to intervene before the ischemic
24:57event the operating room is a unique
24:59environment in this respect however a
25:01temporary aneurysm clip on the middle
25:04cerebral artery is an example of a focal
25:06ischemic insult that could be predicted
25:07and a brief period of circulatory arrest
25:09induced with adenosine to facilitate
25:11clipping of a basilar artery aneurysm as
25:13an example of a global insult the value
25:16of anticipating such events is that it
25:18allows the anesthetist to intervene in
25:19advance despite the luxury of planning
25:22the intervention for the ischemic insult
25:24the options and SSS have for cerebral
25:26protection are few and the evidence for
25:28benefit is modest and much of this
25:30evidence has been extrapolated from
25:31animal research
25:35ischaemic insult to the brain results in
25:37energy failure the brain depends on a
25:40continuous supply of glucose and oxygen
25:41to support aerobic metabolism generation
25:45of adenosine triphosphate and
25:46maintenance of cellular function when
25:49this nutrient supply is interrupted ATP
25:51is depleted cellular processes such as
25:54those to maintain cellular membrane
25:56integrity fail it is reasonable then to
25:58attempt to minimize ischemic insult by
26:00lowering cerebral metabolic rate thus
26:02decreasing the likelihood of exhausting
26:04ATP reserves during the period of
26:06ischemia this has been the traditional
26:08paradigm for approaching the subject of
26:10intraoperative neuroprotection
26:15unfortunately further damage occurs as a
26:17result of processes that are initiated
26:19during the reperfusion stage the
26:21reperfusion injury may be mediated via
26:23the generation of toxic oxygen species
26:25the release of excitotoxin amino acids
26:28such as glutamate the up regulation of
26:30nitric oxide synthase and initiation of
26:32cellular apoptosis further therapeutic
26:35interventions would need to target these
26:37pathways as well to provide protection a
26:39shift in the focus of neuro protection
26:41from metabolic suppression to targeting
26:43ischemic Cascades has recently been
26:44advocated
26:47it is important to distinguish mild and
26:50profound hypothermia as they have
26:51different practical considerations and
26:53they likely modify cerebral function in
26:55different ways profound hypothermia is
26:58well known for its neuroprotective
26:59effects and notes a successful
27:01resuscitation of hypothermic drowning
27:03and Avalanche victims with good
27:05neurologic recovery have been reported
27:07furthermore extensive use of deep
27:09hypothermia with circulatory arrests has
27:11been used interoperative Lee for repair
27:13of aneurysms of the thoracic aorta and
27:14for cerebral aneurysms when core body
27:17temperatures are less than 20 degrees
27:19Celsius circulatory arrests of less than
27:2130 minutes appears to be well tolerated
27:23this level of hypothermia not only
27:25decreases cerebral activity but it also
27:28decreases the energy required for
27:29cellular housekeeping
27:32the practical constraints against using
27:35deep hypothermia in settings in which
27:36cerebral ischemia is anticipated are
27:38numerous for most is a need for
27:41cardiopulmonary bypass during the
27:42cooling and warming portion of the
27:43procedure
27:44hypothermia induced coagulopathy is
27:47another concern during surgical
27:48procedures in the cold patient despite
27:50the drawbacks to this technique it
27:52remains a reasonable anesthetic option
27:54to provide protection for the brain and
27:55other organs when the surgical procedure
27:57necessitates circulatory arrest mild
28:00hypothermia that is 33 to 35 degrees
28:02Celsius not only decreases cerebral
28:04metabolism but likely modulates the
28:06immune and inflammatory response to
28:08ischemia thus affecting the reperfusion
28:10portion of the injury as well animal
28:13studies have shown improved neurologic
28:14function following resuscitation from
28:16arrest this promising result in animals
28:18was later confirmed by two independent
28:20studies in humans demonstrating that
28:21induction of hypothermia in cardiac
28:23arrest patients improved outcome
28:25although mild hypothermia is clearly
28:27beneficial in the setting of cardiac
28:29arrest
28:29cerebral ischemia due to an arrest is an
28:31uncommon occurrence in patients under
28:33anesthesia
28:34in contrast the cerebral ischemia
28:36frequently encountered by the
28:37anesthetist is focal in nature because
28:39of the temporary occlusion of a cerebral
28:41vessel although there is considerable
28:43evidence in rats that mild hypothermia
28:45is beneficial here too there is paucity
28:48of evidence in humans in fact a large
28:51multicenter study the eye has to
28:53evaluating patients undergoing cerebral
28:55aneurysm surgery found no benefit with
28:57mild interoperative hypothermia
28:59nevertheless hypothermia remains our
29:02most promising intervention for cerebral
29:03protection
29:07there is compelling physiologic
29:09rationale for its use it clearly
29:11demonstrated affected animals and human
29:12data showing benefit in the setting of
29:14cardiac arrest
29:15unfortunately inadequate evidence exists
29:18in humans outside cardiac arrest to
29:19recommend its use in the neuro surgical
29:21patient despite the lack of evidence to
29:24support hypothermia in humans for
29:25cerebral protection there is ample
29:27evidence that hyperthermia is associated
29:29with worse outcomes in the setting of
29:30ischemic stroke subarachnoid hemorrhage
29:32cardiac arrest and traumatic brain
29:34injury a common extrapolation from these
29:37studies is a belief that concomitant
29:39hyperthermia and cerebral ischemia is
29:41deleterious it is important to consider
29:44however that these studies demonstrate
29:45an association not a causation a poor
29:48outcome from fever nevertheless it would
29:51seem reasonable to avoid hyperthermia
29:52and treat fever in any setting in which
29:54the brain is at risk in the operating
29:57room during neurosurgical procedures in
29:59which the brain is at risk for ischemic
30:00insult a gold temperature of 35 to 36
30:03degrees Celsius is reasonable mild
30:05hypothermia 33 to 35 degrees may be
30:08appropriate in many patients even
30:10recognizing that there may be no benefit
30:11from this therapy finally deep
30:14hypothermia which is less than 20
30:16degrees Celsius is appropriate in any
30:17situation in which a prolonged cardiac
30:19arrest is required
30:24volatile and intravenous anesthetic
30:26agents decrease cerebral metabolism and
30:28thus seem like appropriate candidates
30:30for cerebral protection however evidence
30:33that the level of metabolic suppression
30:35does not correlate with the degree of
30:36protection has eroded the traditional
30:38belief in the mechanism of protection
30:40nevertheless numerous animal studies
30:42have found protective effects of
30:43volatile anesthetics particularly
30:45isoflurane and mitigating mild to
30:47moderate ischemic insult although this
30:49effect may only be short-lived this
30:51effect may exist when applied during the
30:53insult but also may be effective when
30:55administered prior to the insult as a
30:56pre conditioning therapy barbiturates
30:59such as thiopental have been extensively
31:01researched in regard to cerebral
31:03protection they have been shown to have
31:05at least short-term benefit on focal
31:07cerebral ischemia while benefit in
31:09global ischemia remains controversial
31:11this effect may be mediated through the
31:13reduction in glutamate activity and
31:15intracellular calcium the increase in
31:17gamma-aminobutyric acid activity as well
31:20as NMDA antagonism
31:24although hyperglycaemia has long been
31:26recognized as a frequent occurrence in
31:28critically ill patients it was commonly
31:30viewed as benign or even beneficial
31:33hyperglycemia could facilitate cellular
31:35uptake of glucose through non
31:36insulin-dependent mechanisms and thus
31:38may benefit cellular metabolism a
31:40subsequent recognition of its
31:42association with worse outcome in many
31:44settings including acute coronary
31:46syndrome stroke traumatic brain injury
31:48and critical illness forced the medical
31:50community to reconsider the burden of
31:51hyperglycemia although considerable
31:54evidence accumulated suggesting harm to
31:56hyperglycemia evidence for the benefit
31:58with normalization of serum glucose
31:59using insulin has been somewhat
32:01controversial the most influential
32:03literature is from the intensive care
32:05unit setting not the operating room a
32:07prospective study and surgical ICU
32:09predominantly after cardiac surgery
32:11showed that mortality and morbidity
32:13benefit with tight glycemic control that
32:15a blood sugar of 80 to 110 milligrams
32:17per deciliter this study spurred an
32:20unfettered enthusiasm for aggressive
32:22treatment of hyperglycemia changing
32:23practice not only in the surgical ICU
32:25but the medical ICU and in many cases
32:28the operating room a subsequent study
32:30evaluating this therapy and much sicker
32:32medical ICU population showed no overall
32:34mortality benefit in fact subgroup
32:37analysis revealed increased mortality in
32:39patients who stayed in the ICU less than
32:41three days with an improvement only in
32:43those who had a longer ICU stay in the
32:45heterogeneous patient population who
32:47present for neurological surgery with
32:49operative times of several hours not
32:51several days it is inappropriate to
32:53extrapolate conclusions from a body of
32:55controversial ICU literature to the
32:56anesthetic environment particularly when
32:59there is evidence for harm with short
33:00durations of therapy furthermore a
33:03prospective study of intraoperative
33:04insulin therapy and cardiac surgery
33:06patients further eroded the basis for
33:08translating this ICU literature to the
33:09operating room the insulin group had a
33:12higher incidence of death and stroke
33:14despite our reluctance to embrace enter
33:16operative tight glycemic control given
33:18the current literature it is worthwhile
33:19to consider the patient undergoing
33:21cerebral vascular surgery in particular
33:23given the preponderance of evidence at
33:25hyperglycemia and cerebral ischemia in
33:28combination are harmful changing
33:29practice and these patients may be
33:31warranted hyperglycemia on the day of
33:34surgery for carotid is associated with a
33:36worse outcome
33:37patients who suffer from an ischemic
33:39stroke have an improved outcome if their
33:41glucose is treated aggressively
33:43therefore it may be appropriate to treat
33:45neurosurgical patients who will have a
33:47period of cerebral ischemia due to
33:49temporary vascular occlusion differently
33:51from other neurosurgical patients tight
33:53glycemic control is a reasonable goal in
33:55these patients however we cannot state
33:58at this time that the intervention is
33:59neuroprotective
34:04evaluation of the patient who presents
34:05for neurologic or spine surgery requires
34:07the same thorough assessment appropriate
34:09to any person who will have an
34:10anaesthetic in addition a number of
34:13considerations specific to this patient
34:14population are important it is prudent
34:17to consider the nature of the patient's
34:18disease that brings him or her to the
34:19operating room in the context of his or
34:21her medical and surgical history a
34:23thorough history may be difficult to
34:25obtain from patients whose disease has
34:27resulted in a neurologic decline such as
34:29those obtained from a traumatic brain
34:30injury prior medical records and family
34:33members are both helpful in this context
34:35preoperative risk stratification for
34:37cardiac complications is important to
34:39consider the 2007 American College of
34:42Cardiology and American Heart
34:43Association guidelines has a simplified
34:45algorithm for considering whether a
34:47patient needs preoperative cardiac
34:48testing such as stress echocardiography
34:50or a nuclear medicine evaluation of
34:53myocardial perfusion most spine and
34:55neurosurgical procedures fall into the
34:57intermediate risk procedure category the
35:00decision to perform a non-invasive
35:01cardiac tests in patients with risk
35:03factors for coronary disease and poor
35:04functional status hinges on whether
35:06findings from that evaluation will
35:08affect the management of the patient in
35:09time before surgery
35:14changes in care of the patient include
35:16interventions such as coronary
35:18angiography and scenting which may
35:20significantly delay surgery current
35:22guidelines include delaying surgery for
35:24at least two weeks following simple
35:25balloon angioplasty four to six weeks
35:28for a bare metal stent and a full year
35:29for a drug-eluting stent such a delay
35:32may be reasonable for some surgical
35:33procedures but very few indicated spine
35:36and neurosurgical procedures can be
35:38delayed for a year
35:39furthermore the enthusiasm for
35:41perioperative beta blockade has been
35:42tempered by further studies
35:44demonstrating no obvious benefit as well
35:46as concerning preliminary adverse
35:48results on a large prospective trial
35:49evaluating Perry operatives and Tokra
35:51law on all cause mortality beta blockers
35:54are therefore appropriate primarily in
35:56two types of patients undergoing
35:57intermediate risk surgical procedures
35:59first those already receiving a beta
36:02blocker second those who are at high
36:05risk for perioperative myocardial
36:07infarction due to demonstratable
36:08reversible ischemia on a non-invasive
36:10study furthermore patients previously
36:13receiving a statin should continue their
36:15statin in the perioperative period
36:16further considerations in the
36:18preoperative visit should include issues
36:20that will affect the choice of
36:21medications and anesthetic agents many
36:24patients presenting for spine surgery
36:25have weakness or paralysis that may
36:27present a contraindication to the use of
36:29succinylcholine in addition some
36:32neurosurgical patients may have suffered
36:34from a stroke resulting in a similar
36:35contra indication finally many
36:38neurosurgical patients have been exposed
36:40to antiepileptic medications previous
36:43allergies or reactions to these
36:44medications especially phenytoin should
36:47be elucidated
36:51for most procedures induction of
36:53anesthesia is an uncomplicated process
36:55with great flexibility of drug choice
36:57with the exception of some minimally
36:59invasive spine surgery procedures and
37:01awake craniotomies an endotracheal tube
37:03is essential for most surgical
37:04procedures of the brain and spinal cord
37:06during induction of anesthesia there are
37:09three iatrogenic consequences that may
37:11be significant for the neuro surgical
37:12patient hypotension hypertension and
37:15apnea it is essential to understand how
37:18each of these will be tolerated by the
37:19patient
37:22breathe mild hypotension is frequently
37:25encountered following induction of
37:26anesthesia
37:27although most patients tolerate this
37:29transient phenomenon well it should be
37:31aggressively avoided in patients with
37:33brain injury in which any episode of
37:34hypotension is associated with
37:36unfavorable outcomes hypertension due to
37:39laryngoscopy in contrast would be poorly
37:41tolerated by patients following
37:42aneurysmal subarachnoid hemorrhage a
37:44systolic hypertension is thought to be
37:46the cause of recurrent hemorrhage from
37:48the aneurysm
37:49finally apnea results in a predictable
37:51increase in paco2 and corresponding
37:54cerebral vasospasm crease in cerebral
37:59blood volume patients with intracranial
38:01hypertension may quickly decompensate
38:03from apnea not to mention the decrease
38:05in cerebral perfusion
38:08traumatic brain injury patients in
38:11particular are frequently intolerant of
38:13apnea unfortunately many of these
38:15patients require a rapid sequence
38:17induction to further complicate matters
38:19the presence of a cervical collar for
38:21known or suspected cervical spine injury
38:23may make intubation more difficult
38:25careful preparation for a difficult
38:27airway is essential these patients are
38:30also particularly harmed by periods of
38:31hypotension as noted earlier furthermore
38:34these patients may have concomitant
38:36injuries with significant blood loss
38:38that may predispose to hypotension
38:40vigorous resuscitation with isotonic
38:42fluid and/or blood should be
38:44administered prior to induction and
38:45continued until the patient is euvolemic
38:50a conservative dose of thiopental or
38:53etomidate maybe appropriate for the
38:54induction agent with succinylcholine a
38:56reasonable choice for the muscle
38:57relaxant in the setting of acute injury
38:59because patients with subarachnoid
39:01hemorrhage are at risk for harm from
39:03hypertension it is reasonable to place
39:05an arterial catheter for hemodynamic
39:07monitoring prior to induction
39:09unacceptable increases in blood pressure
39:11during laryngoscopy should result in
39:12discontinuing the attempt returning to
39:15mass cumulation and deepening knee
39:17anesthesia the latter can be
39:19accomplished with either a higher
39:20concentration of inspired anesthetic or
39:22a bolus of an intravenous agent such as
39:24propofol or REM fentanyl in addition
39:27asthma law 0.5 milligrams per kilogram
39:29can be given prior to laryngoscopy to
39:31blunt the hypertensive response
39:36the choice of muscle relaxant used for
39:38induction deserves some consideration
39:40many neurosurgical and spine surgery
39:42patients have conditions in which
39:44succinylcholine is contraindicated
39:46muscle the innervation from stroke
39:48myelopathy or spinal cord injury results
39:50in up regulation of acetylcholine
39:52receptor isoforms across the muscle
39:54belly these receptors can be stimulated
39:56by acetylcholine sexina choline and
39:59choline the profound hyperkalemia that
40:01can result from the use of
40:02succinylcholine described by groaner and
40:04thein in 1975 is potentially lethal
40:07therefore sexina choline should be
40:10avoided in patients with significant
40:11denervation injuries however in the
40:14setting of acute stroke or spinal cord
40:16injury it remains safe to use
40:18succinylcholine for approximately 48
40:19hours from the time of injury a nine-day
40:22polarizing muscle relaxant is therefore
40:24appropriate in many neurosurgical
40:25patients to achieve acceptable
40:27intubation conditions the criteria for
40:30deciding between the available options
40:31are similar to other circumstances
40:33except duration of action is more
40:35significant if mep spontaneous EMG or
40:38cranial nerve monitoring is planned
40:43the primary considerations for
40:45maintenance of anesthesia include the
40:47type of monitoring plan for the
40:48procedure brain relaxation in the
40:51desired level of analgesia at the end of
40:53the surgical procedure most
40:55neurosurgical and spine procedures can
40:57be performed using a balanced anesthetic
40:58with volatile agents opioids are
41:01frequently administered to reduce
41:02volatile anesthetic requirements for
41:05both SSEP monitoring and brain
41:07relaxation less than 1 macca volatile
41:10anesthetic is desired for the opioid an
41:13infusion of remi fentanyl fentanyl Sioux
41:15fentanyl or elephant note are all
41:17reasonable options Remi fentanyl is most
41:20appropriate for neurosurgical procedures
41:22in which extubation is planned at the
41:23end of the surgery and minimal residual
41:25effect is desired to facilitate
41:27neurologic examination other opioids
41:30with longer duration of action are
41:32appropriate in spine surgery in which
41:33reasonable analgesia following the
41:35procedure is required
41:39replacement of the volatile anesthetic
41:41with a continuous infusion of propofol
41:43is desirable in two settings first mep
41:46monitoring virtually requires it to
41:48obtain excellent signal quality second
41:51when brain relaxation is inadequate with
41:53a volatile anesthetic propofol will
41:55provide better relaxation by further
41:57decreasing cerebral blood volume the use
42:00of interoperative muscle relaxant is of
42:02controversial utility for neurosurgical
42:03procedures it should be avoided during
42:06MEP spontaneous EMG or cranial nerve
42:09monitoring it may be used during
42:11isolated SSEP monitoring however some
42:15are more comfortable managing a patient
42:16whose head is held in rigid fixation
42:18with muscle relaxant but adequate
42:20anesthesia and avoidance of stimulating
42:22the airway are far more important and
42:23should prevent unintended patient
42:25movement
42:28hypocapnia cerebral vasospasm our filet
42:33ting cerebral blood flow and cerebral
42:35blood volume hyperventilation is
42:38routinely employed to provide brain
42:39relaxation and optimize surgical
42:41conditions because hyperventilation
42:43decreases cerebral blood flow it has
42:45been theoretical potential for causing
42:47or exacerbating cerebral ischemia
42:49clinically it has been associated with
42:51harm only in the early periods of
42:53traumatic brain injury but it is still
42:55recommended to be avoided in all
42:56patients with traumatic brain injury
42:58except when necessary for a brief period
42:59to manage acute increases in
43:01intracranial pressure in the non trauma
43:04population it is not clear whether there
43:06is harm in mild to moderate
43:07hyperventilation particularly for the
43:09duration of a typical anaesthetic as it
43:12appears to be well tolerated it is
43:13reasonable during neurosurgical
43:15procedures to maintain a paco2 between
43:1730 and 35 millimeters of mercury further
43:20brain relaxation should be accomplished
43:22with other modalities such as mannitol
43:24hypertonic saline or intravenous
43:26anesthesia the duration of effectiveness
43:29of hyperventilation is also
43:30controversial as normalization of
43:32cerebral blood flow and consequently
43:34cerebral blood volume has been reported
43:36to occur within minutes clinically the
43:38effects of cerebral blood volume appear
43:40to be sustained during most
43:41neurosurgical procedures of monestary
43:43ssin
43:47to maintain adequate cerebral perfusion
43:50adequate intravascular volume should be
43:52maintained with perhaps the exception of
43:55healthy patients with AVMs the aim
43:57should always be you bulimia or slight
43:59hypervolemia because of the presence of
44:02the blood-brain barrier movement of
44:04water into the intracellular or
44:05interstitial compartments from the
44:06vascular compartment is primarily
44:08dependent on the osmotic pressure and
44:10not on the oncotic pressure consequently
44:13to minimize brain edema it is important
44:15to maintain serum tonicity
44:19the most important osmotic species in
44:22blood is sodium so it is prudent to
44:23check serum sodium levels on a regular
44:25basis in prolonged surgical procedures
44:27in which mannitol has been given for the
44:30care of the neuro surgical patient
44:31hypotonic fluids including lactated
44:33ringers should not be used and colloid
44:35has proven no advantage over crystalloid
44:38moreover henna starch can result in
44:40coagulopathy and although low doses and
44:43healthy individuals is well tolerated
44:44this may not be the case in patients
44:46undergoing intracranial procedures as
44:48the brain is rich in thromboplastin the
44:50release of which may initiate
44:52coagulation abnormalities
44:56there has been an increasing effort to
44:58conserve the limited resource of banked
45:00blood this effort has been driven in
45:02part by concerns over the complications
45:04associated with transfusion such as
45:06transfusion reaction transfusion related
45:09acute lung injury and the transmission
45:11of infections such as hepatitis and HIV
45:13the lower limit of acceptable hemoglobin
45:16or hematocrit has not been well defined
45:18one study evaluated two different
45:20transfusion thresholds for hemoglobin in
45:22a heterogeneous ICU population either 7
45:25or 10 grams per deciliter and found that
45:27the restrictive use of red cell
45:29transfusions was at least as good as or
45:31superior to a more liberal transfusion
45:33threshold even though the study has been
45:35criticized for various reasons including
45:37the poor representation of neurosurgical
45:39patients it remains the best evidence to
45:41support avoidance of transfusions than
45:43to a hematocrit of approximately 21
45:45percent except in the context of ongoing
45:47hemorrhage and possibly the early phase
45:49of resuscitation for septic patients
45:51despite the lack of evidence to support
45:53the practice many who care for
45:55neurosurgical patients have advocated
45:57for more liberal transfusion practices
45:59to maximize oxygen delivery to the CNS
46:01but there is evidence to support a
46:04similar conservative transfusion
46:05threshold in both traumatic brain injury
46:07and spine patients unfortunately most of
46:10the evidence available on transfusion
46:12thresholds relates to critically ill but
46:14euvolemic patients in the operating room
46:17with patients undergoing neurosurgical
46:18and spine procedures ongoing hemorrhage
46:21may necessitate transfusion well before
46:23the hematocrit Falls to 21% a practical
46:26approach is to consider the rate of
46:27surgical blood loss if it is slow then
46:30it may be appropriate simply to maintain
46:31a normal intravascular volume with
46:33isotonic crystalloid solution or with an
46:35appropriate colloid recognizing that
46:37albumin should be avoided in patients
46:39with traumatic brain injury packed red
46:41blood cells can be administered when the
46:42hematocrit approaches 21% as the rate of
46:46blood loss increases blood transfusion
46:48should begin at a higher hematocrit to
46:49prevent unintended profound anemia in
46:51all circumstances regardless of
46:54transfusion threshold hypovolemia should
46:56be avoided
46:59the decisions that need to be made with
47:01respect to emergence of neurosurgical
47:03and spine surgery patients hinge on
47:05whether the patient is an appropriate
47:06candidate for excavation to determine
47:09this one must evaluate what has changed
47:11over the duration of the procedure with
47:13respect to the patient's airway
47:14oxygenation and ventilation in addition
47:18one must consider whether this patient
47:19will tolerate the hemodynamic changes
47:21that occur with the excavation finally
47:24post-operative plans such as cerebral
47:25angiography should be taken into account
47:27for extensive prone spine surgeries
47:30significant dependent edema frequently
47:32occurs although the predictive value of
47:34a cuff leak from the endotracheal tube
47:36is poor in general the combination of
47:38pronounced facial edema and an absent
47:40cuff League following prone surgery
47:42should make one suspicious for upper
47:43airway edema delaying the excavation of
47:46the trachea under these circumstances is
47:48appropriate other factors that may delay
47:50excavation in these patients include the
47:52development of pulmonary edema and
47:54hypoxemia from fluid administration as
47:56well as persistent hemodynamic
47:58instability
48:02for neurosurgical cases the desire
48:04usually is to allow the patient to
48:06emerge from anesthesia and excavate the
48:08trachea as soon after completion of the
48:09procedure as possible this pathway
48:12provides an immediate neurologic
48:13examination and may obliviate the need
48:15for post-operative CT scans to
48:18facilitate emergence and excavation in
48:19the operating room minimal use of
48:21opioids other than remi fentanyl is
48:23appropriate whether to give a longer
48:25acting opioids such as fentanyl or
48:27morphine prior to emergence to treat
48:29post-operative pain is controversial
48:30opioid administration may delay
48:33emergence and a patient population that
48:34usually requires a relatively small
48:36amount of post-operative opioid for pain
48:38control the anti tesa properties of
48:40opioids may be desirable during
48:42emergence avoiding coughing and
48:44hemodynamic changes with emergence is
48:46important for all neurosurgical patients
48:48and particularly those high at risk for
48:50post-operative hemorrhage such as
48:51patients who have just had resection of
48:53an AVM coughing due to irritation of the
48:56trachea can be minimized with
48:57intravenous lidocaine one to one and a
48:59half milligrams per kilogram and four
49:01percent lidocaine instilled in the cuff
49:03of the endotracheal tube for the
49:04duration of the procedure labetalol
49:06hydralazine and nakata pain are all
49:09reasonable options for controlling
49:10hypertension on emergence
49:16now we will take a look at an aesthetic
49:18implications for some of the more common
49:20neurosurgical procedures
49:23the fundamental an aesthetic
49:25considerations in tumor surgery are
49:27proper positioning of the patient that
49:29facilitate the surgical approach
49:30providing adequate relaxation of the
49:32brain to optimize surgical conditions
49:34and avoiding well known devastating
49:36complications such as venous air
49:38embolism in addition patients with large
49:41tumors resulting in significant
49:42intracranial hypertension are at risk of
49:44cerebral ischemia as well as herniation
49:47preoperative review of level of
49:49consciousness and CT scans should always
49:51be performed and the results taken into
49:52consideration in the anesthetic plan
49:54patient positioning can be very
49:56challenging for any neurosurgical
49:58procedure particularly for surgery in
50:00the posterior fossa lateral park bench
50:03prone and sitting positions are all used
50:05for surgical procedures in this region
50:10when placing a patient in a complicated
50:12position for surgery it is essential for
50:14the safety of the patient that all
50:15catheters and the endotracheal tube are
50:17secured particularly well ample help
50:20should be available at the time of
50:21positioning particularly for large or
50:22obese patients padding adequately to
50:25avoid pressure necrosis is also
50:27essential the head is typically secured
50:30in a Mayfield apparatus nothing should
50:32impinge on the nose eyes or chin as the
50:35sitting position confers a greatest risk
50:37for venous air embolism plans should be
50:39made for treating it should it occur a
50:41multi orifice catheter can be placed in
50:43the right atrium to evacuate air it's
50:46location can be confirmed either
50:47electrocardiographic ly or with an
50:49echocardiogram a patent foramen ovale
50:52increases the risk of paradoxical
50:54embolism transpulmonary passage of air
50:57has been described however and its risk
50:59may be higher with volatile than
51:00intravenous anesthesia patients to be
51:03placed in the sitting position should be
51:04evaluated for a patent foramen ovale and
51:06an alternate position should be
51:08considered for those who have one
51:11the structures in the posterior fossa
51:14most notably the brainstem and cranial
51:16nerves are especially vulnerable and
51:18intolerant of surgical invasion BAE P
51:21and cranial nerve monitoring are
51:23appropriate when the surgical procedure
51:24places the cranial nerves or brain stem
51:26at risk SSCP and MEP monitoring can be
51:30used for any tumor resection whether
51:31supra or m4 tutorial these modalities
51:35may be particularly useful in surgeries
51:36that play specific tracks at risk the
51:39brainstem is intimately involved in
51:41systemic hemodynamics and surgery in
51:43that region may affect rapid changes in
51:45blood pressure and heart rate
51:46hemodynamically abilities should be
51:48anticipated and treated during surgery
51:50in this region bradycardia can be
51:52treated with a drooping but it should
51:54also prompt communication with the
51:55surgeon as its development may affect
51:57surgical technique adequate brain
51:59relaxation is typically achieved with a
52:01standard anesthetic including Sub Mac
52:03volatile anesthesia and opioid infusion
52:05mild to moderate hyperventilation and
52:08mannitol in addition tumor nema may
52:11benefit from the administration of
52:12dexamethasone further relaxation can be
52:15achieved with the discontinuation of the
52:17volatile anesthetic and initiation of a
52:19propofol infusion
52:24hypertonic saline is a reasonable
52:26alternative to mannitol especially in
52:28the setting of an eunuch renal failure
52:29when mannitol is contraindicated a
52:32recent randomized trial showed that 3%
52:34saline and mannitol have equivalent
52:36brain relaxation effects but with the
52:38former having less electrolyte and
52:40vascular volumes Aquila a brain that
52:43remains full may be the result of venous
52:44congestion this problem can be mitigated
52:47with head-up tilt but is best prevented
52:49during the positioning of the patient by
52:50minimizing excessive rotation or
52:52angulation of the neck the central
52:54venous pressure and jugular venous
52:56pressure can be transduced to confirm
52:58the absence of a pressure gradient
52:59across the neck vascular tumors such as
53:02a meningioma may benefit from
53:03preoperative embolization and large ones
53:06or ones that could not be embolized are
53:07still at risk for significant blood loss
53:09coagulopathy can also develop
53:11intraoperatively it is important to
53:14perform frequent coagulation studies and
53:16administer clotting products and
53:17platelets promptly
53:21masses in the region of the cella most
53:23commonly are a pituitary origin although
53:25other benign and malignant tumors may
53:27occur in this region these tumors are
53:30typically recognized as a result of the
53:32neurologic changes they affect as they
53:33compress adjacent structures such as
53:35visual changes with impingement of the
53:37optic chiasm or through the systemic
53:39effects they exert via a change in
53:41hormone secretion although many patients
53:43with Celler tumors may undergo surgical
53:45resection with an uncomplicated general
53:47anesthetic there are several
53:48preoperative considerations that will
53:50affect management of the patient the
53:52patient should undergo a preoperative
53:54evaluation of their hormonal function to
53:56detect hypersecretion of pituitary
53:58hormones common in pituitary adenomas x'
54:00as well as pan hypopituitarism
54:05the hormones that may be secreted by
54:07pituitary tumors include prolactin
54:09growth hormone corticotropin and thyroid
54:13stimulating hormone patients with
54:15excessive growth hormone eventually will
54:17develop acromegaly the anesthetist
54:19should be prepared for a difficult
54:20airway as well as post-operative
54:22respiratory complications in the
54:23acromegaly patient patients with a
54:26corticotropin secreting adenoma will
54:28develop Cushing disease these patients
54:31may have a typical cushingoid habitus
54:32that may make airway management
54:34challenging
54:35in addition venous access may be
54:37difficult an intraoperative
54:38hyperglycemia is likely
54:43patients with thyroid stimulating
54:45hormone hypersecretion will exhibit
54:47signs of hyperthyroidism like
54:49tachycardia and weight loss these
54:51patients should be managed in the
54:52preoperative period with anti thyroid
54:54medications and beta blockade clothes
54:57hemodynamic monitoring during surgery is
54:59essential patients with pan
55:01hypopituitarism will need hormone
55:03replacement including cortisol
55:05levothyroxine and possibly ddavp these
55:09medications should be continued in the
55:10perioperative period small pituitary
55:13tumors can be resected from the trans
55:14final approach and larger tumors may
55:17require a craniotomy
55:21intraoperative monitoring of glucose and
55:23electrolytes is essential particularly
55:26if the patient has pre-existing diabetes
55:27insipidus or if the patient develops
55:29signs of diabetes insipidus during the
55:31surgery diabetes insipidus is a common
55:34complication of pituitary surgery due to
55:36the loss of antidiuretic hormone
55:38production it may be temporary or
55:41permanent and may occur either in the
55:42intraoperative or post-operative period
55:44it is initially suspected on the basis
55:47of copious urine output as well as
55:49rising serum sodium a urine specific
55:52gravity of less than one point zero zero
55:54five is confirmatory although infusions
55:57of intravenous fluids containing free
55:59water may mitigate the electrolyte
56:00changes replacement of the ADH with
56:02ddavp 0.5 to when Mike intravenously or
56:06subcutaneously is an effective therapy
56:08for diabetes insipidus volume
56:10replacement therapy may be guided with
56:12the use of central venous pressure
56:13monitoring as well as the observation of
56:15systolic variations in blood pressure
56:19a cerebral AVM is an abnormal vascular
56:23connection between the arterial and
56:24venous circulation the absence of an
56:27intervening capillary bed results in a
56:29low resistance path for blood flow
56:31patients may present with hemorrhage
56:33seizure or focal neurologic deficits
56:36cerebral angiography remains the gold
56:38standard for AVM diagnosis although
56:41embolization of the AVM is commonly
56:43performed either radiosurgery or an open
56:45surgical procedure is typically required
56:47subsequent to the embolization to cure
56:49the lesion although these lesions may be
56:51adjacent to vital structures and an
56:53immediate post-operative neurologic
56:55examination may be desirable emergence
56:57from anesthesia following resection of
56:59AVM requires particular care
57:04because of local hemodynamic changes as
57:06a result of the AVM the adjacent vessels
57:09must chronically vasodilator preserve
57:11perfusion when the low-resistance AVM
57:14has been occluded or resected the
57:16adjacent vessels are exposed to a higher
57:17pressure than they are accustomed to
57:19these vessels may not be able to auto
57:21regulate appropriately within the normal
57:23blood pressure range and normal
57:25perfusion pressure breakthrough may
57:26occur this phenomenon is defined by the
57:29regional hyperemia at a normal systemic
57:31blood pressure normal perfusion pressure
57:34breakthrough may result in vasogenic
57:35edema and hemorrhage it can be minimized
57:38with careful blood pressure control
57:40preoperative embolization likely
57:42decreases its incidence as well
57:44following the resection of large AVMs
57:46are those in the posterior fossa taking
57:49the patient to the ICU and a ventilated
57:51and sedated state may be appropriate
57:52should the decision be made between the
57:55surgeon and anesthesia to allow
57:56emergence and excavation of the trachea
57:58aggressive management of blood pressure
58:00should be instituted and coughing should
58:02be avoided
58:03intravenous labetalol and hydralazine
58:05may be adequate but an accordeon
58:07infusion may be appropriate for blood
58:09pressure control blood pressure control
58:11needs to be conducted using an
58:12anticipated and prophylactic approach
58:14rather than a reactive one as the delay
58:16in treating hypertension may be
58:18detrimental intravenous lidocaine again
58:20can be used to blunt coughing
58:25cerebral aneurysms which are abnormally
58:28shaped cerebral arteries are relatively
58:30prevalent vascular abnormalities which
58:32means that there is approximately a 5%
58:33incidence at autopsy and they arise from
58:35congenital weakness of the vessel wall
58:37as well as extrinsic influences such as
58:39hypertension and cigarette smoking they
58:42are more prevalent in women than in men
58:43some aneurysms become clinically
58:46significant when they rupture resulting
58:48in arterial bleeding into the
58:49subarachnoid space this event typically
58:52causes severe headache and may also
58:53cause focal neurologic deficit lethargy
58:56and coma
58:58for patients who survived their
59:00hemorrhage surgical or endovascular
59:02intervention to secure the aneurysm is
59:04essential to prevent further hemorrhage
59:06in addition many patients are
59:08incidentally found to have cerebral
59:09aneurysms and they may need intervention
59:12to decrease the risk of an initial
59:13subarachnoid hemorrhage intervention for
59:16a cerebral aneurysm may include a
59:17craniotomy and surgical clipping or
59:19endovascular coiling an aesthetic
59:22considerations for cerebral aneurysm
59:24surgery are somewhat different in those
59:25patients who have experienced a
59:27subarachnoid hemorrhage as compared with
59:29those who present for elective repair
59:31patients with aneurysmal subarachnoid
59:33hemorrhage are at risk for numerous
59:34complications that may affect the
59:36anesthetic plan these include cardiac
59:39dysfunction neurogenic or cardiogenic
59:41pulmonary edema hydrocephalus as well as
59:44further hemorrhage from the aneurysm
59:45this last complication is perhaps the
59:48most devastating careful attention to
59:50hemodynamics particularly during
59:52stimulating procedures is essential to
59:54avoid REO current bleeding
59:58laryngoscopy and placement of the head
1:00:00and the Mayfield devices are two points
1:00:02at which the anesthetist must be
1:00:03particularly vigilant about maintaining
1:00:05adequate depth of anesthesia following
1:00:08subarachnoid hemorrhage cardiac
1:00:09dysfunction and pulmonary edema commonly
1:00:11resolve over time the cardiac
1:00:13dysfunction may be severe resulting in
1:00:15electrocardiogram changes elevated
1:00:17troponin and even cardiogenic shock
1:00:20echocardiography may reveal hypokinesis
1:00:22and a distribution not consistent with
1:00:24an anatomic vascular territory
1:00:26unfortunately the need to secure the
1:00:28aneurysm in a timely fashion may require
1:00:30the anesthetist to provide an anesthesia
1:00:32despite ongoing cardiac and pulmonary
1:00:34issues hemodynamic support with
1:00:36carefully titrated vasopressors may be
1:00:38necessary
1:00:39recognizing the risk of elevating the
1:00:40blood pressure too much hypoxemia can
1:00:43often be managed with increased fio2 and
1:00:45positive end expiratory pressure with
1:00:48the exception of a hemodynamically
1:00:49unstable patient Surgical clipping of a
1:00:52ruptured aneurysm should rarely be
1:00:53postponed
1:00:57once the aneurysm is secured with an
1:00:59aneurysm clip the risk of recurrent
1:01:01hemorrhage from the aneurysm is removed
1:01:03although careful attention to
1:01:05hemodynamics as well as coughing during
1:01:07emergence is still important the concern
1:01:09of devastating hemorrhage is diminished
1:01:11the patient presenting for an elective
1:01:13aneurysm procedure will typically have
1:01:15good brain conditions with easily
1:01:17achievable relaxation using mannitol 0.5
1:01:19to 1 gram per kilogram mild to moderate
1:01:22hyperventilation and sub mak volatile
1:01:24anesthetic in combination with an opioid
1:01:26infusion following subarachnoid
1:01:28hemorrhage brain relaxation may be more
1:01:30difficult to achieve an intravenous
1:01:32anesthesia may be required drainage of
1:01:35CSF via a lumbar drain or external
1:01:37ventricular drain can be used at the
1:01:39discretion of the surgeon
1:01:43carotid stenosis is a common cause of
1:01:45transient ischemic attack and ischemic
1:01:47stroke it is amenable to surgical
1:01:49intervention and endovascular stenting
1:01:52in older studies carotid endarterectomy
1:01:54'z were found to be beneficial in
1:01:56reducing stroke rate and symptomatic
1:01:58patients with around greater than to
1:02:00equal to 70% internal carotid artery
1:02:02stenosis in to a lesser extent in
1:02:04patients with 50 to 69 percent internal
1:02:06carotid artery stenosis in asymptomatic
1:02:09carotid stenosis the benefit of surgical
1:02:12intervention over medical therapy
1:02:13appears to be somewhat smaller and it
1:02:15depends on the incidence of
1:02:16perioperative stroke in addition surgery
1:02:19is associated not only with the risk of
1:02:21stroke but also myocardial infarction
1:02:23wound infection and so forth at the time
1:02:26of the nasa trial medical therapy
1:02:28consisted primarily of daily aspirin
1:02:30with advances and medical therapy
1:02:32including more aggressive lipid lowering
1:02:33drugs as well as other effective
1:02:35antiplatelet agents and better
1:02:37antihypertensive therapy the margin of
1:02:39benefit of surgery may be even less
1:02:41appropriate candidate selection for
1:02:43surgery has therefore become extremely
1:02:45important preoperative evaluation of the
1:02:48asymptomatic patient depends on
1:02:49assessment of the risk for progression
1:02:51to stroke and weighing that risk against
1:02:53the morbidity of the procedure both
1:02:55general and regional anesthesia may be
1:02:57used for a carotid endarterectomy
1:03:01a regional anesthesia is accomplished
1:03:04with a superficial cervical plexus block
1:03:06or a combination of superficial and deep
1:03:09block this technique allows continuous
1:03:11neurologic assessment during the surgery
1:03:13which is especially useful at the time
1:03:15of carotid cross-clamp some patients and
1:03:18surgeons may not be agreeable to this
1:03:20anaesthetic technique however general
1:03:23anesthesia with an endotracheal tube is
1:03:24therefore a more common technique for a
1:03:26carotid these patients are at an
1:03:28increased risk for perioperative
1:03:29complications given their high
1:03:31prevalence of coronary artery disease
1:03:32hypertension COPD diabetes and chronic
1:03:36kidney disease continuation of beta
1:03:39blockers and statins is appropriate for
1:03:41patients who are receiving these
1:03:42medications preoperatively the
1:03:44initiation of a beta blocker prior to
1:03:46surgery although theoretically indicated
1:03:48must be considered within the context of
1:03:50the preliminary results from the poise
1:03:51trial reporting higher mortality from
1:03:53stroke
1:03:56blood pressure should be maintained as
1:03:58close to baseline as possible throughout
1:04:00the surgery without evidence to support
1:04:02it some advocate raising the blood
1:04:04pressure during carotid cross-clamp to
1:04:05improve flow through collateral vessels
1:04:07this practice presupposes that
1:04:09collateralization is marginal and will
1:04:11be helped by the elevation and pressure
1:04:13collateral flow may be marginal but it
1:04:15may also be absent or entirely adequate
1:04:17in the latter two situations elevation
1:04:20and blood pressure through the use of
1:04:21phenylephrine will only increase
1:04:23myocardial oxygen demand however
1:04:25evidence of hypoperfusion EPSA lateral
1:04:28to the cross clamp is reason to consider
1:04:29blood pressure elevation several CNS
1:04:32monitors may be used during carotid
1:04:34under general anesthesia EEG allows for
1:04:37easy detection of decline in spectral
1:04:39power on the hemisphere EPSA lateral to
1:04:41the surgery which would be concerning
1:04:42for ischemia near-infrared spectroscopy
1:04:45is also promising for its ability to
1:04:47demonstrate relative changes between the
1:04:49ipsilateral and contralateral
1:04:50hemispheres but it has not come into
1:04:53common use yet TCD is particularly
1:04:56attractive however as it allows
1:04:57determination of changes in flow during
1:04:59carotid cross-clamp as well as detection
1:05:01of emboli the formal problem can be
1:05:03avoided with a shunt during the surgical
1:05:05procedure but a shunt increases the risk
1:05:07of the latter problem TCD is useful in
1:05:10providing real-time information on the
1:05:12nature of cerebral blood flow during
1:05:13cross-clamp and in guiding the decision
1:05:15on whether to shine a decrease in flow
1:05:18velocity of up to 60% is typically well
1:05:20tolerated in the anesthetized patient
1:05:22during this procedure as the pulsatilla
1:05:24tia flow will decrease when it is
1:05:25supplied via collateral vessels it is
1:05:27important to make decisions based on
1:05:29mean flow velocity not systolic or
1:05:31diastolic velocity should a shunt be
1:05:34needed the development of micro embolic
1:05:36signals can provide feedback to the
1:05:38surgeon if there is a modifiable
1:05:39technique to the surgical procedure at
1:05:42the end of the endarterectomy during
1:05:43surgical closure continued presence of a
1:05:45good flow velocity waveform on TC d
1:05:48provides confirmation of stability of
1:05:50the graph and lack of intimal flap or
1:05:52thrombosis although each monitor has
1:05:54attractive features ultimately user
1:05:56familiarity and comfort will determine
1:05:58its use sustained elevation of flow
1:06:01velocity exceeding 100% of baseline
1:06:03values is highly suggestive of the
1:06:05development of hyper perfusion syndrome
1:06:07and should prompt lowering of the
1:06:08systemic blood pressure immediately
1:06:13rapid emergence and tracheal excavation
1:06:16at the end of the procedure is desirable
1:06:18because it allows immediate neurologic
1:06:20assessment hemodynamic changes can occur
1:06:23in the post-operative period from
1:06:24denervation of the carotid baroreceptor
1:06:26in addition headache optin Dacian and/or
1:06:30focal neurologic deficit in the
1:06:32post-operative period should prompt one
1:06:33to consider hyperemia hemorrhage or
1:06:35ischemic stroke carotid artery stenting
1:06:38may be used to treat carotid stenosis as
1:06:40well it is an attractive procedure in
1:06:43that it is minimally invasive and can be
1:06:44performed under sedation the Sapphire
1:06:47trial has indicated that sensing may be
1:06:49a reasonable option in asymptomatic
1:06:50patients with tight stenosis currently
1:06:53most centers reserved stinting for
1:06:55patients who are poor surgical
1:06:56candidates an aesthetic considerations
1:06:59for this procedure are important even
1:07:01though it is typically performed under
1:07:03sedation these patients tend to have
1:07:05significant medical comorbidities
1:07:07conversion to general anesthesia may
1:07:09incur significant risk furthermore the
1:07:12procedure itself may induce significant
1:07:14hemodynamic changes most notably
1:07:16bradycardia or asystole during balloon
1:07:18angioplasty of the internal carotid
1:07:20artery
1:07:21although pretreatment with atropine may
1:07:23prevent this complication a brisk
1:07:25tachycardia is frequently not desirable
1:07:27in these patients
1:07:31some intracranial neurosurgical
1:07:34procedures are performed on awake
1:07:35patients in order to facilitate
1:07:37monitoring of the region of the brain on
1:07:39which the surgeon is operating these
1:07:41patients are sedated and pain-free yet
1:07:43able to respond to verbal or visual
1:07:45commands these procedures require a
1:07:47particular attention on the part of the
1:07:49anesthetist to provide patient comfort
1:07:50and safety
1:07:51typically these surgeries are for tumors
1:07:53adjacent to eloquent cortex or for
1:07:56resection of an epileptic focus
1:07:58frequently the decision to perform the
1:08:00procedure awake has been made by the
1:08:01neurosurgeon prior to the patient
1:08:03meeting the anesthetist it is the role
1:08:05of the anesthetist to determine whether
1:08:07the patient is an appropriate candidate
1:08:08for an awake procedure to coordinate
1:08:10with a neurosurgeon the anesthetic plan
1:08:12and to support and reassure the patient
1:08:14through the process although the patient
1:08:16with a difficult airway obstructive
1:08:18sleep apnea or orthopnea may present a
1:08:20relative contraindication to an awake
1:08:22craniotomy it is the patient with severe
1:08:24anxiety claustrophobia or other
1:08:26psychiatric disorders who may be
1:08:28particularly inappropriate for this type
1:08:30of procedure
1:08:34preoperative evaluation should be
1:08:36complete and should include a thorough
1:08:37airway examination conversion to a
1:08:40general anesthetic remains a possibility
1:08:41at any point during the procedure
1:08:44extensive discussion with the patient
1:08:46regarding the plan is essential to
1:08:47prepare him or her for the experience in
1:08:49the operating room although the patient
1:08:51may be kept awake for the entire surgery
1:08:53to facilitate patient tolerance of the
1:08:55procedure and asleep awake asleep
1:08:57pathway is often chosen this anaesthetic
1:09:01plan involves a general anesthesia for
1:09:02the skin incision initial craniotomy and
1:09:05then for the closure in the end while
1:09:07the patient is allowed to emerge from
1:09:08anesthesia for the middle portion of the
1:09:10surgery in which the surgeon is working
1:09:11around important structures this general
1:09:14pathway can take on many forms however
1:09:16in particular the asleep portion of the
1:09:19procedures may be performed without an
1:09:20airway with an LMA or with an
1:09:22endotracheal tube in place for suitable
1:09:25candidates spontaneous ventilation with
1:09:27propofol anesthesia is an attractive
1:09:29option as it allows straightforward
1:09:30emergence with minimal coughing gagging
1:09:32or straining in addition propofol
1:09:35provides a nice anesthetic for these
1:09:37patients because of its low incidence of
1:09:38nausea and vomiting during the awake
1:09:40period benzodiazepines should be avoided
1:09:43as they may interfere with the electrode
1:09:44choreography during epilepsy surgery an
1:09:47LMA is a suitable alternative to no
1:09:49airway as it can frequently be removed
1:09:51with little movement of the patient as
1:09:52he or she emerges from anesthesia
1:09:55topical application of lidocaine to the
1:09:57airway prior to the insertion of the LMA
1:09:59supplemented with lidocaine jelly on the
1:10:01LMA may improve patient tolerance during
1:10:03emergence an endotracheal tube provides
1:10:06the most secure airway but it is also
1:10:07the most difficult to remove during the
1:10:09procedure particularly with the
1:10:11patient's head secured in rigid fixation
1:10:13if this pathway is chosen several
1:10:15options exist to minimize coughing as
1:10:17the patient emerges prior to placement
1:10:19of the endotracheal tube the larynx and
1:10:21trachea may be localized with lidocaine
1:10:23in addition the cuff of the endotracheal
1:10:25tube can be filled with 4% lidocaine
1:10:27rather than air finally allowing the
1:10:30patient to emerge on an infusion of
1:10:32low-dose rummy fentanyl or Dex
1:10:34mediterrenean may facilitate excavation
1:10:36with little movement
1:10:39during the awake portion of the
1:10:41procedure all sedatives are typically
1:10:43withheld for particularly simulating
1:10:45events like drilling and in coordination
1:10:47with a surgeon small boluses of propofol
1:10:49may be given antiemetics may be given
1:10:52for nausea and small doses of fentanyl
1:10:54for discomfort following this critical
1:10:56portion of the surgery the patient may
1:10:58be fully anesthetized for the surgical
1:10:59closure initiating a propofol infusion
1:11:02and continuing with spontaneous
1:11:04ventilation is again a good option
1:11:06otherwise manipulation of the airway to
1:11:08place an LMA or endotracheal tube will
1:11:10be necessary while avoiding the sterile
1:11:12field
1:11:12for procedures in which the patient is
1:11:14kept awake throughout the process
1:11:15planning and collaboration with a
1:11:17surgeon should include a discussion of
1:11:19sedation that allows continued
1:11:20participation of the patient in the
1:11:22neuro monitoring Dex Mehta Tama Dean a
1:11:25central alpha-2 agonist is a useful
1:11:27medication that can be used as an
1:11:28infusion in these patients it provides
1:11:31good sedation and blood pressure control
1:11:32without respiratory depression and it
1:11:34allows the patient to respond to
1:11:36commands appropriately
1:11:39the presence of traumatic brain injury
1:11:41is a primary determinant and quality of
1:11:43outcome for patients suffering from
1:11:44trauma and necess are involved in the
1:11:47care of these patients in many different
1:11:48settings including the initial
1:11:50resuscitation in the emergency
1:11:51department anaesthetic management in the
1:11:53operating room and ongoing care in the
1:11:55ICU secondary injury includes insults
1:11:58resulting from inflammation super oxide
1:12:00production excitotoxin amino acid
1:12:03release and apoptosis these mechanisms
1:12:06are not preventable at this point
1:12:10the initial approach to patients with
1:12:12the traumatic brain injury should be
1:12:13similar to that of any trauma patient as
1:12:15outlined in the advanced trauma life
1:12:17support by the American College of
1:12:19Surgeons airway and breathing are
1:12:21obviously of paramount importance and in
1:12:23a critically ill patient but even more
1:12:25so in patients with head injuries given
1:12:27the sensitivity of the brain to
1:12:28hypoxemia and hypercapnia pre-hospital
1:12:31intubation of the patient with traumatic
1:12:33brain injury is controversial outcome
1:12:36may be worsened by ultra early
1:12:37hyperventilation if the patient arrives
1:12:40in the emergency department intubated
1:12:42one must confirm proper placement of the
1:12:44endotracheal tube with a carbon dioxide
1:12:46detector if the patient is not intubated
1:12:48immediate attention should be focused on
1:12:50assessing the airway and making
1:12:52preparations for intubation patients
1:12:54with traumatic brain injury usually have
1:12:56several indications for intubation
1:12:57including a decreased level of
1:12:59consciousness increased risk of
1:13:01aspiration as well as a concern for
1:13:03hypoxemia and hypercarbia sometimes
1:13:06these patients must be intubated and
1:13:07sedated simply to allow further
1:13:09diagnostic studies
1:13:13patience with the traumatic brain injury
1:13:15have a five to six percent incidence of
1:13:17an unstable cervical spine injury risk
1:13:20factors include a motor vehicle accident
1:13:22and a GCS less than eight therefore all
1:13:24attempts at intubation should include
1:13:26in-line neck stabilisation to decrease
1:13:28the chance of worsening a neurologic
1:13:29injury
1:13:30this maneuver may worsen the view of the
1:13:32glottis making intubation more difficult
1:13:34therefore one must always have a back-up
1:13:36plan and device in mind when performing
1:13:38an emergency intubation including two
1:13:40but not limited to an LMA and fiber
1:13:42optic or video technology patients with
1:13:46TBI should generally be intubated orally
1:13:48as the potential presence of a basilar
1:13:50skull fracture could increase the risk
1:13:51associated with the nasal intubation
1:13:53a surgical airway remains an appropriate
1:13:56procedure for patients with severe
1:13:57facial trauma and a difficult airway
1:14:00minimizing the risk of aspiration during
1:14:02airway procedures is essential the
1:14:04efficacy of cricoid pressure has not
1:14:06been demonstrated and it may displace
1:14:08cervical fractures nevertheless it
1:14:10remains the standard of care during
1:14:11rapid sequence intubation another
1:14:14important consideration is a choice of
1:14:16drugs to facilitate your intubation
1:14:18hypotension is extremely detrimental to
1:14:20the injured brain as discussed
1:14:22previously therefore the choice of drugs
1:14:24must be tailored to each individual
1:14:26patient pentathol in a dose of three to
1:14:28six milligrams per kilogram is a useful
1:14:30drug in you bulimic hemodynamically
1:14:32stable patients through its pharmacology
1:14:35this drug decreases cerebral blood flow
1:14:37cerebral blood volume and ICP however it
1:14:41also causes a large decrease in systemic
1:14:43vascular resistance which may be
1:14:45deleterious to blood pressure in a
1:14:46hypovolemic patient propofol has similar
1:14:49effects another choice to facilitate
1:14:52intubation is etomidate in doses of 0.2
1:14:55to 0.3 milligrams per kilogram this drug
1:14:58also decreases cerebral metabolic
1:14:59requirements and cerebral blood flow but
1:15:02has a less effect on the patient's blood
1:15:03pressure care must be taken in the
1:15:06acutely unstable patient with the
1:15:08administration of any potent sedative
1:15:09hypnotic drug as even etomidate can
1:15:12produce profound hypotension another
1:15:14drug that is useful to blunt the effects
1:15:16of laryngoscopy and intubation on ICP is
1:15:18lidocaine in doses of 1.5 milligrams per
1:15:22kilogram this drug decreases ICP with
1:15:24minimal hemodynamic effects
1:15:26finally the choice of muscle relaxant is
1:15:28somewhat controversial administering
1:15:31muscle relaxants prevent coughing and
1:15:33the resultant spikes of ICP the main
1:15:35choice is between six enol choline and
1:15:37rocuronium the two agents with the
1:15:39fastest onset the main drawback to
1:15:41rocuronium is a prolonged effect when a
1:15:44rapid sequence dose of 1.2 milligrams
1:15:46per kilogram is used while the argument
1:15:48against sexina choline is a potential
1:15:50increase in ICP however Kovarik and
1:15:53others studied the effects of this drug
1:15:55and neurologically injured patients and
1:15:57found no increase on ICP once the
1:16:00trachea is intubated the initial
1:16:01ventilation parameters should include a
1:16:02hundred percent oxygen and arterial
1:16:05carbon dioxide should be maintained in
1:16:06the lower normal range of around 35
1:16:08millimeters of mercury and should be
1:16:10guided by arterial blood gas analysis
1:16:15the goal of resuscitation in any trauma
1:16:18patient is to establish adequate
1:16:20circulation so that the organ perfusion
1:16:21may be maintained the long-standing
1:16:24belief that aggressive resuscitation in
1:16:26traumatic brain injury patients should
1:16:27be avoided to minimize cerebral edema is
1:16:29no longer considered appropriate the
1:16:32overwhelming evidence of harm from
1:16:33hypotension necessitates restoration of
1:16:35intravascular volume isotonic fluid
1:16:38should be used to accomplish this goal
1:16:40and it should be noted that lactated
1:16:42ringers solution is slightly hypotonic
1:16:44the goal is to maintain a cerebral
1:16:46perfusion pressure in the range of 50 to
1:16:4870 millimeters of mercury as recommended
1:16:50by the guidelines from the brain trauma
1:16:52foundation in 2007 hypertonic fluids
1:16:55such as 3% saline may be useful in this
1:16:58setting although there is insufficient
1:16:59evidence to justify its routine use
1:17:01phase oppressors and inotropes may be
1:17:04needed after fluid resuscitation to
1:17:05achieve the desired CPP or to treat
1:17:08hypotension while volume restoration is
1:17:10ongoing they should be used judiciously
1:17:12as they are thought to increase the
1:17:14incidence of acute respiratory distress
1:17:16syndrome in the absence of ICP
1:17:18monitoring but with known traumatic
1:17:20brain injury and ICP of at least 20
1:17:22should be assumed and the map should be
1:17:24kept above 60 patients with TBI are
1:17:27typically described by their localized
1:17:29GCS score this simple test facilitates
1:17:32communication between providers and it
1:17:34provides prognostic information a mild
1:17:37head injury is represented by a score of
1:17:3913 to 15 moderate head injury by a score
1:17:42of 9 to 12 and severe head injury by a
1:17:45score of less than or equal to 8 the
1:17:47score should be determined on post
1:17:49resuscitation information as hypotension
1:17:51may depress Mental Status in any patient
1:17:53even those without a traumatic brain
1:17:55injury
1:17:58the people examination is also useful
1:18:01the presence of a unilateral dilated
1:18:04pupil suggests brainstem compression and
1:18:06is a surgical emergency and the presence
1:18:08of dilated pupils bilaterally pretends a
1:18:11dismal prognosis intracranial
1:18:13hypertension predisposes patients to
1:18:15poor outcomes an elevated ICP refractory
1:18:18to therapy is associated with the worst
1:18:20prognosis some controversy exists
1:18:23regarding what constitutes the optimal
1:18:24ICP and CPP prior recommendations were
1:18:28to maintain the CPP at 70 or above and
1:18:30to lower the ICP when it exceeded twenty
1:18:32to twenty five subsequently CPP goals
1:18:35were redefined to the range between 60
1:18:37and 70 to avoid increased morbidity
1:18:39associated with a RDS the range was
1:18:42subsequently broadened to 50 to 70
1:18:47reduction of ICP and patients with head
1:18:49injuries can be accomplished effectively
1:18:51using osmotic diuretics mannitol is the
1:18:54most commonly used agent and is
1:18:55available for intravenous administration
1:18:57in either a 20 or 25% solution common
1:19:01dosages range from 0.25 to one gram per
1:19:04kilogram of body weight mannitol may be
1:19:06used on a repeated schedule but the
1:19:08serum osmolarity should not be allowed
1:19:10to exceed 320 furthermore intravascular
1:19:13volume depletion should be avoided the
1:19:16mechanism of ICP reduction by mannitol
1:19:18may be related to its osmotic effect and
1:19:21shifting fluid from the brain tissue
1:19:22compartment to the intravascular
1:19:23compartment as well as its ability to
1:19:26decrease blood viscosity the latter
1:19:28effect has been postulated to cause
1:19:30reflex vasoconstriction which keeps
1:19:32cerebral blood flow constant while
1:19:33reducing cerebral blood volume in ICP in
1:19:36addition man at all like other
1:19:38hypertonic fluids decreases the
1:19:40production of CSF some individuals may
1:19:43benefit from the use of lasix in
1:19:45combination with mannitol as the
1:19:47combination appears to increase the
1:19:48duration of their effect on ICP both
1:19:51hypertonic saline and HS dextran have
1:19:54been used to manage elevated ICP
1:19:56primarily in the setting of intracranial
1:19:58hypertension refractory to mannitol
1:20:00therapy
1:20:03as the blood brain barrier reflection
1:20:06coefficient to sodium ions is
1:20:08approximately 1 H s establishes a
1:20:10gradient that facilitates the movement
1:20:11of water from the brain into the
1:20:13intravascular space recent evidence
1:20:16indicates that H s may be more effective
1:20:18in controlling ICP than mannitol in
1:20:20addition to its efficacy the proposed
1:20:23benefit of hypertonic saline is the lack
1:20:25of severe electrolyte disturbance which
1:20:27is common with me and at all the brisk
1:20:29diuresis seen with mannitol is absent
1:20:31from H s therapy although H s has been
1:20:34administered both as a bolus and as a
1:20:36continuous infusion currently no firm
1:20:38guidelines have been established for its
1:20:40use in addition no standard
1:20:42concentration has been established for
1:20:44clinical use although most clinical
1:20:46studies have used either 7.5 or 3% at an
1:20:49infusion rate of 20 to 40 mils per hour
1:20:54following prolonged infusions in the ICU
1:20:57hypertonic saline should be tapered off
1:20:59slowly to prevent subsequent
1:21:00hyponatremia and rebound edema in
1:21:03addition h s should be administered
1:21:05through a central line in situations in
1:21:08which hypertonic saline causes an
1:21:09unacceptable hyperchloremia casa dosis a
1:21:12mixture of sodium chloride and sodium
1:21:14acetate can be used as we have mentioned
1:21:17hyperventilation is an effective way to
1:21:18reduce ICP it is useful in the setting
1:21:21of an acutely increased ICP that needs
1:21:23to be controlled until a more definitive
1:21:25therapy can be initiated
1:21:27hyperventilation may be useful in the
1:21:29initial stages of resuscitation of head
1:21:31injured patients or in a patient who
1:21:33suddenly demonstrates signs of
1:21:34herniation hyperventilation causes
1:21:37cerebral basically
1:21:38primarily in the small regulatory
1:21:40arteries in the brain this
1:21:42vasoconstriction rapidly reduces the
1:21:44cerebral blood volume and therefore the
1:21:46ICP the reduction and cbv
1:21:48is achieved at the expense of cerebral
1:21:50blood flow however hyperventilation in
1:21:53the setting of traumatic brain injury
1:21:54remains controversial the degree of
1:21:57hyperventilation that is acceptable is
1:21:59unknown and the duration of
1:22:00hyperventilation that can be used safely
1:22:02and effectively is uncertain the primary
1:22:05concern with hyperventilation is that it
1:22:06may exacerbate cerebral ischemia current
1:22:09recommendations are that the patient who
1:22:11are head injured should be maintained at
1:22:13a normal cap Nia except when hypocapnia
1:22:15is necessary to control acute increases
1:22:17in intracranial pressure
1:22:19chronic hyperventilation should be
1:22:21avoided if possible
1:22:25patients with traumatic brain injury
1:22:27requiring surgery can be subdivided into
1:22:29two major groups with different
1:22:30perioperative concerns these groups
1:22:33include those who require emergent
1:22:34surgery and those who require non
1:22:36emergent surgery the emergent group can
1:22:39be subdivided into neurosurgical
1:22:40procedures and non neurosurgical
1:22:42procedures we will address the emergent
1:22:45neurosurgical patient these patients
1:22:47commonly arrive in the operating room
1:22:49with an endotracheal tube in place if
1:22:51their airway has not yet been secured in
1:22:53the same principles that were discussed
1:22:55in the airway section should be applied
1:22:57often there is little time allotted for
1:22:59the preoperative assessment and once
1:23:01approach must be concise and focused to
1:23:02obtain the pertinent information in a
1:23:04brief amount of time these patients may
1:23:07have other injuries that will affect
1:23:08their care the neurologic condition of
1:23:10the patient can be determined rapidly by
1:23:12obtaining the GCS score examining the
1:23:14peoples and reviewing the CT scan the
1:23:17hemodynamic status of the patient is
1:23:19also extremely important patients may
1:23:22demonstrate Cushing response of
1:23:23hypertension and bradycardia which
1:23:25signifies brainstem compression from
1:23:27raised intracranial pressure
1:23:29however these classic findings may be
1:23:31masked by hypovolemia and their absence
1:23:34does not rule out brain stem compression
1:23:39an estimation of volume status is
1:23:42appropriate other important information
1:23:44includes oxygenation which may be
1:23:46compromised because of pulmonary
1:23:48contusion hematocrit which may be low in
1:23:51the presence of additional injuries and
1:23:52the extent of the evaluation obtained
1:23:55prior to the decision to proceed to the
1:23:56operating room
1:23:57an incomplete evaluation radiographic or
1:24:00otherwise should leave one highly
1:24:02suspicious for missed injuries such as a
1:24:04pneumothorax or intra-abdominal
1:24:06hemorrhage appropriate monitoring must
1:24:09be established rapidly so as to not
1:24:10delay surgical intervention standard
1:24:13monitors should be applied including an
1:24:15ECG pulse oximetry capnography and
1:24:18non-invasive blood pressure measurement
1:24:20two large-bore intravenous catheters are
1:24:23required at a minimum the delay for
1:24:25placement of a central line should occur
1:24:27only if adequate peripheral access
1:24:29cannot be obtained consideration for a
1:24:31femoral line should be made as it can be
1:24:33placed while preparation of the head for
1:24:35surgery is ongoing and a line is
1:24:38desirable but it is secondary priority
1:24:40after venous access
1:24:44these patients usually do not have ICP
1:24:47monitors in place but one can assume the
1:24:49presence of an intracranial hypertension
1:24:50in the setting of an acute
1:24:52space-occupying lesion the presence of
1:24:55midline shift on CT scan and pupillary
1:24:57abnormalities on physical examination
1:24:59reinforce this diagnosis moderate
1:25:02hyperventilation should be used in these
1:25:04patients until the dura is opened as the
1:25:06elevation in ICP is likely more
1:25:08detrimental than the short-term
1:25:09hyperventilation blood pressure
1:25:11management in these patients is critical
1:25:12they may arrive in the operating room in
1:25:15a hypertensive state the hypertension is
1:25:18often a response to the stress of the
1:25:19injury as well as the elevated ICP
1:25:22unfortunately this hypertension may mask
1:25:24an underlying volume deficit due to
1:25:26hemorrhage or high urine output from
1:25:28mannitol administration prior to arrival
1:25:30in the operating room profound
1:25:32hypotension may follow anesthesia
1:25:34induction or more likely after the
1:25:36craniectomy when the intrinsic stimulus
1:25:38for blood pressure elevation is
1:25:39diminished risk factors for post
1:25:42decompressive hypotension include a low
1:25:44GCS score absence of basal cisterns on
1:25:47CT and bilateral dilated pupils to avoid
1:25:51hypotension an intravenous volume
1:25:53loading in the early stages of the
1:25:54anaesthetic is essential particularly in
1:25:56patients with other injuries and
1:25:58significant blood loss
1:26:01the choice of anesthetic agent should be
1:26:04based on the clinical condition of the
1:26:06patient anesthetic requirement for the
1:26:08traumatized CNS is lower adequate
1:26:11anesthesia should be administered
1:26:12without compromising hemodynamics
1:26:14volatile anesthesia is acceptable as it
1:26:17is easily titratable whereas intravenous
1:26:19agents have the benefit of a greater
1:26:21reduction in CB v and ICP nitrous oxide
1:26:25should be avoided as it increases
1:26:26cerebral blood flow and ICP and head
1:26:28injured patients narcotics can be used
1:26:31safely in these patients as long as the
1:26:33blood pressure is not compromised and
1:26:34the patient is mechanically ventilated
1:26:39patients with chronic spinal cord
1:26:41lesions above the level of t7 may
1:26:43develop autonomic hyperreflexia when
1:26:45stimulated below the site of the lesion
1:26:47this is a condition characterized by
1:26:49intense vasoconstriction below the site
1:26:51of the lesion accompanied by cutaneous
1:26:53vaso dilatation above the site
1:26:55hypertension and bradycardia this is a
1:26:58result of the reflex sympathetic
1:26:59stimulation below the lesion unmodulated
1:27:02by super spinal influence from above in
1:27:04severe cases cerebral hemorrhage and
1:27:07myocardial ischemia can occur to reduce
1:27:10the incidence of this complication
1:27:12suppression of the afferent pathway by
1:27:14deepening anesthesia is necessary to
1:27:17this end a spinal anesthetic if possible
1:27:19may be the ideal anesthetic
1:27:24although there are many potential
1:27:25complications of Neurosurgery including
1:27:28massive hemorrhage venous air embolism
1:27:30myocardial infarction pulmonary edema
1:27:33and pressure necrosis the complication
1:27:35of post-operative visual loss is a
1:27:37particular concern in prone spine
1:27:39surgery although it can occur in other
1:27:41settings the visual loss is commonly
1:27:44bilateral and due to ischemic optic
1:27:46neuropathy
1:27:46although retinal artery occlusion and
1:27:48cortical blindness may also occur these
1:27:51incidents of visual loss occurred
1:27:53despite the absence of pressure on the
1:27:54eyes from positioning errors which would
1:27:56result in central retinal artery
1:27:58thrombosis and non anterior or posterior
1:28:00ischemic optic neuropathy ischemic optic
1:28:04neuropathy is associated with blood loss
1:28:06hypotension and more importantly long
1:28:08duration and most certainly has a
1:28:10multifactorial etiology including
1:28:12anatomic variation in the vasculature of
1:28:15individual patients
1:28:19given the increasing recognition of this
1:28:21problem determining whether a patient
1:28:23has experienced any visual changes is an
1:28:26integral part of the post-operative
1:28:27evaluation visual complaints warrant an
1:28:30immediate retinal examination and
1:28:31ophthalmology console currently there is
1:28:34no proven method to prevent it nor is
1:28:36there any reliable method to monitor
1:28:38visual function during these procedures
1:28:39a number of investigators have focused
1:28:42on monitoring interoperative intraocular
1:28:44pressure which is unlikely to yield
1:28:45meaningful results on the other hand
1:28:48monitoring a VP may provide more useful
1:28:51information staging of a complex spine
1:28:54procedure may be the most effective
1:28:55means of preventing this devastating
1:28:57complication as limiting the duration of
1:28:59the procedure would also limit the risk
1:29:01of hypotension and blood loss
1:29:05in conclusion in the absence of
1:29:08compelling evidence in humans regarding
1:29:10the benefit of one practice or another
1:29:11it is difficult to present firm
1:29:13guidelines with respect to the
1:29:14prevention of intraoperative ischemic
1:29:16insult for patients undergoing surgical
1:29:19procedures with an anticipated period of
1:29:21cerebral ischemia such as cerebral
1:29:23aneurysm surgery or cerebral vascular
1:29:25bypass procedures either volatile
1:29:27anesthesia or an intravenous technique
1:29:29is appropriate it is reasonable to
1:29:31administer additional propofol or
1:29:33thiopental prior to vessel occlusion
1:29:35even though this intervention can be
1:29:37guided by EEG monitoring with the goal
1:29:39of achieving birth suppression this that
1:29:41may not be necessary or even beneficial
1:29:43you glycemia prior to vessel occlusion
1:29:46is desirable but frequent glucose checks
1:29:48are essential throughout the anesthetic
1:29:50to avoid episodes of hypoglycemia if
1:29:52insulin is administered finally
1:29:55hyperthermia should be avoided during
1:29:56this time with the temperature kept at
1:29:58or below 36 degrees Celsius
1:30:03the following are key concepts either
1:30:06covered or relevant to this lecture
1:30:10most of the physiologic parameters
1:30:13necessary to measure are not easy to
1:30:14record in the clinical setting such as
1:30:16the effect of drugs on cerebral blood
1:30:18flow cerebral metabolic rate or
1:30:20intracranial pressure to understand ICP
1:30:23the anesthetist should remember the
1:30:25analogy of the brain as a closed box out
1:30:27of which something must leave if
1:30:29something else goes in although blood is
1:30:31the smallest of the four kinds of
1:30:33tissues located in the brain its
1:30:35importance lies in the fact that the
1:30:36cerebral blood volume and hence ICP can
1:30:39be changed very rapidly
1:30:43different anesthetics like barbiturates
1:30:45propofol etomidate the volatile agents
1:30:48and narcotics all have somewhat
1:30:50different effects on cerebral blood flow
1:30:51and cerebral metabolic rate in general
1:30:54intravenous drugs reduce cerebral blood
1:30:56flow whereas volatile agents are
1:30:58vasodilators essentially all agents
1:31:01except perhaps ketamine reduce cerebral
1:31:03metabolic rate nitrous oxide is not
1:31:06benign and its effects on the brain it
1:31:09can considerably increase cerebral blood
1:31:11flow
1:31:13all volatile anesthetics can increase
1:31:16cerebral blood flow and ICP in some
1:31:19cases dramatically nonetheless these
1:31:21drug induced ICP increases have never
1:31:24been demonstrated to be detrimental and
1:31:25are relatively easily counteracted by
1:31:27other ICP control measures including
1:31:29hyperventilation in a patient who is
1:31:32severely hypertensive because of
1:31:33intracranial hypertension
1:31:34it is probably unwise to aggressively
1:31:37lower the blood pressure
1:31:40the five factors that control cerebral
1:31:42blood flow cerebral blood volume and
1:31:44intracranial pressure are paco2 pao2
1:31:48plus arterial content autoregulation
1:31:50cerebral flow metabolism coupling and
1:31:53autonomic nervous system control while
1:31:56mechanisms controlling flow metabolism
1:31:58coupling are unknown it is important to
1:32:00recognize that anesthetics do not
1:32:02uncouple flow and metabolism
1:32:06the sitting position although
1:32:08accompanied by hazards of air embolism
1:32:10and other problems is still in use in
1:32:12neurosurgery in spite of the problems it
1:32:15is not clear that alternative positions
1:32:17are any better continued reduction of
1:32:19brain swelling even though the cranium
1:32:21is open is key to successful neuro
1:32:23anesthesia relaxation of the brain
1:32:26during the neurosurgical procedure
1:32:27itself may require not only
1:32:28hyperventilation but also osmotic
1:32:31diuretic such as man at all during
1:32:33cerebral aneurysm surgery there must be
1:32:35a plan for dealing with a sudden rupture
1:32:37and the blood loss that can occur from
1:32:39such a tiny operative site
1:32:48you

About this transcript

This page contains the full transcript of A Look at Neuroanesthesia by Sterling Anesthesia Education Solutions, Inc., generated from the public captions YouTube serves with the video. The transcript has 14,198 words across 2,541 segments, with the original timestamps preserved so you can click any line to jump to that moment in the embedded player.

What you can do with it

Use the transcript to take notes, quote the speaker, build a study guide, generate a summary with ChatGPT or Claude via the YouTube Summary tool, or export it as a timed subtitle file with YouTube to SRT. You can also re-open it in the transcriber to translate the transcript into 100+ languages.

Free YouTube transcript tool

YouTube2Text is a free YouTube transcript generator — no signup, no daily limit. Paste any YouTube link and get the full transcript instantly, with timestamps, click-to-jump, translation to 100+ languages, AI prompts for ChatGPT, Claude, and Gemini, and exports to TXT, SRT, VTT, or Markdown.