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Ventilator Modes (Mechanical Ventilation - Lecture 7) — Transcript

by Strong Medicine · 2,940 words · 499 segments · language en · Watch on YouTube

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0:00[Music]
0:14hello this is Eric strong again and this
0:16is the seventh lecture in this series on
0:19mechanical ventilation the topic uh
0:22today is ventilator modes if you've
0:24watched the first six lectures you may
0:26have started wondering when I was ever
0:28going to get around to discuss how
0:30actually to program the ventilator well
0:32this is the point where I will begin to
0:34make the transition from the physiology
0:36background to the practical
0:39application the learning objectives of
0:42this lecture are as follows uh first to
0:45understand the three variables which
0:46Define a v ventilator
0:49mode next to be familiar with the most
0:52commonly used ventilator modes that is
0:54assist control simv pressure control and
0:59pressure support
1:01ventilation finally to know how to
1:03choose modes for a specific situation
1:06and be aware of the advantages and
1:07disadvantages of each before I go any
1:10further I want to point out two things
1:13uh first the topics of ventilator modes
1:16and ventilator options are each
1:18difficult to understand without first
1:19learning about the other uh therefore
1:21you may not fully appreciate this
1:23lecture until after seeing the next one
1:25um and may even benefit from watching it
1:27again afterwards um second as
1:30problematic and inconsistent as
1:32terminology is within the general
1:33subject of mechanical ventilation I I
1:36find personally that there's no specific
1:38Topic in which this is more of an issue
1:39than with ventilator modes uh people
1:42will often use different terms to refer
1:44to the exact same form of ventilation
1:46and sometimes even use the same term to
1:49refer to different forms of ventilation
1:51this is a partially a consequence of
1:53different brands of ventilators coining
1:55fancy sounding names for a basic vent
1:57mode which could be known by a different
1:59name when by a different brand of vent
2:02the most common place this becomes a
2:04difficulty is when Physicians and
2:05respiratory therapists uh disagree with
2:07a terminology about what mode a
2:09particular patient is on uh usually a
2:12physician bases their language on what
2:13they've read in the textbook about
2:15ventilators in the general sense while
2:17the RT bases the language on the
2:19specific ventilator being used now
2:22whenever there seems to be a
2:23disagreement I advised just discussing
2:26the specifics of the form of ventilation
2:28being delivered and usually you'll
2:30discover that you are both describing
2:31the same
2:33thing patterns of ventilation also known
2:36as ventilator modes can be partially
2:38defined based on the following three
2:40variables the trigger variable the
2:43control variable and the cycling
2:45variable I will discuss each one at a
2:49time the trigger variable defines how
2:52the ventilator determines when to
2:54initiate a machine driven breath for
2:57patients with no spontaneous
2:58respirations the passage of a specific
3:01duration of time is a trigger for a
3:03breath for patients with spontaneous
3:06respirations this variable may be set to
3:08either pressure triggered or flow
3:10triggered such that the ventilator
3:12delivers a breath when either a
3:14threshold negative pressure or a
3:16threshold degree of flow is detected
3:19both pressure triggered and flow
3:20triggered ventilation are equally
3:22effective and there is rarely an
3:23advantage to switching from one to the
3:25other uh with the exception of patients
3:27with COPD who may do better with a flow
3:30trigger uh triggering options used less
3:33frequently include volume uh chest wall
3:36electrical impedance and
3:38motion the control variable defines what
3:42aspect of inspiration is the primary
3:44variable controlled by the venator uh
3:47during inspiration uh the most common
3:49options include pressure controlled and
3:52flow controlled in pressure controlled
3:55ventilation the pressure delivered to
3:57the airway is constant regardless of aay
4:00resistance or
4:01compliance therefore if pressure is
4:03preset and constant changing resistance
4:06or compliance results in changing title
4:09volumes in flow controlled ventilation
4:12the flow is controlled as a function of
4:14the preset title volume and the
4:16additional designation of flow pattern a
4:19flow pattern also known as flow Contour
4:21will be discussed later in this lecture
4:23as well as in lecture eight um if flow
4:26is predetermined changes in resistance
4:28or compliance result in changes in area
4:32pressures it should be noted that most
4:34clinicians and even many textbooks refer
4:36to flow controlled volume cycled
4:39ventilation such as most forms of assist
4:42control and
4:43SV as quote volume controlled although
4:47this seems like a logical description as
4:49the clinician is directly specifying and
4:52thus controlling the title volume to be
4:54delivered the ventilator is actually
4:56controlling the flow rate um some people
4:58would therefore state that usage of the
5:00term volume control is technically
5:02erroneous While others would brush off
5:04this criticism as inconsequential
5:08semantics the cycling variable now
5:11defines what signals the ventilator to
5:12terminate inspiration for example with
5:15volume cycled ventilation the ventilator
5:18will cease inspiration after a preset
5:20volume has been delivered with flow
5:23cycled ventilation the ventilator will
5:24cease inspiration after air flow drops
5:27below a preset threshold uh typically
5:29approximately 25% Peak flow ventilators
5:33can also be time cycled or pressure
5:35cycled the cycling variable has some
5:38overlap with the control variable and as
5:41kind of mentioned already the two are
5:43frequently um used interchangeably
5:45although some would say this is uh
5:47incorrect usage of the
5:49terms so here is a summary of common
5:52options for the three mode variables uh
5:55a mode can either be time triggered flow
5:57triggered or pressure triggered uh it
6:00could be flow controlled or pressure
6:02controlled and it could be flow cycled
6:05volume cycled or time
6:08cycled typical settings for these
6:10variables are listed to the
6:15right to add to the confusion of
6:17ventilator modes you should also be
6:19familiar with the terms volume targeted
6:23and pressure
6:24targeted volume targeted ventilation
6:27describes any mode in which the
6:28clinician is able to ensure the patient
6:30receives a specific tital
6:32volume pressure Target ventilation
6:35describes any mode in which the
6:37clinician is able to ensure the
6:38patient's inspiratory pressure does not
6:41exceed a maximum pre-specified
6:44value remember from lecture two that
6:47lung compliance is equal to the change
6:49in volume over the change in pressure
6:51therefore by setting either volume or
6:53pressure lung compliance will dictate
6:56the value of the remaining
6:58parameter for example
7:00in volume targeted ventilation High
7:02compliance of the lung will result in
7:04low AA pressures and low lung compliance
7:07will result in high airway pressure
7:09while in pressure targeted ventilation
7:11High lung compliance will result in high
7:13lung volume while low lung compliance
7:16result in low lung
7:18volume ventilator modes also distinguish
7:21between three different types of
7:23breaths mandatory breaths are triggered
7:25by the ventilator and receive full level
7:28of support the number of mandatory
7:30breaths per minute is dictated by the
7:32respiratory rate set by the
7:34clinician assisted breaths are triggered
7:37by the patient and may receive either
7:39full support or only partial
7:42support finally spontaneous breaths are
7:46those which the patient takes which do
7:48not trigger any response from the
7:50ventilator and therefore receive no
7:52support most modes allow fully supported
7:55assisted breaths to count as mandatory
7:58breaths in order to improve patient
8:00Comfort uh prevent patient ventilator D
8:02synchrony and prevent
8:07hyperventilation there are four basic
8:09common ventilator modes used in the ICU
8:11which I will talk about in this lecture
8:14they are assist control synchronized
8:17intermittent mandatory ventilation more
8:20commonly known as
8:21simv a pressure controlled ventilation
8:24and pressure support
8:26ventilation a fifth common mode is
8:28control mandatory ventilation or CMV
8:32however as this mode usually requires
8:34deep sedation and paralysis it is uh
8:37rarely done outside of the O and I won't
8:39be talking about it more at this
8:41time as I mentioned at the beginning of
8:43this lecture there is a severe lack of
8:45consistency with mode terminology
8:48therefore what I am referring to as
8:49pressure controlled ventilation may be
8:51called something different by other
8:53clinicians also in the common usage he
8:55terms assist control and SV imply volume
8:59targeted ventilation however pressure
9:02targeted assist control and SV also
9:04exist though they are much more rarely
9:08used to best understand the differences
9:11between these four ventilator modes I
9:13will first provide a brief description
9:15of the mode as well as its possible
9:18options for the trigger control and
9:20cycling variables I will then dis
9:22display graphs of air flow AA pressure
9:25and volume as functions of time here
9:28each hash mark along the horizontal axis
9:31will represent 1 second a peep for all
9:34examples will be set at 5 cm of water uh
9:37lastly I will list the advantages
9:39disadvantages and indications for each
9:44mode the first mode I will discuss is
9:47assist control this mode has a mix of
9:50mandatory and fully supported assisted
9:53breaths all breaths once triggered are
9:56treated the same and have a consistent
9:57title volume the trigger for assist
10:00control may be either time pressure or
10:03flow the control variable is technically
10:06flow though as mentioned before many
10:08people will refer to this as volume
10:11control and the cycling variable is
10:13typically
10:15time here is the typical appearance of
10:18flow pressure and volume graphs for the
10:21assist control mode in assist control
10:24the ventilator senses an inspiratory
10:25effort by the patient and responds to
10:28each by delivering a preset title volume
10:31the patient is able to vary respiratory
10:33rate but a backup rate is set to prevent
10:35hypo ventilation such that if a certain
10:38period of time passes without the
10:40patient initiating a breath the
10:42ventilator will give one in this
10:44particular example in the pressure
10:46tracing you can see the small downward
10:48deflections just before the first third
10:51and fourth breaths these are the result
10:53of the patient trying to take a breath
10:55on his own uh this small but relatively
10:58negative pressure pressure triggers the
11:00ventilator to deliver a fully supported
11:02breath the second breath has no such
11:05proceeding downward deflection but the
11:07ventilator has given a breath anyway
11:09because the maximum allowable time
11:11without a breath has
11:13elapsed in this case you can see that
11:15just over 4 seconds elapses between the
11:18moment when the first breath is pressure
11:19triggered by the patient and the moment
11:21when the second breath is time triggered
11:24by the ventilator uh thus you can infer
11:26that the backup rate for this patient is
11:29about 14 breaths per
11:31minute also you can see that the
11:33clinician has preset the title volume to
11:36be about 600 milliliters the ventilator
11:39then calculates the necessary flow rate
11:41to provide that tital volume in the
11:43amount of time designated for each
11:46breath advantages of assist control are
11:49that it guarantees a minimum minute
11:51ventilation and requires low work of
11:53breathing on the part of the
11:55patient disadvantages are that it can
11:58lead to respiratory alcal osis an auto
12:01Peep and hypotension in hyperventilating
12:04patients now this is because every
12:06patient triggered breath receives a full
12:08level of support therefore if the rate
12:11is set to 14 and the patient suddenly
12:14begins to breathe at a rate of 28 the
12:17patient's minute ventilation will
12:18literally double leading to a drop in
12:20arterial pco2 and significantly
12:23increased airay
12:26pressures indications for assist control
12:28are critically ill patients requiring
12:30full vent support and in whom
12:32fluctuations in title volume are
12:36undesirable assist control is overall
12:39the most commonly used vent mode in the
12:43world let's now talk about
12:46simv although SV can be used without
12:48pressure support it is very rarely done
12:51so the following will assume that
12:52pressure support is being
12:55used SB is a mix of mandatory breaths
12:59some of which are synchronized with
13:01spontaneous breaths and assisted breaths
13:05the mandatory non-synchronized breaths
13:07are time triggered flow controlled and
13:10time cycled while the assisted breaths
13:13which are synchronized with the
13:14mandatory breaths are pressure or flow
13:17triggered flow controlled and time
13:20cycled lastly the nonsynchronized
13:23assistant breaths are pressure or flow
13:26triggered pressure controlled and flow
13:30cycled these non-synchronized assistant
13:32breaths are only partially supported
13:36breaths in this example of simv the
13:39first and fourth inflations are
13:41synchronized pressure triggered fully
13:43supported breaths tial volume has been
13:46set for approximately 600
13:48milliliters the second third and fifth
13:51inflations are spontaneous
13:54nonsynchronized breaths assisted by
13:56pressure
13:57support in this particular particular
13:59case pressure support has been set for
14:0110 cm of water which is the difference
14:04between the maximum inspiratory pressure
14:06on the partially supported breaths and a
14:09peep one advantage of simv is that like
14:13assist control it guarantees a minimum
14:16minute
14:17ventilation however unlike assist
14:19control it generally leads to lower mean
14:22airr pressure and can provide a wide
14:24range of respiratory support depending
14:27upon the set respiratory rate
14:30a major disadvantage of sinb is that it
14:32requires more work of breathing on part
14:34of the patient um I've also heard
14:37multiple clinicians and textbooks claim
14:39that SV can reduce cardiac output in
14:41patients with LV dysfunction um however
14:44I am unaware of both the original source
14:46of this claim and the mechanism by which
14:48this would occur uh so therefore I I
14:50can't vouch for its
14:52accuracy the indication for simb is a
14:56critically ill patient who requires a
14:57relatively high level of vent supports
15:00but who are hyperventilating or
15:02otherwise prone to Auto peep or high
15:04Airway
15:08resistance the major difference between
15:10assist control and SV is that in assist
15:12control spontaneous breaths in excess of
15:15the set respiratory rate receive full
15:17support while in SV excess spontaneous
15:22breaths receive partial support
15:24therefore in a patient with no
15:26spontaneous breaths for example a
15:28patient who is deep sedated and
15:29paralyzed assist control and SB are
15:35identical the next mode to discuss is
15:37pressure control
15:38ventilation this mode has only mandatory
15:41breaths in its more common formulation
15:44and the patient is unable to trigger the
15:46ventilator it is time triggered pressure
15:49controlled and time
15:52cycled here are the flow pressure and
15:54volume
15:56tracings the inspiratory pressure is set
15:58at 25 cm of water the flow starts high
16:02with each inspiration then rapidly
16:04declines this is known as a decelerating
16:07flow contour and with pressure control
16:09ventilation it is simply a consequence
16:11of lung
16:12mechanics there are many advantages to
16:15pressure control ventilation it helps
16:17prevent excessive Airway pressures
16:20avoids Regional alveolar overd
16:22distension which can lead to lung injury
16:24and worsened VQ mismatch and may lead to
16:28earlier Liberation for mechanical
16:30ventilation unfortunately it is very
16:33uncomfortable and requires uh deep
16:35sedation plus minus paralysis it is also
16:38unable to guarantee a minimum minute
16:42ventilation the major indication for
16:44pressure control ventilation is a
16:46particularly high risk of barot
16:49trauma the final mode I will discuss in
16:52any detail is pressure support
16:54ventilation in pressure support
16:56ventilation there are no mandatory
16:59thus every breath must be triggered by
17:01the patient by pressure or flow it is
17:04pressure controlled and flow
17:07cycled in this example the pressure
17:09support is set at approximately 15 cm of
17:13water remember remember that the
17:15pressure support will be equal to the
17:17maximum inspiratory pressure minus the
17:19peep which in all these examples has
17:21been set at five also notice in the flow
17:24tracing how inspiration is terminated by
17:27removal of the precious port once the
17:29flow drops to 25% its
17:32maximum the major advantage of pressure
17:35support ventilation is that it is
17:37probably the most comfortable mode for
17:38the awake conscious
17:40patient there are many disadvantages
17:43including the patient must trigger each
17:45breath a minimum minute ventilation
17:48cannot be guaranteed it is associated
17:51with poor quality sleep than other modes
17:54and is generally incapable of providing
17:56full ventilator
17:57support it's indications are conscious
18:00patience and as a stepping stone
18:02immediately prior to
18:06exavation here is just one more
18:08relationship between vent modes which
18:10may help you to understand them a little
18:11bit better so pressure support
18:14ventilation with peep uh Bap or BiPAP as
18:17it may um discussed in the last lecture
18:21and simv with Peep and with respiratory
18:24rate set to zero are all equivalent they
18:27will provide the exact same form of
18:29ventilatory support to a
18:34patient recent advances in technology
18:36have given rise to an entirely new class
18:38of ventilator modes known as dual
18:41control modes dual control modes use
18:44instantaneous feedback to control
18:46aspects of long volume and airway
18:48pressure
18:50simultaneously remember that traditional
18:53modes uh Target only volume or pressure
18:56but not both and allow lung compliance
18:58to fully dictate the other parameter
19:01examples of dual control modes include
19:04pressure regulated volume control this
19:06mode is a form of pressure control
19:08ventilation in which the pressure limit
19:10is continuously adjusted in small
19:11increments to maintain a tital volume as
19:14close as possible to the desired volume
19:17some ventilators have a setting known as
19:19autoflow which is very similar to
19:22prvc there's also volume support this is
19:25a form of pressure support ventilation
19:27when the amount of pressure pressure
19:29support is adjusted up or down with each
19:31breath in order to maintain a minimum
19:33minute ventilation and minimum title
19:35volume there's volume assured pressure
19:39support this is a form of pressure
19:41support ventilation where the ventilator
19:43will provide additional air flow near
19:45the end of an individual inspiration if
19:48it predicts that the title volume from
19:50that inspiration will otherwise fall
19:52below a preset minimum allowed
19:55volume dual control modes are somewhat
19:57more complicated to use uh and
20:00understand than the other modes we've
20:02discussed in more detail and are best
20:04left to clinicians and respiratory
20:06therapists with significant ventilator
20:09experience I hope you found this lecture
20:12on vent modes to be both interesting and
20:14useful please continue to lecture eight
20:17on ventilator options which will further
20:19round out your understanding of how
20:21ventilators work and how they can be
20:23programmed to treat patients with a
20:25variety of clinical disorders
20:28[Music]

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