How does Pulse Oximetry work? — Transcript

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1. 0:00can you imagine working in a critical
2. 0:02care environment without pulse oximetry
3. 0:05oximetry was invented in 1935 but it
4. 0:09wasn't until the pulse oximeter was
5. 0:11developed in 1972 and released
6. 0:14commercially in 1980 that this became a
7. 0:16universal critical care monitoring
8. 0:19modality my name is Ken Hoffman I'm an
9. 0:22intensive care specialist at the Alfred
10. 0:25hospital in Melbourne in this video
11. 0:27we're going to review how this
12. 0:29incredible piece of technology works
13. 0:31this video is made for those sitting the
14. 0:34Australian and New Zealand College of
15. 0:36intensive care medicine first part
16. 0:38examination to demonstrate the expected
17. 0:40level of knowledge as with all
18. 0:43measurement and monitoring devices there
19. 0:45are four components to the system one a
20. 0:48biological variable - a sensor 3 an
21. 0:54integrator and 4 an output
22. 0:59firstly the biological variable that we
23. 1:02are measuring is the percentage
24. 1:04saturation of hemoglobin molecules in
25. 1:07the bloodstream
26. 1:09to calculate this we rely on the
27. 1:11principle that oxygenated and
28. 1:13deoxygenated hemoglobin have different
29. 1:17light absorption characteristics
30. 1:18depending on the light wavelength we can
31. 1:23see this visually because oxygenated
32. 1:25hemoglobin is a bright red color and
33. 1:28deoxygenated hemoglobin is a deep dark
34. 1:31red
35. 1:33the next component is the sensor system
36. 1:36this consists of a pair of
37. 1:39light-emitting diodes which emit
38. 1:41monochromatic red light at a wavelength
39. 1:44of 660 nanometers and monochromatic
40. 1:47infrared light which we cannot see
41. 1:50because it's outside the visible
42. 1:52spectrum at a wavelength of 914
43. 1:55nanometers
44. 1:56the reason these wavelengths were
45. 1:59selected is because at these wavelengths
46. 2:01oxygenated hemoglobin and deoxygenated
47. 2:04hemoglobin have very different absorb
48. 2:07ative properties this allows us to
49. 2:10calculate the proportion of oxygenated
50. 2:13hemoglobin and deoxygenated hemoglobin
51. 2:15that is present we then have a pair of
52. 2:18photo diodes or light sensors which
53. 2:22detect how much light of each wavelength
54. 2:25has passed through the tissues and
55. 2:27converts this to an electrical signal
56. 2:30these sensors are surrounded by an
57. 2:33opaque case to minimize the ambient
58. 2:36light signal next we have the integrator
59. 2:40or computer which processes the
60. 2:42electrical signal this is where pulse
61. 2:45oximetry gets very clever
62. 2:50the signal we need to review to physics
63. 2:52laws beers law and Lambert's law these
64. 2:56are often combined and termed beer
65. 2:59Lambert's law beers law states that the
66. 3:03absorption or attenuation of life as it
67. 3:07passes through a substance is
68. 3:09proportional to the concentration of
69. 3:11that substance this is how the pulse
70. 3:14oximeter calculates the concentration or
71. 3:17relative proportions of oxygenated
72. 3:21hemoglobin and deoxygenated hemoglobin
73. 3:25the second law is Lambert's law this
74. 3:28states that the absorption or
75. 3:30attenuation of light as it passes
76. 3:32through a substance is proportional to
77. 3:35the distance the light has to travel
78. 3:37this is how the pulse oximeter can
79. 3:41isolate the pulsatile component of the
80. 3:44signal as blood moves into the tissues
81. 3:47with each cardiac contraction there is a
82. 3:50tiny but measurable increase in the
83. 3:53distance between the light emitting
84. 3:55diodes and the photodiode detectors we
85. 3:59can then isolate the pulsatile component
86. 4:01of the signal which reflects arterial
87. 4:04blood entering the tissues
88. 4:07we do this by assessing that aec or
89. 4:10pulsatile component of the signal
90. 4:12compared to the ratio of DC or
91. 4:16non-pulsatile component of the signal we
92. 4:20do this for both six hundred and sixty
93. 4:22nanometer and 940 nanometer light
94. 4:25wavelengths we then combine these ratios
95. 4:28for both six hundred and sixty nanometer
96. 4:30and 940 nanometer light into another
97. 4:34ratio termed the R value once the
98. 4:38computer has calculated the R value and
99. 4:41then converts the R value into an oxygen
100. 4:44saturation level obtained from
101. 4:46experimental data where healthy
102. 4:48volunteers braved hypoxic gas mixtures
103. 4:51this is the reason why pulse oximeters
104. 4:54are calibrated down to a saturation
105. 4:57level of around 75 percent beyond this
106. 5:01point the curve is extrapolated as it's
107. 5:03very hard to get ethics approval or
108. 5:05volunteers willing to become this
109. 5:07hypoxic
110. 5:09at this point we should mention a couple
111. 5:11of clever things
112. 5:12the pulse oximeter does to clean up the
113. 5:14signal
114. 5:16the first is to remove interference from
115. 5:19ambient light sources
116. 5:21it does this bicycling through which
117. 5:24light emitting diode is actually
118. 5:26emitting light 660 nanometer and then
119. 5:30940 nanometer and then having both of
120. 5:34the light sources turned off we can't
121. 5:37actually see this as the light sources
122. 5:39are flickering at up to 900 Hertz during
123. 5:44the time that both light emitting diodes
124. 5:45are off the computer calculates the
125. 5:48effect of ambient light on the
126. 5:50photodiode detector it could then remove
127. 5:53ambient light from the analysis of the
128. 5:56signal the second is that after ambient
129. 5:59light and non-pulsatile signal is
130. 6:01removed
131. 6:02the pulse oximeter amplifies the
132. 6:04positive signal and takes an average
133. 6:06over a few seconds before calculating
134. 6:09the saturation this is to try and
135. 6:12identify and compensate for any movement
136. 6:15artifact that has occurred
137. 6:18the final component of the system is the
138. 6:20output this includes a numerical display
139. 6:23usually with the saturations and the
140. 6:26heartrate and on higher quality monitors
141. 6:29a graph called a plasma graph which
142. 6:32means a graph of the changing volume
143. 6:35this allows an assessment of the quality
144. 6:38of the signal
145. 6:39so to recap our pulse oximeter consists
146. 6:43of four components one a biological
147. 6:47variable which is the percentage
148. 6:50saturation of hemoglobin 2 a sensor
149. 6:54which is assessing the transmission of
150. 6:57660 nanometer and 940 nanometer
151. 7:01wavelengths of light through the tissues
152. 7:043 an integrator which is analyzing the
153. 7:09signal from the sensor and separating
154. 7:11out the pulsatile component of the
155. 7:13signal and 4 an output which is a
156. 7:18numerical display of the saturations and
157. 7:20heart rate and a graphical display of
158. 7:22the quality of the signal
159. 7:26thank you for listening if this was
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