. MEDICAL SCIENCE SERIES II
`
`DESIGN
`OF PULSE
`OXIMETERS
`
`'
`' '
`,I
`' '
`; I
`
`EDITED BY
`
`JG WEBSTER
`IaP
`
`1
`
`Masimo Ex. 2012
`Apple v. Masimo
`IPR2021-00208
`
`

`

`Design of Pulse Oximeters
`
`2
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`

`

`
`
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`Other lit/es of" interest
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`Prevention of Pressure Sores: Engineering and Clinical Aspects
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`
`JG Webster
`
`3
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`

`

`
`
`
`
`Medical Science Series
`
`Design of Pulse Oximeters
`
`
`
`Edited by
`
`JG Webster
`
`Department of Electrical and Computer Engineering
`
`
`
`
`University of Wisconsin-Madison
`
`Institute of Physics Publishing
`
`
`
`Bristol and Philadelphia
`
`4
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`

`

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`
`CJ lOP Publishing Ltd 1997
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`All rights reserved. No part of this publication may he reproduced, stored in a
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`British Librarv Cataloguing-in-Publication Data
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`ISBN 0 7503 0467 7
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`tables reproduced in this publication and apologizes to copyright holders if
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`Series Editors:
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`Croydon, UK
`RF Mould,
`Detroit, USA C G Orton, Karamanos Cancer Institute,
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`The Netherlands J A E Spaan, University of Amsterdam,
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`USA J G Webster, University of Wisconsin-Madison,
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`Published by Institute of Physics Publishing, wholly owned by The Institute of
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`Institute of Physics Publishing, Dirac House, Temple Back, Bristol BS 1 6BE, UK
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`7
`
`

`

`CONTENTS
`
`PREFACE
`
`NORMAL OXYGEN TRANSPORT
`
`Susanne A Clark
`
`xv
`
`l
`
`1.1 Ventilatory control
`
`1
`1.1.1 Neural control
`1
`
`1.1.2 Respiratory feedback
`2
`
`1.2 Ventilatory mechanics
`2
`1.2.2 Expiration
`4
`
`1.3 Diffusion to blood
`5
`1.3.1 The alveoli
`5
`1.3.2 Gas exchange
`5
`1.4 Bind to hemoglobin
`6
`
`1.4.1 Characteristics of hemoglobin
`6
`1.4.2 Oxyhemoglobin dissociation curves
`7
`
`1.5 Dissolved in plasma
`8
`1.6 Circulation
`8
`1.6.1 The heart
`8
`
`1.6.2 Pulmonary circulation
`9
`
`1.6.3 Systemic circulation
`9
`
`1.6.4 Cardiac output
`9
`
`1.7 Diffusion to tissue
`10
`1.7.1 Diffusion into interstitial fluid and cell
`
`
`11
`1.7.2 Oxygen delivered
`11
`1.7.3 Myoglobin
`11
`1.8 Use in cell
`11
`References
`12
`
`Instructional objectives
`12
`
`2 MOTIVATION OF PULSE OXIMETRY
`
`
`Daniel J Sebald
`
`2.1 Pulse oximeter principles
`
`
`2.2.1 Comprehensive approach
`
`2.2.2 Arterial oxygen saturation
`
`2.2.3 Hypoxia and hypoxemia
`
`2.2.4 Role of SpO2 in avoiding hypoxia
`
`13
`
`13
`15
`15
`15
`16
`
`8
`
`

`

`-
`
`Vlll Contents
`
`2.2.5 Photoplethysmography 18
`2.2.6 Hyperoxia
`lO
`2.3 Limitations
`19
`2.3.1 Instrument
`and operation
`limitations 19
`2.3.1 Limitations
`in Sp02
`19
`References
`20
`Instructional
`objectives
`20
`
`3 BLOOD OXYGEN MEASUREMENT
`James Farmer
`
`21
`
`21
`3.1 Chemical
`methods
`22
`3.1.1 Van Slyke method
`23
`3.1.2 Mixing syringe
`method
`3.1.3 The Clark electrode
`23
`3.1.4 The galvanic
`electrode
`25
`25
`3.2 Transcutaneous
`P02 sensor
`26
`3.3 In vitro oximeters
`26
`3.3.1 Spectrophotometers
`3.3.2 The CO-oximeter
`28
`30
`3.4 In vivo two-wavelength
`oximeters
`30
`3.4.1 The first
`in vivo oximeters
`30
`3.4.2 The cyclops
`30
`3.5 Fiber optic oximeters
`30
`3.5.1 In vitro reflectance
`oximeter
`31
`3.5.2 In vivo reflectance
`catheter
`oximeter
`32
`3.5.3 In vivo chemical
`oximeter
`32
`3.6 In vivo eight-wavelength
`oximeter
`34
`3.7 Pulse oximeters
`34
`3.7.1 Overview
`35
`3.7.2 LEDs
`36
`3.7.3 Photodiode
`36
`3.7.4 Probes
`3.7.5 Analog amplifier
`and signal
`processing 37
`3.7.6 A three-wavelength
`pulse oximeter
`for
`COHb determination
`3.7.7 Comparison
`of pulse oximetry
`to
`transcutaneous
`P02 electrodes
`References
`Instructional
`objectives
`
`37
`
`38
`38
`39
`
`'I
`I
`
`4 LIGHT ABSORBANCE
`IN PULSE OXIMETRY
`Oliver
`Wieben
`
`4.1 Beer's
`Law
`4.1.1 Transmittance
`and absorbance
`of light
`4.1.2 Multiple
`absorbers
`
`40
`
`40
`41
`41
`
`9
`
`

`

`Contents
`lX
`
`4.2 Hemoglobin
`coefficients
`extinction
`42
`4.2.l Functional
`hemoglobins
`42
`hemoglobins
`4.2.2 Dysfunctional
`42
`spectra
`absorbance
`4.2.3 Hemoglobin
`44
`law in pulse oximetry
`4.3 Beer's
`44
`of wavelengths 45
`for the choice
`4.3.1 Criteria
`solutions
`in hemoglobin
`4.3.2 Absorbance
`45
`of the blood
`4.3.3 Pulsation
`46
`of pulse oximeters
`4.3.4 Measurement
`48
`49
`4.4 Saturation
`normalized
`ratio
`versus
`4.4.1 Normalization
`49
`4.4.2 Ratio of normalized signals 49
`4.4.3 Theoretic calibration curve
`50
`51
`law in pulse oximetry
`of Beer's
`4.5 Validity
`4.6 Light Scattering
`52
`in whole blood
`4.6.1 Light absorbance
`52
`scattering 52
`including
`4.6.2 Models for light absorbance
`on pulse oximeter readings
`of scattering
`4.6.3 Influence
`53
`used for pulse oximeters 54
`4.6.4 Calibration
`curves
`54
`References
`Instructional objectives 55
`
`DIODES AND THEIR CONTROL
`5 LIGHT-EMITTING
`Brad W J Bourgeois
`
`56
`
`5.1 An introduction
`to light-emitting
`diodes
`56
`5.1.1 Description, materials,
`and operation
`57
`5.1.2 Bandwidth
`considerations
`57
`5.2 Light-emitting
`57
`diode specifications
`voltage
`5.2.1 Forward
`58
`5.2.2 Forward
`current
`58
`59
`5.2.3 Power dissipation
`voltage
`breakdown
`5.2.4 Reverse
`60
`current
`5.2.5 Reverse
`60
`temperature
`5.2.6 Operating
`60
`times
`5.2.7 Switching
`61
`5.2.8 Beam angle
`61
`5.2.9 Pulse capability
`61
`LED wavelengths 62
`5.3 Measuring
`and identifying
`circuit
`5.4 LED driver
`64
`shift with temperature 66
`5.5 LED peak wavelength
`heating
`5.5.1 p-n junction
`66
`5.5.2 Studies
`66
`5.5.3 Two methods
`to compensate
`for
`changes
`LED temperature
`of burns in pulse oximetry
`5.6 Prevention
`5.7 LED packaging
`References
`objectives
`Instructional
`
`68
`69
`69
`70
`70
`
`10
`
`

`

`X
`
`Contents
`
`6 PHOTODETECTORS AND AMPLIFIERS
`
`S Schowalter
`Jeffi·ey
`
`71
`
`6.1 Photodetection devices
`
`71
`6.1.1 Photocells
`71
`6.1.2 Photodiodes
`72
`6.1.3 Phototransistors
`76
`
`
`6.1.4 Integrated circuit (IC) sensors
`76
`6.2 Photodiode characteristics 76
`
`6.2.1 Junction capacitance
`76
`6.2.2 Dark current
`77
`77
`6.2.3 Sensitivity
`6.2.4 Spectral response
`
`77
`6.2.5 Packaging
`77
`6.3 Optical Concerns
`79
`6.3.1 Optical filtering 79
`
`6.3.2 Optical interference
`79
`6.4 Amplifiers
`79
`
`
`
`6.4.1 Standard transimpedance amplifier configuration
`80
`
`6.4.2 Differential transimpedance
`amplifier 82
`
`6.4.3 Zeroing circuit
`83
`6.4.4 Future trends
`
`84
`References
`84
`
`Instructional objectives
`84
`
`7 PROBES
`
`Moala Venkata Subba Reddy
`
`86
`
`7.1 Transmittance Probes
`
`7.1.1 Principle
`86
`
`7.1.2 Sensor placement
`87
`
`7.2 Reflectance Probes
`87
`7.2.1 Principle
`88
`
`7.2.2 Sensor placement
`88
`
`
`7.2.3 Effect of multiple photodiode
`arrangement 90
`
`7.2.4 Effect of skin temperature
`90
`
`
`7.2.5 Advantages and disadvantages of
`
`
`
`reflectance probes over transmittance probes 91
`7.3 MIR probes
`91
`7.4 Probe connectors
`92
`
`7.5 Reusable probes
`93
`
`7.6 Disposable probes
`94
`
`
`7.7 Sources of errors due to probes
`and placement 94
`
`7.7.1 Ambient light interference
`94
`
`7.7.2 Optical shunt
`95
`7.7.3 Edema
`95
`7.7.4 Nail Polish
`95
`References
`96
`
`Instructional objectives
`96
`
`11
`
`

`

`8
`
`ELECTRONIC
`INSTRUMENT
`CONTROL
`Ketan S Paranjape
`
`97
`
`Contents xi
`
`General
`theory
`of operation
`8.1
`97
`Historic
`perspective
`8.1.1
`98
`Main block diagram
`99
`8.2
`Input module
`8.2.1
`100
`Digital
`processor
`system
`8.3
`101
`Microprocessor
`subsection
`8.3.1
`101
`General
`block description
`8.3.2
`102
`Wait state generator
`103
`8.3.3
`Clock generator,
`timer circuit
`and UART
`103
`8.3.4
`Pattern
`generator
`104
`8.3.5
`Analog processing
`system (Nellcor
`®)
`105
`8.4
`Analog signal flow
`105
`8.4.1
`Coding resistor,
`temperature
`sensor,
`and prefiltering
`105
`8.4.2
`Preamplifier
`105
`8.4.3
`Demodulator
`and filtering
`106
`8.4.4
`DC offset elimination
`107
`8.4.5
`®)
`Timing diagram
`(Nellcor
`109
`8.4.6
`LED driver
`circuit
`110
`8.4.7
`system (Ohmeda®)
`Analog processing
`111
`8.4.8
`ECG section
`113
`8.5
`Active
`filters
`114
`8.5.1
`Offset amplifiers
`114
`8.5.2
`Detached
`lead indicator
`114
`8.5.3
`Power line frequency
`sensing
`115
`8.5.4
`ECG output
`115
`8.5.5
`Signal
`conversion
`116
`8.6
`Analog-to-digital
`conversion
`technique
`116
`8.6.1
`Digital-to-analog
`conversion
`117
`8.6.2
`Sample-and-hold circuit
`117
`8.6.3
`Timing and control
`117
`8.7
`Polling
`and interrupt
`117
`8.7.1
`Power Supply
`118
`8.8
`Alarms
`119
`8.9
`Storage
`119
`8.10
`Front end display
`120
`8.11
`Front end driver
`circuit
`120
`8.11.1
`Front panel control
`121
`8.11.2
`Power up display
`tests
`121
`8.11.3
`Speakers
`121
`8.12
`References
`122
`Instructional
`objectives
`122
`
`9
`
`SIGNAL PROCESSING
`ALGORITHMS
`Surekha
`Palreddy
`
`Sources
`of errors
`9.1
`Beer-Lambert
`law
`9.2
`
`124
`
`124
`125
`
`12
`
`

`

`xii Contents
`
`9 .2.1 Estimation of oxygen saturation
`
`
`using the Beer-Lambert law
`126
`9.3
`129
`Ratio of ratios
`
`9.3.1 Peak and valley method
`129
`software 9.3.2 Derivative method: noise reduction
`
`
`
`130
`
`
`
`General processing steps of oximetry signals
`9.4
`133
`134
`9 .4.1 Start up software
`
`Transient conditions
`135
`9.5
`143
`
`ECG synchronization algorithms
`9.6
`
`9.6.1 Nellcor® system
`144
`9.6.2 Criticare
`149
`® system
`9.7
`
`
`
`Spectral methods of estimating SpO2
`157
`References
`158
`158
`
`Instructional objectives
`
`CALIBRATION
`10
`Jeffrey S Schowalter
`
`10.1 Calibration methods
`
`
`10.1.1 Traditional in vivo calibration
`
`10.1.2 In vitro calibration using blood
`
`10.2 Testing simulators
`
`10.2.1 Simulators using blood
`
`10.2.2 Nonblood simulators
`
`10.2.3 Electronic simulators
`10.3 Standards
`10.3.1 ASTM F1415
`10.3.2 ISO 9919
`10.3.3 Other standards
`References
`
`Instructional objectives
`
`ACCURACY AND ERRORS
`11
`Supan Tungjitkusolmun
`
`159
`
`159
`159
`162
`163
`164
`168
`173
`172
`173
`173
`174
`174
`175
`
`176
`
`11.1
`Evaluation of pulse oximeters
`
`176
`
`
`
`and confidence limit 11.1.1 Accuracy, bias, precision,
`177
`
`
`11.1.2 What do pulse oximeters really measure?
`178
`
`
`11.1. 3 Pulse oximeter versus CO-oximeter
`179
`
`11.1.4 Pulse oximeter versus
`
`in vivo eight-wavelength ear oximeter
`11.2
`
`
`Accuracy versus saturation
`
`
`11.2.1 High saturation (greater than 97 .5%)
`11.2.2 Normal saturation (90 to 97.5%)
`
`11.2.3 Low saturation (less than 90%)
`
`11.3
`
`
`Accuracy versus perfusion
`11.3.1 Venous congestion
`11.4
`
`
`Accuracy versus motion artifacts
`
`
`
`Accuracy versus optical interference
`11.5
`11.6
`
`
`
`Accuracy versus intravenous dyes
`
`179
`180
`180
`181
`181
`182
`182
`183
`184
`185
`
`13
`
`

`

`xiii
`Contents
`
`11.7 Effect of dyshemoglobins and fetal hemoglobin 187
`
`11.7.1 Carboxyhemoglobin (COHb)
`
`187
`188
`
`11.7.2 Methemoglobin (MetHb)
`11.7.3 Fetal hemoglobin
`189
`190
`11.7.4 Bilirubin
`190
`11.8 Effect of temperature
`11.8.1 Ambient temperature
`
`190
`191
`11.8.2 Patient temperature
`
`192
`
`
`
`11.9 Accuracy versus medical conditions
`192
`11.9.1 Cardiac arrhythmia
`
`192
`11.9.2 Myxoma
`193
`
`11.10 Accuracy versus probe position
`
`194
`11.11 Electromagnetic interference
`
`
`11.11.1 Interference from
`(MRI)
`
`
`magnetic resonance imaging
`11.12 Other effects on accuracy
`
`
`11.12.1 Exercise
`
`11.12.2 Dried blood
`
`11.12.3 Pigments
`References
`
`Instructional objectives
`
`194
`195
`195
`196
`196
`197
`198
`
`USER INTERFACE FOR A PULSE OXIMETER
`
`12
`
`Albert Lozano-Nieto
`
`12.1 Introduction
`12.2 Front Panel
`12.2.1 Graphical displays
`
`12.2.2 Numerical displays
`
`
`12.3 Function controls
`12.4 Alarm controls
`12.5 Communicative functions
`12.6 Cables and Connectors
`
`12.7 Other features
`12.8 Compliance requirements
`
`References
`
`Instructional objectives
`
`13
`APPLICATIONS OF PULSE OXIMETRY
`
`Joanna B Ruchala
`
`199
`
`199
`200
`201
`203
`204
`206
`209
`210
`210
`211
`212
`213
`
`214
`
`214
`13 .1 Anesthesia
`215
`13 .1.1 Problems encountered during induction to anesthesia
`
`
`
`
`216
`
`13.1.2 Surgery under anesthesia
`
`and organ viability 217
`13.2 Monitoring tissue blo d supply
`13.2.1 Intestinal blood flow and
`
`217
`
`
`bowel viability following surgery
`
`
`
`13.2.2 Tissue transfer and setting of limb fractures 218
`
`and viability 218
`13.2.3 Dental pulp blood supply
`13.3 Monitoring on the road and in the air
`
`219
`
`14
`
`

`

`xiv Contents
`
`13.3. l Ambulances
`219
`13.3.2 Flight
`220
`13.4
`Childbirth
`221
`13 .4.1 Causes of desaturation in mother and fetus
`
`221
`222
`
`
`13.4.2 Special apparatus for fetal monitoring
`224
`
`
`Neonatal and pediatric care
`13.5
`227
`
`
`
`Sleep studies and physical stress testing
`13.6
`227
`13.6.1 Sleep
`13.6.2 Exercise
`231
`
`Management of cardiopulmonary resuscitation
`231
`13.7
`232
`
`Computer-controlled oxygen weaning
`13.8
`232
`
`
`Systolic blood pressure measurement
`13.9
`
`Cerebral oxygen measurement
`232
`13.10
`233
`
`Veterinary care
`13.11
`234
`
`Future improvements for pulse oximetry
`13.12
`234
`References
`236
`
`Instructional objectives
`
`GLOSSARY
`
`INDEX
`
`237
`
`243
`
`15
`
`

`

`PREFACE
`
`Pulse oximetry was introduced in 1983 as a noninvasive method for monitoring
`
`
`
`
`
`
`
`
`the arterial oxygen saturation of a patient's blood. Recognized worldwide as the
`
`
`
`standard of care in anesthesiology, it is widely used in intensive care, operating
`
`
`
`
`
`rooms, emergency, patient transport, general wards, birth and delivery, neonatal
`
`
`
`
`care, sleep laboratories, home care and in veterinary medicine. It provides early
`
`
`
`
`information on problems in the delivery of oxygen to the tissue. Those problems
`
`
`
`
`may arise because of improper gas mixtures, blocked hoses or airways,
`
`
`
`
`inadequate ventilation, diffusion, or circulation, etc. More than 35 companies
`
`
`
`
`manufacture and distribute the more than 300 000 pulse oximeters presently in
`use in the USA.
`This book emphasizes the design of pulse oximeters. It details both the
`
`
`
`
`
`
`
`
`hardware and software required to fabricate a pulse oximeter as well as the
`
`
`
`
`
`equations, methods, and software required for effective functioning.
`
`
`
`
`
`Additionally, it details the testing methods and the resulting accuracy. The book
`
`
`
`
`
`should be of interest to biomedical engineers, medical physicists, and health care
`
`
`
`providers who want to know the technical workings of their measuring
`instruments.
`Chapter 1 reviews the methods of transport of oxygen to the tissue by
`
`
`
`
`
`
`
`ventilation, perfusion to the blood, binding to hemoglobin in the red blood cells,
`
`
`
`
`and transport through the blood circulation. Chapter 2 describes the problems
`
`
`
`and diseases that can occur in oxygen transport, which motivate us to measure
`
`
`
`oxygenation. In chapter 3, we review the many ways oxygenation has been
`
`
`
`
`
`measured in the past, the CO-oximeter used as the gold standard, and provide an
`
`introduction to the pulse oximeter.
`Chapter 4 begins with Beer's law for the absorption of light by hemoglobin
`
`
`
`
`
`
`and oxyhemoglobin, and develops the equations required for converting
`
`
`
`
`
`
`measured light transmission through the tissue to display the hemoglobin oxygen
`
`
`
`
`saturation. The light-emitting diodes, which alternately emit red light at 660 nm
`
`
`
`
`
`and infrared light at 940 nm and require precise wavelength control, are
`
`
`
`
`
`described in chapter 5. Chapter 6 covers the variety of light sensors, with
`
`
`
`
`emphasis on the single photodiode typically used.
`Chapter 7 details the design of reusable and disposable probes and their
`
`
`
`
`
`
`
`
`
`flexible cables. The probes can transmit light through either the finger or ear, or
`
`
`
`use reflected light from the scalp or other skin surface. Chapter 8 covers the
`
`
`
`
`
`hardware, with block diagrams showing how red and infrared signals are
`
`
`
`
`amplified to yield the ratio of pulse-added red absorbance to the pulse-added
`
`
`
`
`
`infrared absorbance. These signals are used to control light-emitting diode levels
`
`
`and the ratio is used to calculate oxygen saturation. The flow charts and
`
`16
`
`

`

`xvi Preface
`
`algorithms to perform oxygen aturation calculations are given in chapter 9, with
`
`
`
`
`
`
`
`worked out examples. ynchronizali.on with the electrocardiogram improves
`
`
`accuracy during patient movement.
`Chapter JO describes ways to lest performance of pulse oximeters: the
`
`
`
`
`
`
`
`
`
`techojcian's finger, electronic simulator , in vitro test systems and optoelectronic
`
`
`
`
`
`simu.lalors. In chapter 11 we find tbe resulting accuracies and descriptions of the
`
`
`
`
`inaccurac.ies caui;ed by alternative forms of hemoglob.in, optical and electrical
`
`
`
`
`
`
`mterference, colored nail polisb, etc. Chapter 12 describes the interface between
`
`
`
`
`the pulse oximeter, the operator, and the external world. Chapter 13 covers the
`
`
`
`many appli ations for pulse oximetry in intensive care, operating rooms,
`
`
`
`
`
`
`emergency, patient transport, general wards, biith and delivery, neonatal care,
`
`
`sleep labor·1t rics, home care, and in veterinary medicine.
`A glossary provides definitions of terms from both the medical and the
`
`
`
`
`
`
`
`engineering world. We also provide instructional objectives as a means of
`
`
`
`
`
`
`provoking further thought toward leamjng tbe information. We gleaned much of
`
`
`
`the design information from operator's manual and from patents; periodical
`
`
`
`
`literature provided more general information. Rather than giving an exhau tive
`
`
`
`list of references, we have included review articles and books that can serve as an
`
`
`
`
`entry into further study. All contributors are from the Department of Electrical
`
`
`
`
`
`and Computer Engineering at the University of Wisconsin, Madison, WI, USA,
`
`and worked as a team to write this book. We would welcome suggestions for
`
`
`
`improvement of subsequent printings and editions.
`
`John G. Webster
`
`Department of Electrical and Computer Engineering
`
`
`
`
`University of Wisconsin-Madison
`
`Madison WI, USA
`August 1997
`
`17
`
`

`

`CHAPTERl
`
`NORMAL OXYGEN TRANSPORT
`
`
`
`Susanne A Clark
`
`Oxygen is viral I tbc functioning of each cell in the human body. rn U,e ab ·encc
`
`
`
`
`of oxygen for a prolonged amount of Lime, cells will die. Tims, oxygen ddivery
`
`
`
`to cells is an important indicator of a patient's health.
`Several meUiods have been developed to analyze oxygen delivery. Pulse
`
`
`
`
`
`
`
`
`
`
`oximetry is a common, noninvasive method nsed io clinical environments. This
`
`
`
`
`
`book discusses pulse oximetry, from applications to signal processing. Before
`
`
`
`
`continuing, it is essential to understand normal oxygen transport, which is the
`
`subject of this chapter.
`Oxygen delivery to ceJls requires the use of the respiratory system as well as
`
`
`
`
`
`
`the circulatory system. Ventilation is the initial step, moving air into and out of
`
`
`
`
`the lungs. Within the lungs, gas exchange occurs. Oxygen is diffused inr.o the
`
`
`
`
`blood, while carbon dioxide, a byproduct of cellular respiration, diifuses into the
`
`
`
`lungs. The oxygenated blood circulates around the body until 1r reache oxygen
`
`
`
`
`
`depleted areas, where oxygen is diffused to cells, and carbon dioxide is
`
`
`
`
`
`transferred to the blood returning to the lungs. The ventilatory process is
`
`
`
`controlled by neurons in tbe brain stem. The circulatory system also can
`
`
`
`
`modulate cardiac output to effect the oxygen delivery.
`
`
`
`1.1 VENTILATORY CONTROL
`
`Ventilation is the involuntary, rhythmic process of moving air in and out of the
`
`
`
`
`
`
`
`
`
`
`lungs. This process is controlled by respiratory neurons in the brain stem. The
`
`
`
`
`
`respiratory neurons excite motor neurons, which in tum cause the movement of
`
`
`
`
`
`respiratory muscles. The output of the respiratory neurons is modulated by
`
`chemoreceptors and mechanoreceptors.
`
`1.1.l Neural control
`
`The respiratory neurons in the brain stem are responsible for the pattern
`
`
`
`
`
`
`are modulated generation in normal breathing. The rate and depth of ventilation
`
`
`
`
`
`by these neurons. The respiratory neurons excite motor neuroas in the spinal
`
`
`
`cord. The excitation of Lbe motor neurons causes the contraction of the
`
`
`
`
`
`diaphragm, pectoral muscles, and intercostal muscles. All of these muscles
`
`18
`
`

`

`
`
`2 Design o_f'pulse oximeters
`
`combine efforts puJling the ribcage up and out, expanding the lungs, causing
`
`
`
`
`
`
`
`
`
`
`
`inspiration. The activity of respiratory neurons is thought to occur sp otaneously,
`
`
`
`
`
`
`with occasional inhibition nllowing the respiratory muscles to relax. Thi causes
`
`
`the rib cage to contract which yields expiration.
`
`
`
`1.1.2 Respiratory feedback
`
`The brain stem receives feedback from many mechanical and chemical i:ccepl rs.
`
`
`
`
`
`
`
`
`
`The input from these neurons is analyzed by the respiratory neuron. to determine
`
`
`the appropriate rate and depth f ventilation. Mechanoreceptors give feedback
`
`
`
`
`
`
`related to mechanical aspects of breathing. For example stretch receptors are
`
`
`
`
`
`mechanoreceptors that provide feedback on the expansion of the Jung and he l
`
`
`
`
`during both inspiration and expiration. An inflation index is the level of feedback
`
`
`
`
`
`
`provided that causes inhibition of inspiration, preventing overinflation of the
`
`
`
`
`
`
`lungs. A deflation index serves a similar purpose in expiration hindering the
`
`c llapse of the lungs.
`Chemoreceplors provide information on the level of carbon dioxide, oxygen,
`
`
`
`
`
`
`
`and hydrogen ions in U1e blood. Chemoreceptors are located in the carotid
`
`
`
`ai:teries, as the oxygenated blood is being sent to tbe brain, and in the aorta,
`
`h rtly after Lhe oxygenated blood is being pumped from tbe heart to U1e body.
`
`
`
`
`Oxygen levels under normal conditions are bigh in the , ystcmic arteries, and
`
`
`
`
`carbon dioxide and hydrogen levels are low.
`The brain stem must process all of the information it receives and n single
`
`
`
`
`
`
`
`
`fa ·tor controls ventilation. Under normal breathing conditions, the brain stem is
`
`
`
`
`most se11Sitive to the 1.evels of carbon dioxide and hydrogen. The oxygen
`
`
`
`
`concentrations are only important when the level is extremely low. Consider an
`
`
`
`
`
`
`extremely high level of carbon dioxide present iu the bJ0ocl, such as would ccur
`
`
`
`
`
`during maximal exercise. However stretch receptors indicate that the lung and
`
`
`
`
`chest arc at maximal expansion, meaning the inflation index has been reached.
`
`
`
`
`Thus, the rate of breathing increases t compensate without a proportional
`
`increase in chest and lung ex_pansion.
`An unusual feature of ventilation is that breathing can be brought under
`
`
`
`
`
`
`
`
`
`
`
`voluntary control to some extent. However, iL is nol possible to commit suicide by
`
`
`
`
`refusing to breathe. Once the individual lo es consciousness, the input from
`
`
`chemorcceplors will cause ventilation to be restored.
`
`
`
`1.2 VENTILATORY MECHANICS
`
`Ventilatory mechanics are based on the principle of air flow from areas of h.igh
`
`
`
`
`
`
`
`pressure to areas of .lower pressure. The contraction of the intercostal muscles,
`
`
`
`pectoral muscles and Lhc diaphragm causes the thoracic cavity to expand,
`
`
`
`
`
`
`decreasing the pressure in the thoracic cavity. The atmospheric pressure i higher
`
`
`
`
`
`than the pressure inside the lungs, causing air to flow into the lungs, which is
`
`
`
`
`termed inspiration. The relaxation of the intercostal muscles and the diaphragm
`
`
`
`ause · the volume of the lungs to decrease, increasing the pressure in the thoracic
`
`
`
`
`
`cavit-y. As the pressure in the lungs increases reaching levels above U1e
`
`
`
`
`atmospheric pressure, air flows out of the lungs, which is referred to ,as
`expiration.
`
`19
`
`

`

`Normal oxygen transport 3
`
`1.2.J Inspiration
`
`As discussed
`previously,
`the brain stem excites
`motor neurons
`in the spinal
`cord.
`which,
`in Lurn, causes
`the conlracLion
`of the diaphragm,
`the pectoral
`muscles,
`and
`tlllercostaJ muscles, located
`between
`the ribs. The contraction
`of the diaphragm
`causes the flattening
`and lenglhening
`of the thoracic
`cavity.
`The intercostaJ
`muscles
`and pectoral
`muscles
`pull the ribcage
`up and ouL. Both of 1J1ese
`sets or
`muscles
`work to expand the lungs.
`This means that pressure
`will be reduced
`within
`the lungs,
`since rbe air present
`will have a greater
`volume to expand in.
`This will create
`a pressure
`differential
`between
`the air outside
`the b dy and tbe
`air inside
`Lhe body. Thus, ait flows lnto the body (see figure I. I (a)).
`
`(a)Inspiration
`
`Air drawn into lungs
`
`(b)Expiration
`
`
`
`Air forced out ot lungs
`
`Pectoralis mlhor
`
`
`muscles contrAct
`
`lntercostal
`muscles
`contract
`
`1,1ttrcostal
`11\usctes
`relM
`
`Diaphragm contracts
`
`and flattens
`
`Diaphragm rela:-:es
`
`and moues up
`
`Figure 1. I During inspiration, (a), the diaphragm, intercostul muscles :md pcctor.tlis minor
`
`
`
`
`
`
`
`
`
`
`
`muscles conlrocl, causing the lungs to cxpnnd and air 10 enter U1e lungs. /\s Ilic dlnphrngm,
`
`
`
`
`
`
`
`111tcrcosrnl muscles and pectoralis minor relax. th� lungs contract, causing uir lo leave the lungs (h),
`
`
`which is referred to as c.,piration (from Micro. oft Encarrn).
`
`Air travels through the nasaJ cavity. Cilia nre microscopic hairs within the
`
`
`
`
`
`
`
`
`
`
`nasal cavity thal acr to eliminate pollutnnts from ente,ing the r� piratory tracl.
`Air nnd food both go through
`the pharynx. When food is swallowed, the
`
`
`
`
`
`to shut work together pharynx, and momh cavity epiglottis (part of the larynx),
`
`
`
`off lhe opening to tJ1e trachea to avoid the entry of food particles into the lungs.
`
`
`
`
`
`The larynx is commonly referred LO as the voice box. Besides assisting with
`
`
`
`
`
`cparnt1on of food particles from air, the larynx contains the cricoid cartilage
`
`
`
`which reinforces the airway and assists in keeping it open. The larynx also
`
`
`contains Lhe vocal cords. As air vibrates over the vocal cords, a sound i�
`
`
`
`
`produced. The variation in elasticity and ten ion of the vocal cords determines the
`pitch of lhc sound.
`The trachea i composed of ribbed cartilttge which extends 10 cm to the
`
`
`
`
`
`
`
`
`
`bronchj. The trachea also contain cilia which act to filter out further pollutants.
`
`
`1.2). Two broil hi provide a path to each lung (see figure
`
`
`
`Each bron hiole ha Each