`OF PULSE
`OXIMETERS
`
`Apple Inc.
`APL102O Part 1 of2
`
`U.S. Patent No. 8,923,941
`
`0001
`
`Apple Inc.
`APL1020 Part 1 of 2
`U.S. Patent No. 8,923,941
`
`
`
`Design of Pulse Oximeters
`
`0002
`
`
`
`Other books in the series
`
`The Physics and Radiobiology of Fast Neutron Beams
`D K Bewley
`
`Biomedical Magnetic Resonance Technology
`C-N Chen and D I Hoult
`
`Rehabilitation Engineering Applied to Mobility and Manipulation
`R A Cooper
`
`Linear Accelerators for Radiation Therapy, second edition
`D Greene and P C Williams
`
`Health Effects of Exposure to Low-Level Ionizing Radiation
`W R Hendee and F M Edwards
`
`Introductory Medical Statistics, third edition
`R F Mould
`
`Radiation Protection in Hospitals
`R F Mould
`
`RPL Dosimetry-Radiophotoluminescence in Health Physics
`J A Perry
`
`Physics of Heart and Circulation
`J Strackee and N W esterhof
`
`The Physics of Medical Imaging
`SWebb
`
`The Physics of Three-Dimensional Radiation Therapy: Conformal
`Radiotherapy, Radiosurgery and Treatment Planning
`SWebb
`
`The Physics of Conformal Radiotherapy: Advances in Technology
`SWebb
`
`Other titles of interest
`
`Prevention of Pressure Sores: Engineering and Clinical Aspects
`J G Webster
`
`0003
`
`
`
`Medical Science Series
`
`Design of Pulse Oximeters
`
`Edited by
`J G Webster
`
`Department of Electrical and Computer Engineering
`University of Wisconsin-Madison
`
`Institute of Physics Publishing
`Bristol and Philadelphia
`
`0004
`
`
`
`© lOP Publishing Ltd 1997
`
`All rights reserved. No part of this publication may be reproduced, stored in a
`retrieval system or transmitted in any form or by any means, electronic,
`mechanical, photocopying, recording or otherwise, without the prior permission
`of the publisher. Multiple copying is permitted in accordance with the terms of
`licences issued by the Copyright Licensing Agency under the terms of its
`agreement with the Committee of Vice-Chancellors and Principals.
`
`British Library Cataloguing-in-Publication Data
`
`A catalogue record for this book is available from the British Library.
`
`ISBN 0 7503 0467 7
`
`Library of Congress Cataloging-in-Publication Data are available
`
`The Editor has attempted to trace the copyright holder of all the figures and
`tables reproduced in this publication and apologizes to copyright holders if
`permission to publish in this form has not been obtained.
`
`Series Editors:
`R F Mould, Croydon, UK
`C G Orton, Karamanos Cancer Institute, Detroit, USA
`J A E Spaan, University of Amsterdam, The Netherlands
`J G Webster, University of Wisconsin-Madison, USA
`
`Published by Institute of Physics Publishing, wholly owned by The Institute of
`Physics, London
`
`Institute of Physics Publishing, Dirac House, Temple Back, Bristol BS 1 6BE, UK
`
`US Editorial Office: Institute of Physics Publishing, The Public Ledger Building,
`Suite 1035? 150 South Independence Mall West, Philadelphia, PA 19106, USA
`
`Prepared by the Editor using Microsoft Word 6
`
`Printed in Great Britain by J W Arrowsmith Ltd, Bristol
`
`0005
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`
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`the
`the official book series of
`is
`The Medical Science Series
`International Federation for Medical and Biological Engineering
`(IFMBE) and the International Organization for Medical Physics
`(IOMP).
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`IFMBE
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`to provide medical and biological
`in 1959
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`3891, Fax: (46) 90 785 1588, email: Hans.Svensson@radfys.umu.se.
`
`0007
`
`
`
`CONTENTS
`
`PREFACE
`
`1
`
`NORMAL OXYGEN TRANSPORT
`Susanne A Clark
`
`1.1
`
`1.2
`
`1.3
`
`1.4
`
`1.5
`1.6
`
`1.7
`
`1.8
`
`Ventilatory control
`Neural control
`1.1.1
`1.1.2
`Respiratory feedback
`Ventilatory mechanics
`1.2.2
`Expiration
`Diffusion to blood
`The alveoli
`1.3.1
`1.3.2 Gas exchange
`Bind to hemoglobin
`Characteristics of hemoglobin
`1.4.1
`1.4.2 Oxyhemoglobin dissociation curves
`Dissolved in plasma
`Circulation
`The heart
`1.6.1
`1.6.2
`Pulmonary circulation
`1.6.3
`Systemic circulation
`Cardiac output
`1.6.4
`Diffusion to tissue
`Diffusion into interstitial fluid and cell
`1.7.1
`1.7.2 Oxygen delivered
`1.7.3 Myoglobin
`Use in cell
`References
`Instructional objectives
`
`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 Sp02 in avoiding hypoxia
`
`XV
`
`1
`
`1
`1
`2
`2
`4
`5
`5
`5
`6
`6
`7
`8
`8
`8
`9
`9
`9
`10
`11
`11
`11
`11
`12
`12
`
`13
`
`13
`15
`15
`15
`16
`
`0008
`
`
`
`Vlll
`
`Contents
`
`2.3
`
`2.2.5
`Photoplethysmography
`2.2.6
`I--Iyperoxia
`Limitations
`2.3.1
`Instrument and operation limitations
`2.3.1
`Limitations in Sp02
`References
`Instructional objectives
`
`3
`
`BLOOD OXYGEN MEASUREMENT
`James Farmer
`
`3.1
`
`3.2
`3.3
`
`3.4
`
`3.5
`
`3.6
`3.7
`
`Chemical methods
`3.1.1
`Van Slyke method
`3.1.2 Mixing syringe method
`3.1.3
`The Clark electrode
`3.1.4
`The galvanic electrode
`Transcutaneous P02 sensor
`In vitro oximeters
`3.3.1
`S pectrophotometers
`3.3.2
`The CO-oximeter
`In vivo two-wavelength oximeters
`3.4.1
`The first in vivo oximeters
`3.4.2
`The cyclops
`Fiber optic oximeters
`3.5.1
`In vitro reflectance oximeter
`3.5.2
`In vivo reflectance catheter oximeter
`3.5.3
`In vivo chemical oximeter
`In vivo eight-wavelength oximeter
`Pulse oximeters
`3.7.1
`Overview
`3.7.2
`LEDs
`3.7.3
`Photodiode
`3.7.4
`Probes
`3.7.5
`Analog amplifier and signal processing
`3.7.6 A three-wavelength pulse oximeter for
`COHb determination
`Comparison of pulse oximetry to
`transcutaneous P02 electrodes
`References
`Instructional objectives
`
`3.7.7
`
`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
`
`18
`10
`19
`19
`19
`20
`20
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`21
`
`21
`22
`23
`23
`25
`25
`26
`26
`28
`30
`30
`30
`30
`30
`31
`32
`32
`34
`34
`35
`36
`36
`37
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`37
`
`38
`38
`39
`
`40
`
`40
`41
`41
`
`0009
`
`
`
`Contents
`
`IX
`
`4.2
`
`4.3
`
`4.4
`
`4.5
`4.6
`
`Hemoglobin extinction coefficients
`4.2.1
`Functional hemoglobins
`4.2.2 Dysfunctional hemoglobins
`4.2.3 Hemoglobin absorbance spectra
`Beer's law in pulse oximetry
`4.3.1
`Criteria for the choice of wavelengths
`4.3.2 Absorbance in hemoglobin solutions
`4.3.3
`Pulsation of the blood
`4.3.4 Measurement of pulse oximeters
`Saturation versus normalized ratio
`4.4.1
`Normalization
`4.4.2
`Ratio of normalized signals
`4.4.3
`Theoretic calibration curve
`Validity of Beer's law in pulse oximetry
`Light Scattering
`4.6.1
`Light absorbance in whole blood
`4.6.2 Models for light absorbance including scattering
`4. 6. 3
`Influence of scattering on pulse oximeter readings
`4.6.4
`Calibration curves used for pulse oximeters
`References
`Instructional objectives
`
`5
`
`LIGHT-EMITTING DIODES AND THEIR CONTROL
`Brad W J Bourgeois
`
`5.1
`
`5.2
`
`5.3
`5.4
`5.5
`
`5.6
`5.7
`
`An introduction to light-emitting diodes
`5 .1.1
`Description, materials, and operation
`5 .1.2
`Bandwidth considerations
`Light-emitting diode specifications
`5 .2.1
`Forward voltage
`5 .2.2
`Forward current
`5.2.3
`Power dissipation
`5 .2.4
`Reverse breakdown voltage
`5.2.5
`Reverse current
`5.2.6 Operating temperature
`5 .2. 7
`Switching times
`5.2.8
`Beam angle
`5.2.9
`Pulse capability
`Measuring and identifying LED wavelengths
`LED driver circuit
`LED peak wavelength shift with temperature
`5.5.1
`p-n junction heating
`5.5.2
`Studies
`5.5.3
`Two methods to compensate for
`LED temperature changes
`Prevention of burns in pulse oximetry
`LED packaging
`References
`Instructional objectives
`
`42
`42
`42
`44
`44
`45
`45
`46
`48
`49
`49
`49
`50
`51
`52
`52
`52
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`54
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`57
`57
`57
`58
`58
`59
`60
`60
`60
`61
`61
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`62
`64
`66
`66
`66
`
`68
`69
`69
`70
`70
`
`0010
`
`
`
`X
`
`6
`
`Contents
`
`PHOTODETECTORS AND AMPLIFIERS
`JeffreyS Schowalter
`
`6.2
`
`6.1 Photodetection devices
`6.1.1
`Photocells
`6.1.2
`Photodiodes
`6.1.3
`Phototransistors
`6.1.4
`Integrated circuit (IC) sensors
`Photodiode characteristics
`6.2.1
`Junction capacitance
`6.2.2 Dark current
`6.2.3
`Sensitivity
`6.2.4
`Spectral response
`6.2.5
`Packaging
`Optical Concerns
`6.3.1
`Optical filtering
`6.3.2 Optical interference
`6.4 Amplifiers
`6.4.1
`Standard transimpedance amplifier configuration
`6.4.2 Differential transimpedance amplifier
`6.4.3
`Zeroing circuit
`6.4.4
`Future trends
`References
`Instructional objectives
`
`6.3
`
`7
`
`PROBES
`Moola Venkata Subba Reddy
`
`7.1
`
`7.2
`
`Transmittance Probes
`7.1.1
`Principle
`7.1.2
`Sensor placement
`Reflectance Probes
`7.2.1
`Principle
`7.2.2
`Sensor placement
`7.2.3
`Effect of multiple photodiode arrangement
`7.2.4
`Effect of skin temperature
`Advantages and disadvantages of
`7.2.5
`reflectance probes over transmittance probes
`7.3 MIR probes
`7.4
`Probe connectors
`7.5
`Reusable probes
`7.6
`Disposable probes
`7.7
`Sources of errors due to probes and placement
`7.7.1
`Ambient light interference
`7.7.2 Optical shunt
`7.7.3
`Edema
`7.7.4 Nail Polish
`References
`Instructional objectives
`
`71
`
`71
`71
`72
`76
`76
`76
`76
`77
`77
`77
`77
`79
`79
`79
`79
`80
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`83
`84
`84
`84
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`86
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`86
`87
`87
`88
`88
`90
`90
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`91
`91
`92
`93
`94
`94
`94
`95
`95
`95
`96
`96
`
`0011
`
`
`
`8
`
`ELECTRONIC INSTRUMENT CONTROL
`l(etan S Paranjape
`
`Contents
`
`Xl
`
`97
`
`97
`98
`99
`100
`101
`101
`102
`103
`103
`104
`105
`105
`105
`105
`106
`107
`109
`110
`111
`113
`114
`114
`114
`115
`115
`116
`116
`117
`117
`117
`117
`118
`119
`119
`120
`120
`121
`121
`121
`122
`122
`
`124
`
`124
`125
`
`8,1
`
`8.3
`
`8.4
`
`General theory of operation
`8.1.1
`Historic perspective
`8.2 Main block diagram
`Input module
`8.2.1
`Digital processor system
`8.3.1 Microprocessor subsection
`8.3.2 General block description
`8.3.3 Wait state generator
`8.3.4
`Clock generator, timer circuit and UART
`Pattern generator
`8.3.5
`Analog processing system (Nell cor®)
`8.4.1
`Analog signal flow
`8.4.2
`Coding resistor, temperature sensor, and prefiltering
`8.4.3
`Preamplifier
`8.4.4 Demodulator and filtering
`8.4.5 DC offset elimination
`Timing diagram (N ellcor®)
`8.4.6
`LED driver circuit
`8.4.7
`8.4.8 Analog processing system (Ohmeda®)
`ECG section
`8.5.1
`Active filters
`8.5.2 Offset amplifiers
`8.5.3 Detached lead indicator
`8.5.4
`Power line frequency sensing
`8.5.5
`ECG output
`Signal conversion
`Analog-to-digital conversion technique
`8.6.1
`8.6.2 Digital-to-analog conversion
`8.6.3
`Sample-and-hold circuit
`Timing and control
`Polling and interrupt
`8.7.1
`Power Supply
`8.8
`Alarms
`8.9
`8.10 Storage
`8.11 Front end display
`8.11.1 Front end driver circuit
`8.11.2 Front panel control
`8.11.3 Power up display tests
`8.12 Speakers
`References
`Instructional objectives
`
`8.5
`
`8.6
`
`8.7
`
`9
`
`SIGNAL PROCESSING ALGORITHMS
`Surekha Palreddy
`
`9.1
`9.2
`
`Sources of errors
`Beer-Lambert law
`
`0012
`
`
`
`xu
`
`Contents
`
`9.3
`
`9.4
`
`9.5
`9.6
`
`9.7
`
`9 .2.1
`
`Estimation of oxygen saturation
`using the Beer-Lambert law
`Ratio of ratios
`9. 3.1
`Peak and valley method
`9.3 .2 Derivative method: noise reduction software
`General processing steps of oximetry signals
`9.4.1
`Start up software
`Transient conditions
`ECG synchronization algorithms
`9 .6.1
`Nellcor® system
`9.6.2
`Criticare® system
`Spectral methods of estimating Sp02
`References
`Instructional objectives
`
`10
`
`CALIBRATION
`Jeffrey S Schowalter
`
`10.1 Calibration methods
`1 0.1.1 Traditional in vivo calibration
`1 0.1.2
`In vitro calibration using blood
`10.2 Testing simulators
`1 0.2.1 Simulators using blood
`1 0.2.2 Non blood simulators
`1 0.2.3 Electronic simulators
`10.3 Standards
`10.3.1 ASTM F1415
`10.3.2
`ISO 9919
`1 0. 3. 3 Other standards
`References
`Instructional objectives
`
`11
`
`ACCURACY AND ERRORS
`Supan Tungjitkusolmun
`
`11.1
`
`11.2
`
`11.3
`
`11.4
`11.5
`11.6
`
`Evaluation of pulse oximeters
`11.1.1 Accuracy, bias, precision, and confidence limit
`11.1.2 What do pulse oximeters really measure?
`11. 1. 3 Pulse oximeter versus CO-oximeter
`11.1.4 Pulse oximeter versus
`in vivo eight-wavelength ear oximeter
`Accuracy versus saturation
`11.2.1 High saturation (greater than 97 .5o/o)
`11.2.2 Normal saturation (90 to 97 .5o/o)
`11.2.3 Low saturation (less than 90o/o)
`Accuracy versus perfusion
`11.3.1 Venous congestion
`Accuracy versus motion artifacts
`Accuracy versus optical interference
`Accuracy versus intravenous dyes
`
`126
`129
`129
`130
`133
`134
`135
`143
`144
`149
`157
`158
`158
`
`159
`
`159
`159
`162
`163
`164
`168
`173
`172
`173
`173
`174
`174
`175
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`176
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`176
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`
`179
`180
`180
`181
`181
`182
`182
`183
`184
`185
`
`0013
`
`
`
`Contents
`
`xn1
`
`11.7 Effect of dyshemoglobins and fetal hemoglobin
`11.7.1 Carboxyhemoglobin (COHb)
`11.7. 2 Methemoglobin (MetHb)
`11.7. 3 Fetal hemoglobin
`11.7 .4 Bilirubin
`11.8 Effect of temperature
`11.8 .1 Ambient temperature
`11.8.2 Patient temperature
`11.9 Accuracy versus medical conditions
`11.9. 1 Cardiac arrhythmia
`11.9.2 Myxoma
`11.10 Accuracy versus probe position
`11. 11 Electromagnetic interference
`11.11.1 Interference from
`magnetic resonance imaging (MRI)
`11. 12 Other effects on accuracy
`11.12.1 Exercise
`11.12.2 Dried blood
`11.12.3 Pigments
`References
`Instructional objectives
`
`12
`
`USER INTERFACE FOR A PULSE OXIMETER
`Albert Lozano-Nieto
`
`Introduction
`12.1
`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
`
`13 .1 Anesthesia
`13 .1.1 Problems encountered during induction to anesthesia
`~3.1.2 Surgery under anesthesia
`13.2 Monitoring tissue blood supply and organ viability
`Intestinal blood flow and
`13 .2.1
`bowel viability following surgery
`13.2.2 Tissue transfer and setting of limb fractures
`13.2.3 Dental pulp blood supply and viability
`13.3 Monitoring on the road and in the air
`
`187
`187
`188
`189
`190
`190
`190
`191
`192
`192
`192
`193
`194
`
`194
`195
`195
`196
`196
`197
`198
`
`199
`
`199
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`201
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`204
`206
`209
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`210
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`213
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`214
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`214
`215
`216
`217
`
`217
`218
`218
`219
`
`0014
`
`
`
`XIV
`
`Contents
`
`13.4
`
`13.5
`13.6
`
`13.7
`13.8
`13.9
`13.10
`13.11
`13.12
`
`13.3. 1 Ambulances
`13.3.2 Flight
`Childbirth
`13 .4.1 Causes of desaturation in mother and fetus
`13 .4.2 Special apparatus for fetal monitoring
`Neonatal and pediatric care
`Sleep studies and physical stress testing
`13.6.1 Sleep
`13.6.2 Exercise
`Management of cardiopulmonary resuscitation
`Computer-controlled oxygen weaning
`Systolic blood pressure measurement
`Cerebral oxygen measurement
`Veterinary care
`Future improvements for pulse oximetry
`References
`Instructional objectives
`
`GLOSSARY
`
`INDEX
`
`219
`220
`221
`221
`222
`224
`227
`227
`231
`231
`232
`232
`232
`233
`234
`234
`236
`
`237
`
`243
`
`0015
`
`
`
`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
`and
`software
`required
`for
`effective
`functioning.
`equations, methods,
`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
`
`0016
`
`
`
`xv1
`
`Preface
`
`algorithms to perform oxygen saturation calculations are given in chapter 9, \Vith
`worked out examples. Synchronization with the electrocardiogram itnproves
`accuracy during patient movement.
`Chapter 10 describes ways to test performance of pulse oximeters: the
`technician's finger, electronic simulators, in vitro test systems, and optoelectronic
`simulators. In chapter 11, we find the resulting accuracies and descriptions of the
`inaccuracies caused by alternative forms of hemoglobin, optical and electrical
`interference, colored nail polish, etc. Chapter 12 describes the interface between
`the pulse oximeter, the operator, and the external world. Chapter 13 covers the
`many applications for pulse oximetry
`in intensive care, operating rooms,
`emergency, patient transport, general wards, birth and deli very, neonatal care,
`sleep laboratories, 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 learning the information. We _gleaned much of
`the design information from operator's manuals and from patents; periodical
`literature provided more general information. Rather than giving an exhaustive
`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
`
`0017
`
`
`
`CHAPTER!
`
`NORMAL OXYGEN TRANSPORT
`
`Susanne A Clark
`
`Oxygen is vital to the functioning of each cell in the human body. In the absence
`of oxygen for a prolonged amount of time, cells will die. Thus, oxygen delivery
`to cells is an important indicator of a patient's health.
`Several methods have been developed to analyze oxygen delivery. Pulse
`oximetry is a common, noninvasive method used in 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 cells 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 into the
`blood, while carbon dioxide, a byproduct of cellular respiration, diffuses into the
`lungs. The oxygenated blood circulates around the body until it reaches 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 the brain stem. Tpe 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 turn cause the movement of
`respiratory muscles. The output of the respiratory neurons is modulated by
`chemoreceptors and mechanoreceptors.
`
`1.1.1 Neural control
`
`the pattern
`The respiratory neurons in the brain stem are responsible for
`generation in normal breathing. The rate and depth of ventilation are modulated
`by these neurons. The respiratory neurons excite motor neurons in the spinal
`cord. The excitation of the motor neurons causes the contraction of the
`diaphragm, pectoral muscles, and intercostal muscles" All of these muscles
`
`1
`
`0018
`
`
`
`2
`
`Design of pulse oximeters
`
`combine efforts pulling the ribcage up and out, expanding the lungs, causing
`inspiration. The activity of respiratory neurons is thought to occur spontaneously,
`with occasional inhibition allowing the respiratory muscles to relax. This causes
`the rib cage to contract which yields expiration.
`
`1.1.2 Respiratory feedback
`
`The brain stem receives feedback from many mechanical and chemical receptors.
`The input from these neurons is analyzed by the respiratory neurons to determine
`the appropriate rate and depth of ventilation. Mechanoreceptors give feedback
`related to mechanical aspects of breathing. For example, stretch receptors are
`mechanoreceptors that provide feedback on the expansion of the lung and chest
`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 sin1ilar purpose in expiration, hindering the
`collapse of the lungs.
`Chemoreceptors provide information on the level of carbon dioxide, oxygen,
`and hydrogen ions in the blood. Chemoreceptors are located in the carotid
`arteries, as the oxygenated blood is being sent to the brain, and in the aorta,
`shortly after the oxygenated blood is being pumped from the heart to the body.
`Oxygen levels under normal conditions are high in the systemic arteries, and
`carbon dioxide and hydrogen levels are low.
`The brain stem must process all of the information it receives and no single
`factor controls ventilation. Under normal breathing conditions, the brain stem is
`most sensitive to the levels 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 in the blood, such as would occur
`during maximal exercise. However, stretch receptors indicate that the lung and
`chest are at maximal expansion, meaning the inflation index has been reached.
`Thus, the rate of breathing increases to compensate without a proportional
`increase in chest and lung expansion.
`An unusual feature of ventilation is that breathing can be brought under
`voluntary control to some extent. However, it is not possible to commit suicide by
`refusing to breathe. Once the individual loses consciousness, the input from
`chemoreceptors will cause ventilation to be restored.
`
`1.2 VENTILATORY MECHANICS
`
`Ventilatory mechanics are based on the principle of air flow from areas of high
`pressure to areas of lower pressure. The contraction of the intercostal muscles,
`pectoral muscles, and the diaphragm causes the thoracic cavity to expand,
`decreasing the pressure in the thoracic cavity. The atmospheric pressure is 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
`causes the volume of the lungs to decrease, increasing the pressure in the thoracic
`cavity. As the pressure
`in the lungs increases reaching
`levels above the
`atmospheric pressure, air flows out of the lungs, which is referred
`to , as
`expiration.
`
`0019
`
`
`
`Normal oxygen transport
`
`3
`
`1.2.1 Inspiration
`
`As discussed previously, the brain stem excites motor neurons in the spinal cord,
`which, in turn, causes the contraction of the diaphragm, the pectoral muscles, and
`intercostal muscles, located between the ribs. The contraction of the diaphragm
`causes the flattening and lengthening of the thoracic cavity. The intercostal
`muscles and pectoral muscles pull the ribcage up and out. Both of these sets of
`muscles work to expand the lungs. This means that pressure will be reduced
`within the lungs, since the air present will have a greater volume to expand in.
`This will create a pressure differential between the air outside the body and the
`air inside the body. Thus, air flows into the body (see figure 1.1 (a)).
`
`(a) Inspiration
`
`Air drawn into lungs
`
`(b) Expiration
`
`Air forced out ot lungs
`
`u.mgs expa.nd
`
`lnte reo stal
`muscles
`contract
`
`1 nte r costal
`muscles
`relax
`
`Diaphragm contracts
`and flattens
`
`Diaphragm relaxes
`and moves up
`
`Figure 1.1 During inspiration, (a), the diaphragm, intercostal muscles and pectoralis minor
`muscles contract, causing the lungs to expand and air to enter the lungs. As the diaphragm,
`intercostal muscles and pectoralis minor relax, the lungs contract, causing air to leave the lungs (b),
`which is referred to as expiration (from Microsoft Encarta).
`
`Air travels through the nasal cavity. Cilia are microscopic hairs within the
`nasal cavity that act to eliminate pollutants from entering the respiratory tract.
`Air and food both go through the pharynx. When food is swallowed, the
`epiglottis (part of the larynx), pharynx, and mouth cavity work together to shut
`off the opening to the trachea to avoid the entry of food particles into the lungs.
`The larynx is commonly referred to as the voice box. Besides assisting with
`separation 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 the vocal cords. As air vibrates over the vocal cords, a sound is
`produced. The variation in elasticity and tension of the vocal cords determines the
`pitch of the sound.
`The trachea is composed of ribbed cartilage which extends 10 em to the
`bronchi. The trachea also contain cilia which act to filter out further pollutants.
`Two bronchi provide a path to each lung (see figure 1.2).
`Each bronchus divides into even narrower bronchioles. Each bronchiole has
`five or more alveolar ducts at the end, which, in turn, end in alveolar sacs. Each
`alveolar sac contain several alveoli (see figure 1.3). Alveoli are the site of gas
`exchange.
`
`0020
`
`
`
`4
`
`Design of pulse oximeters
`
`Figure 1.2 Air travels through the nasal cavity, into the pharynx, trachea, bronchi, and finally the
`lungs. The bronchi, bronchioles, alveolar ducts and alveoli compose the pulmonary tree with its
`branch like system (adapted from Corel Corporation).
`
`Alveolar duct
`
`Figure 1.3 Ten or more alveoli are in one alveolar sac (adapted from Corel Corporation).
`
`1.2.2 Expiration
`
`Neurons in the brain stem cyclically inhibit the motor neurons in the spinal cord
`that cause muscle contraction in the diaphragm, the pectoral muscles, and
`intercostal muscles. The muscles then relax, causing the rib cage to contract,
`decreasing the amount of air space. This causes air to flow out of the lungs when
`the pressure inside the lungs is greater than the pressure ou