RX-0035
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`RX-0035.0001
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`111 -
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`RX-0035.0002
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`APL_MAS_ITC_00015619
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`L
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`1
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`Design of Pulse Oximeters
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`RX-0035.0003
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`APL_MAS_ITC_00015620
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`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 Hotilt
`
`Rehabilitation Engineering Applied to Mobility and 1Manipulation
`R A Cooper
`
`I,inear Accelerators for Radiation Therapy, second edition
`D Greene und P C Williains
`
`Health Effects of Exposure to Low-Level Ionizing Radiation
`W R Hendee and F M Edwards
`
`Introductory 1Medical Statistics, third edition
`R F Mould
`
`Radiation Protection in Hospitals
`R F Mould
`
`RPI. Dosimetry-Radiophotoluminescence in Health Physics
`.1 A Perry
`
`Physics of Heart and Circulation
`J Strackee and N Westerhof
`
`The Physics of Medical Imaging
`S Webb
`
`The Physics of Three-Dimensional Radiation Therapy: Conformal
`Radiotherapy, Radiosurgery and Treatment Planning
`S Webb
`
`The Physics of Conformal Radiotherapy: Advances in Technology
`S Webb
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`Other titles of interest
`
`Prevention of Pressure Sores: Engineering and Clinical Aspects
`J G Webster
`
`i
`
`3
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`RX-0035.0004
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`APL_MAS_ITC_00015621
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`Medical Science Series
`
`l
`
`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
`
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`RX-0035.0005
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`APL_MAS_ITC_00015622
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`1
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`1
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`4
`
`O ]OP 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 ollierwise, 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 Libran? Cataloguing-in-Publication Data
`
`A catalogue record for this book is available from the British Library.
`
`[SBN () 75()3 ()467 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 i f
`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
`JAE Spann, 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
`
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`RX-0035.0006
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`The Medical Science Series is the official book series of the
`International Federation for Medical and Biological Engineering
`(IFMBE) and the International Organization for Medical Physics
`(IOMP).
`
`IFMBE
`
`The IFMBE was established in 1959 to provide medical and biological
`engineering with an international presence. The Federation has a long history of
`encouraging and promoting international cooperation and collaboration in the use
`of technology for improving the health and life quality of man.
`
`The IFMBE is an organization that is mostly an affiliation of national societies.
`Transnational organizations can also obtain membership. At present there are 42
`national members, and one transnational member with a total membership in
`excess of 15 000. An observer category is provided to give personal status to
`groups or organizations considering formal affiliation.
`Objectives
`
`• To reflect the interests and initiatives of the affiliated organizations.
`• To generate and disseminate information of interest to the medical and
`biological engineering community and international organizations.
`• To provide an international forum for the exchange of ideas and concepts.
`• To encourage and foster research and application of medical and biological
`engineering knowledge and techniques in support of life quality and cost-effective
`health care.
`• To stimulate international cooperation and collaboration on medical and
`biological engineering matters.
`• To encourage educational programmes which develop scientific and technical
`expertise in medical and biological engineering.
`
`Activities
`The IFMBE has published the journal Medical and Biological Engineering and
`Computing for over 34 years. A new journal Cellular Engineering was established
`in 1996 in order to stimulate this emerging field in biomedical engineering. In
`IFMBE News members are kept informed of the developments in the Federation .
`Clinical Engineering Update is a publication of our division of Clinical
`Engineering. The Federation also has a division for Technology Assessment in
`Health Care.
`
`Every three years, the IFMBE holds a World Congress on Medical Physics and
`Biomedical Engineering, organized in cooperation with the IOMP and the
`IUPESM. In addition, annual, milestone, regional conferences are organized in
`different regions of the world, such as the Asia Pacific, Baltic, Mediterranean,
`African and South American regions.
`
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`
`The administrative council of the IFMBE meets once or twice a year and is the
`steering body for the IFMBE. The council is subject to the rulings of the General
`Assembly which meets every three years.
`
`For further information on the activities of the IFMBE, please contact Jos A E
`Spaan, Professor of Medical Physics, Academic Medical Centre, University of
`Amsterdam, PO Box 22660, Meibergdreef 9, 1105 AZ, Amsterdam, The
`Netherlands. Tel: 31 (0) 20 566 5200. Fax: 31 (0) 20 6917233. Email:
`IFMBE@amc.uva.nl. WWW: http:Uvub.vub.ac.be/-ifmbe.
`IOMP
`
`The IOMP was founded in 1963. The membership includes 64 national societies,
`two international organizations and 12 000 individuals. Membership of IOMP
`consists of individual members of the Adhering National Organizations. Two
`other forms of membership are available, namely Affiliated Regional Organization
`and Corporate Members. The IOMP is administered by a Council, which consists
`of delegates from each of the Adhering National Organizations; regular meetings
`of Council are held every three years at the International Conference on Medical
`Physics (ICMP). The Officers of the Council are the President, the Vice-President
`and the Secretary-General. IOMP committees include: developing countries,
`education and training; nominating; and publications.
`
`Objectives
`
`• To organize international cooperation in medical physics in all its aspects,
`especially in developing countries.
`• To encourage and advise on the formation of national organizations of medical
`physics in those countries which lack such organizations.
`
`Activities
`Official publications of the IOMP are Physiological Afeasurement, Physics iii
`Medicine and Biology andthe A·ledical Science Series, all published by Institute of
`Physics Publishing . The IOMP publishes a bulletin Medical Physics World twice
`a year.
`Two Council meetings and one General Assembly are held every three years at
`the ICMP. The most recent ICMPs were held in Kyoto, Japan ( 1991) and Rio de
`Janeiro, Brazil (1994). Future conferences are scheduled for Nice, France (1997)
`and Chicago, USA (2()00). These conferences are normally held in collaboration
`with the IFMBE to form the World Congress on Medical Physics and Biomedical
`Engineering. The IOMP also sponsors occasional international conferences.,
`workshops and courses.
`
`For further information contact: Hans Svensson, PhD, DSc, Professor, Radiation
`Physics Department, University Hospital, 90185 UmeA, Sweden, Tel: (46) 90 785
`3891, Fax: (46) 90 785 1588, email: Hans.Svensson®radfys.umu.se.
`
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`CONTENTS
`
`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
`
`1
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`1
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`1.d . 1
`
`11.1
`
`PREFACE
`NORMAL OXYGEN TRANSPORT
`Susanne A Ctark
`
`1.1 Ventilatory control
`1.1.1 Neural control
`1.1.2 Respiratory feedback
`1.2 Ventilatory mechanics
`1.2.2 Expiration
`1.3 Diffusion to blood
`1.3.1 The alveoli
`1.3.2 Gas exchange
`1.4 Bind to hemoglobin
`1.4.1 Characteristics of hemoglobin
`1.4.2 Oxyhemoglobin dissociation curves
`1.5 Dissolved in plasma
`1.6 Circulation
`1.6.1 The heart
`1.6.2 Pulmonary circulation
`1.6.3 Systemic circulation
`1.6.4 Cardiac output
`1.7 Diffusion to tissue
`1.7.1 Diffusion into interstitial fluid and cell
`1.7.2 Oxygen delivered
`1.7.3 Myoglobin
`1.8 Use in cell
`References
`Instructional objectives
`
`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
`
`1
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`2
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`111
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`18
`10
`19
`19
`19
`20
`20
`21
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`21
`22
`23
`23
`25
`25
`26
`26
`28
`30
`30
`30
`30
`30
`31
`32
`32
`34
`34
`35
`36
`36
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`37
`38
`38
`39
`40
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`40
`41
`41
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`V iii
`
`Contents
`
`2.2.5 Photoplethysmography
`2.2.6 Hyperoxia
`2.3 Limitations
`2.3.1 Instrument and operation limitations
`2.3.1 Limitations in SpO2
`References
`Instructional objectives
`
`3
`
`BLOOD OXYGEN MEASUREMENT
`Jarnes Farmer
`
`3.1 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
`3.2 Transcutaneous PO2 sensor
`3.3 In vitro oxirneters
`3.3.1 Spectrophotometers
`3.3.2 The CO-oximeter
`3.4 In vivo two-wavelength oximeters
`The first in vivo oximeters
`3 .4 . 1
`3.4.2
`The cyclops
`3.5 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
`3.6 In vivo eight-wavelength oximeter
`3.7 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
`3.7.7 Comparison of pulse oximetry to
`transcutaneous P02 electrodes
`References
`Instructional objectives
`
`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
`
`4
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`I
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`1X
`42
`42
`42
`44
`44
`45
`45
`46
`48
`49
`49
`49
`50
`51
`52
`52
`52
`53
`54
`54
`55
`56
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`56
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`60
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`69
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`70
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`Contents
`
`4.2 Hemoglobin extinction coefficients
`4.2.1 Functional hemoglobins
`4.2.2 Dysfunctional hemoglobins
`4.2.3 Hemoglobin absorbance spectra
`4.3 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
`4.4 Saturation versus normalized ratio
`4.4.1 Normalization
`4.4.2 Ratio of normalized signals
`4.4.3 Theoretic calibration curve
`4.5 Validity of Beer's law in pulse oximetry
`4.6 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 An introduction to light-emitting diodes
`5.1.1 Description, materials, and operation
`5.1.2 Bandwidth considerations
`5.2 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
`5.3 Measuring and identifying LED wavelengths
`5.4 LED driver circuit
`5.5 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
`5.6 Prevention of burns in pulse oximetry
`5.7 LED packaging
`References
`Instructional objectives
`
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`X
`6
`
`7
`
`Contents
`
`PHOTODETECTORS AND AMPLIFIERS
`Jeffrey S Schowatter
`
`6.1 Photodetection devices
`6.1.1 Photocells
`6.1.2 Photodiodes
`6.1.3 Phototransistors
`6.1.4 Integrated circuit (IC) sensors
`6.2 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
`6.3 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
`PROBES
`Moola Venkata Subba Reddy
`
`7.1 Transmittance Probes
`7.1.1 Principle
`7.1.2 Sensor placement
`7.2 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
`7.2.5 Advantages and disadvantages of
`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
`
`1
`
`71
`
`71
`71
`72
`76
`76
`76
`76
`77
`77
`77
`77
`79
`79
`79
`79
`80
`82
`83
`84
`84
`84
`86
`
`86
`87
`87
`88
`88
`90
`90
`91
`91
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`93
`94
`94
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`96
`96
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`RX-0035.0012
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`111.
`
`l
`
`Contents
`
`Xi
`97
`
`Ill,
`
`8
`
`ELECTRONIC INSTRUMENT CONTROL
`Ketan S Paranjape
`
`97
`8.1 General theory of operation
`98
`8.1.1 Historic perspective
`99
`8.2 Main block diagram
`100
`8.2.1 Input module
`101
`8.3 Digital processor system
`101
`8.3.1 Microprocessor subsection
`102
`8.3.2 General block description
`103
`8.3.3 Wait state generator
`103
`8.3.4 Clock generator, timer circuit and UART
`104
`8.3.5 Pattern generator
`105
`8.4 Analog processing system (Nellcor®)
`105
`8.4.1 Analog signal flow
`8.4.2 Coding resistor, temperature sensor, and prefiltering 105
`105
`8.4.3 Preamplifier
`106
`8.4.4 Demodulator and filtering
`107
`8.4.5 DC offset elimination
`109
`8.4.6 Timing diagram (Nellcor®)
`110
`8.4.7 LED driver circuit
`111
`8.4.8 Analog processing system (Ohmeda®)
`113
`8.5 ECG section
`114
`8.5.1 Active filters
`114
`8.5.2 Offset amplifiers
`114
`8.5.3 Detached lead indicator
`115
`8.5.4 Power line frequency sensing
`115
`8.5.5 ECG output
`116
`8.6 Signal conversion
`116
`8.6.1 Analog-to-digital conversion technique
`117
`8.6.2 Digital-to-analog conversion
`117
`8.6.3 Sample-and-hold circuit
`117
`8.7 Timing and control
`117
`8.7.1 Polling and interrupt
`118
`8.8 Power Supply
`119
`8.9 Alarms
`119
`8.10 Storage
`120
`8.11 Front end display
`120
`8.11.1 Front end driver circuit
`121
`8.11.2 Front panel control
`121
`8.11.3 Power up display tests
`121
`8.12 Speakers
`122
`References
`122
`Instructional objectives
`124
`
`SIGNAL PROCESSING ALGORITHMS
`Surekha Patreddy
`
`9
`
`9.1 Sources of errors
`9.2 Beer-Lambert law
`
`12
`
`124
`125
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`
`Xii
`
`Contents
`
`9.2.1 Estimation of oxygen saturation
`using the Beer-Lambert law
`9.3 Ratio of ratios
`9.3,1 Peak and valley method
`9.3.2 Derivative method: noise reduction software
`9.4 General processing steps of oximetry signals
`9.4.1 Start up software
`9.5 Transient conditions
`9.6 ECG synchronization algorithms
`9.6.1 Nellcor® system
`9.6.2 Criticare® system
`9.7 Spectral methods of estimating SpO2
`References
`Instructional objectives
`
`10 CALIBRATION
`Jeffrey S Schowalter
`
`10.1 Calibration methods
`10.1.1 Traditional in vivo calibration
`In vitro calibration using blood
`10 . 1 .2
`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
`
`11 ACCURACY AND ERRORS
`Supan Ttingjitklisolmitn
`
`11.1 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
`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
`11.5 Accuracy versus optical interference
`11.6 Accuracy versus intravenous dyes
`
`126
`129
`129
`130
`133
`134
`135
`143
`144
`149
`157
`158
`158
`159
`
`159
`159
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`163
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`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 o f 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
`
`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
`
`Contents Xiii
`187
`187
`188
`189
`190
`190
`190
`191
`192
`192
`192
`193
`194
`194
`195
`195
`196
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`197
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`200
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`210
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`214
`
`214
`13.1 Anesthesia
`13.1.1 Problems encountered during induction to anesthesia 215
`216
`13.1.2 Surgery under anesthesia
`217
`13.2 Monitoring tissue blocd supply and organ viability
`13.2.1 Intestinal blood flow and
`217
`bowel viability following surgery
`218
`13.2.2 Tissue transfer and setting of limb fractures
`218
`13.2.3 Dental pulp blood supply and viability
`219
`13.3 Monitoring on the road and in the air
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`Contents
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`13.3.1 Ambulances
`13.3.2 Flight
`13.4 Childbirth
`13.4.1 Causes of desaturation in mother and fetus
`13.4.2 Special apparatus for fetal monitoring
`13.5 Neonatal and pediatric care
`13.6 Sleep studies and physical stress testing
`13.6.1 Sleep
`13.6.2 Exercise
`13.7 Management of cardiopulmonary resuscitation
`13.8 Computer-controlled oxygen weaning
`13.9 Systolic blood pressure measurement
`13.10 Cerebral oxygen measurement
`13.11 Veterinary care
`13.12 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
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`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
`
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`Preface
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`algorithms to perform oxygen saturation calculations are given in chapter 9, with
`worked out examples. Synchronization with the electrocardiogram improves
`accuracy diiring patient movement.
`Chapter 10 describes ways to test performance of pulse oximeters: the
`
`technician's finger, electronic simulators, in vitro test systems , and optoclectronic
`
`simulalors. 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 words, birth and delivery, neonatal care,
`sleep laboralories, 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
`
`L___
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`CHAPTER 1
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`1
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`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 Litne. cells will die. 7'hus, oxygen delivery
`to ccils is an important indicator of a patieni's health.
`Several methods have been developed to analyze oxygen delivery. Pulse
`oximetry is a common, noninvasive method used in clinical environmenis. 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 ilse 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 u reaches oxygen
`depicted areas, wheie 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 steni. The circulalory 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
`chemoreceptons and nzeehanoreceptors.
`
`1.1.1 Neural control
`
`The respiratory neurons in the brain stem are responsible for the paltern
`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
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`Design of pulse oximeters
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`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. 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 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 similar 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
`arleries, 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 stein 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 iii the blood, such as would ircur
`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 breaching 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 lo some extent. However. it is not possible to commit suicide by
`refusing to breathe, Once the individual loses consciousness, the input from
`chemorcceplors will cause ventilation to he 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 diaphragin 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 intercostai 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.
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`Normal oxygen transport
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`3
`
`Ill Inspiration
`As discussed prevjousiy, the brain stem exciles motel· neurons in [lie spinal cord,
`which, in turn, causes die contraction of the diaphragm, the pectoral muscles, and
`tiltercostal muscles, Jocated hetween the ribs. The contraction Of the diaphragm
`causes the flattening and lengthening of the thoracic cavity. The intercostal
`muscles and pectorn] muscles pull the ribcage up and out. Both of these sets of-
`muscles work to expand the lungs. This means thal pressure will be reduced
`williin the lungs, shice the air present will have a greate, volume to expand in.
`This will create a pressure differential between the air outside the body and the
`air inside the body. Thus, ail flows into the body (see figure 1.1 (a)).
`
`(a) Inspiration
`Air drawn into lungs
`
`(b) Expiration
`Air forced out orlungs
`
`. pectoralis mlner
`1 muscles contract
`
`,r·--~7.7 rectoralls minor
`Tracheti6--~ ~ „1~"Cle, Felix
`
`tungs expand ,
`
`i. 0~11 Rip '*
`
`intercostal
`muscles
`contract
`
`2-EP-Whi-~-/tik-- -/ 1•terrow/1
`mUfC|43
`relax
`
`Diaphragm contracts
`and flattens
`
`.I---.---,
`Didphragm relaxes
`anal moves up
`
`Figure 1.] During inspiration, (a), Ike diaphragm. intercostal muscles .ind pectoralis minor
`muscles contracl, causing the lungs to expand ami air to enter the lungs. As Ilic diaphragm,
`inte,·cosial muscles and pectoralis minor relax, Ihe lungs contract. causillgairto leave the I ings (b),
`which is referird to as expiration (from Micir).oft Encart.0.
`
`Air travels through the nasa] cavity. Cilia are Inicroscopic hairs within the
`nasal cavity [liat act to eliminate pollutants from entering the respiratory trac[.
`Air and food both go through the pharynx. When food is swallowed, the
`epiglottis ( par[ of the tarynx), 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 reterred M as the voice box. Besides assisting with
`Separation ot food particles trom air, the larynx contains the cricoid cartilage
`which reinforces the airway and assists iii keeping it open. The lai·ynx 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 ic composed of ribbed cartilage which extends 10 cm to the
`bronchi. The trachea also contai,1 Cilia which act to filter out furlher pollutants.
`Two bronchi provide a path to each lung (see figure 1.2).
`Each bronchus divides into even narrower brotichioles. Each bron ,hiole ha
`in lurn , end in alveola r sacs. Each
`five or more alveolar duas at ilie end. which ,
`alveolar sac contain several alveoli (see figure 1 . 3 ). Aiveoli are the site of gas
`exchange.
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`Design of pulse oximeters
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`Lar ynx --:,A_~J;;4
`
`Trachea
`
`~~-~~--B ronchus
`
`Bronchioles
`
`l
`
`Diaphragm
`
`Figure 1.2 Air travels through me nasal cavity. into the pharynx, trachea, bro