I
`
`.
`
`,,_
`
`DESIGN
`0F PULSE
`
`OXIMHERS
`
`
`
`1
`
`APPLE 1010
`
`

`

`Design of Pulse Oximeters
`
`
`
`
`
`2
`
`

`

`
`
`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
`RFMould
`
`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 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
`
`Other titles of interest
`
`Prevention of Pressure Sores: Engineering and Clinical Aspects
`J G Webster
`
`.
`
`[i
`i
`
`I
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`l
`'
`
`|
`
`|
`
`I.
`
`.;
`
`
`
`3
`
`

`

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

`

`
`
`C?) IOP 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—Piiblication Data
`
`A catalogue record for this book is available from the British Library.
`
`ISBN 0 7503 0467 7
`
`Library of Congress Cataloging—in,—Publicati(m Data are air'ailablc
`
`the figures and
`The Editor has attempted to trace the copyright holder of all
`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 BSl 6BE, UK
`
`US Editorial Office: Institute of Physics Publishing, The Public Ledger Building,
`Suite 1035, 150 South Independence Mall West, Philadelphia, PA 19106, USA
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`Prepared by the Editor using Microsoft Word 6
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`Printed in Great Britain by J W Arrowsmith Ltd, Bristol
`
`
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`5
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`

`

` The Medical Science Series
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`The IFMBE was established in 1959 to provide medical and biological
`engineering with an international presence. The Federation has a long history of
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`
`
`
`
`7
`
`

`

` CONTENTS
`
`
`
`
`\OKDOOOOOOONQONLIILJIUl-lkaNr—‘r—t
`
`
`PREFACE
`
`1
`
`NORMAL OXYGEN TRANSPORT
`Susanne A Clark
`
`
`
`
`
`
`
`1.1
`
`1.2
`
`1.3
`
`1.4
`
`h—h—t
`
`0L1]
`
`1.7
`
`1.8
`
`Ventilatory control
`1.1.1
`Neural control
`
`Respiratory feedback
`1.1.2
`Ventilatory mechanics
`1.2.2
`Expiration
`Diffusion to blood
`1.3.1
`The alveoli
`
`Gas exchange
`1.3.2
`Bind to hemoglobin
`1.4.1
`Characteristics of hemoglobin
`1.4.2
`Oxyhemoglobin dissociation curves
`Dissolved in plasma
`Circulation
`1.6.1
`The heart
`
`Pulmonary circulation
`1.6.2
`Systemic circulation
`1.6.3
`Cardiac output
`1.6.4
`Diffusion to tissue
`1.7.1
`Diffusion into interstitial fluid and cell
`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
`
`
`13
`
`13
`15
`15
`15
`16
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`8
`
`

`

`
`
`viii
`
`Contents
`
`2.3
`
`Photoplethysmography
`2.2.5
`Hyperoxia
`2.2.6
`Limitations
`2.3.1
`Instrument and operation limitations
`2.3.1
`Limitations in SPOZ
`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. l
`Spectrophotometers
`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
`
`21
`
`21
`22
`23
`23
`25
`25
`26
`26
`28
`30
`30
`30
`30
`30
`31
`32
`32
`34
`34
`35
`3 6
`36
`37
`
`37
`
`38
`38
`39
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`40
`
`40
`41
`41
`
`'
`
`'-
`
`1
`
`
`
`9
`
`

`

`4.2
`
`4.3
`
`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
`
`4.4
`
`4.3.4 Measurement of pulse oximeters
`Saturation versus normalized ratio
`4.4. 1
`Normalization
`
`4.4.2
`4.4.3
`
`Ratio of normalized signals
`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
`
`LIGHT—EMITTING DIODES AND THEIR CONTROL
`Brad W J Bourgeois
`
`Contents
`ix
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`42
`42
`42
`44
`44
`45
`45
`46
`48
`49
`49
`49
`50
`51
`52
`52
`52
`53
`54
`54
`55
`
`56
`
`56
`57
`57
`57
`58
`58
`59
`60
`60
`60
`61
`61
`61
`62
`64
`66
`66
`66
`
`68
`69
`69
`70
`70
`
`
`
`
`
`
`
`5.1
`
`5.2
`
`9‘9"?“LII-PU)
`
`”‘8"\IO\
`
`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
`5.2.4
`5.2.5
`
`Power dissipation
`Reverse breakdown voltage
`Reverse current
`
`Operating temperature
`5.2.6
`Switching times
`5.2.7
`Beam angle
`5.2.8
`Pulse capability
`5.2.9
`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
`
`
`
`
`
`10
`
`10
`
`

`

` x
`
`Contents
`
`6
`
`PHOTODETECTORS AND AMPLIFIERS
`Jeffrey S Schowalter
`
`6.1 Photodetection devices
`6.1. 1
`Photocells
`6. 1 .2
`Photodiodes
`6.1.3
`Phototransistors
`
`6.2
`
`Integrated circuit (IC) sensors
`6.1.4
`Photodiode Characteristics
`
`6.2.1
`6.2.2
`
`Junction capacitance
`Dark current
`
`6.3
`
`6.4
`
`Sensitivity
`6.2.3
`Spectral response
`6.2.4
`Packaging
`6.2.5
`Optical Concerns
`6.3.1
`Optical filtering
`6.3.2
`Optical interference
`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
`
`7
`
`PROBES
`
`Moola Venkata Subba Reddy
`
`7.1
`
`Transmittance Probes
`
`Principle
`7.1.1
`Sensor placement
`7.1.2
`Reflectance Probes
`
`7.2
`
`7.2.1
`7.2.2
`7.2.3
`7.2.4
`7.2.5
`
`Principle
`Sensor placement
`Effect of multiple photodiode arrangement
`Effect of skin temperature
`Advantages and disadvantages of
`reflectance probes over transmittance probes
`7.3 MIR probes
`7.4
`Probe connectors
`
`7.5
`7.6
`7.7
`
`Reusable probes
`Disposable probes
`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
`82
`83
`84
`84
`84
`
`86
`
`86
`87
`87
`88
`88
`90
`9O
`
`91
`91
`92
`93
`94
`94
`94
`95
`95
`95
`96
`96
`
`11
`
`| l
`
`11
`
`

`

`I
`
`
`
`i
`
`8
`
`ELECTRONIC INSTRUMENT CONTROL
`Ketan S Paranjape
`
`Contents
`
`8.1
`
`8.3
`
`8.4
`
`General theory of operation
`8.1.1
`Historic perspective
`8.2 Main block diagram
`8.2.1
`Input module
`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
`8.3.5
`Pattern generator
`Analog processing system (Nellcor®)
`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
`8.4.6
`Timing diagram (Nellcor®)
`8.4.7
`LED driver circuit
`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
`8.6.1
`Analog-to-digital conversion technique
`8.6.2
`Digital-to-analog conversion
`8.6.3
`Sample-and—hold circuit
`Timing and control
`8.7.1
`Polling and interrupt
`Power Supply
`Alarms
`Storage
`Front end display
`8.11.1 Front end driver circuit
`8.11.2 Front panel control
`8.11.3 Power up display tests
`Speakers
`References
`Instructional objectives
`
`8.5
`
`8.6
`
`8.7
`
`8.8
`8.9
`8.10
`8.11
`
`8.12
`
`9
`
`SIGNAL PROCESSING ALGORITHMS
`Surekha Palreddy
`9.1
`Sources of errors
`9.2
`Beer-Lambert law
`
`H
`
`
`
`|
`
`I
`
`.
`l
`
`'
`
`Xi
`
`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
`l 19
`119
`120
`120
`121
`121
`121
`122
`122
`
`124
`
`124
`125
`
`
`
`12
`
`12
`
`

`

`
`
`xii
`
`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
`9.3.2
`
`Peak and valley method
`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
`
`l
`
`I
`
`10
`
`CALIBRATION
`
`Jeflrey S Schowalter
`
`10.1 Calibration methods
`10.1.1 Traditional in viva 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
`Standards
`10.3.1 ASTM F1415
`10.3.2
`ISO 9919
`10.3.3 Other standards
`References
`
`10.3
`
`Instructional objectives
`
`11
`
`ACCURACY AND ERRORS
`
`Supan Tungjitkusolmun
`
`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
`1 1.3.1 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
`162
`163
`164
`168
`173
`172
`173
`173
`174
`174
`175
`
`176
`
`176
`177
`178
`179
`
`179
`180
`180
`181
`181
`182
`182
`183
`184
`185
`
`
`
`13
`
`13
`
`

`

`Contents
`
`xiii
`
`187
`187
`188
`189
`190
`190
`190
`191
`192
`192
`192
`193
`194
`
`194
`195
`195
`196
`196
`197
`198
`
`199
`
`11.7 Effect of dyshemoglobins and fetal hemoglobin
`11.7.1 Carboxyhemoglobin (COHb)
`11.7.2 Methemoglobin (MetHb)
`1 1.7.3
`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.] Exercise
`11.12.2 Dried blood
`11.123 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
`Function controls
`12.3
`12.4 Alarm controls
`12.5 Communicative functions
`12.6 Cables and Connectors
`12.7 Other features
`12.8 Compliance requirements
`References
`Instructional objectives
`
`i
`
`‘
`
`13
`
`APPLICATIONS OF PULSE OXIMETRY
`Joanna B Ruchala
`
`13.1 Anesthesia
`13.1.1 Problems encountered during induction to anesthesia
`13.1.2 Surgery under anesthesia
`13.2 Monitoring tissue blood supply and organ viability
`13.2.1
`Intestinal blood flow and
`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
`
`219
`
`199
`200
`201
`203
`204
`206
`209
`210
`210
`211
`212
`213
`
`214
`
`214
`215
`216
`217
`
`217
`218
`218
`
`
`
`
`
`
`14
`
`14
`
`

`

`
`
`xiv
`
`Contents
`
`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
`
`13.7
`13.8
`13.9
`13.10
`13.11
`13.12
`
`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
`
`
`
`15
`
`15
`
`

`

`l
`
`I
`
`.
`
`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.
`
`It details both the
`This book emphasizes the design of pulse oximeters.
`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 car, 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
`
`
`
`16
`
`

`

`
`
`xvi
`
`Preface
`
`algorithms to perform oxygen saturation calculations are given in chapter 9, with
`worked out examples. Synchronization with the electrocardiogram improves
`accuracy during patient movement.
`the
`Chapter
`l0 describes ways to test performance of pulse oximeters:
`technician‘s finger. electronic simulators. in win-o test systems. and optoelectronic
`simulators. In chapter ll, 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 oximctry in intensive care, operating rooms,
`emergency, patient transport, genera] wards, birth and delivery, 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 diat 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
`
`17
`
`

`

`CHAPTER 1
`—————____________
`
`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
`10 cells is an important indicator of a patient's health.
`Several mediods 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. The circulatory system also can
`modulate cardiac output to effect the oxygen delivery.
`
`l.l 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
`dwmorecepronr and mecharmreceprors.
`
`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
`
`18
`
`
`
`18
`
`

`

` 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 Respiratoryfeedback
`
`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. Mcchanoreceptors give feedback
`related to mechanical aspects of breathing. For example. stretch receptors are
`tnechanoreceptors that provide feedback on the expansion of the lung and chest
`during both inspiration and expiration. An inflation index is the levei of feedback
`provided that causes inhibition of inspiration, preventing overinfiation 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
`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 Ihe 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 die 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.
`
`
`
`19
`
`19
`
`

`

`
`
`
`
`
`1.2.1 Inspiration
`
`Normal oxygen transport
`
`3
`
`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 Mini}.
`
`(b) Expiration
`(8) inspiration
`
`Air drawn into lungs
`Air forced out or lungs
`Pectoralis minor
`__
`1 Pectoralls minor
`
`muscles contract
`-
`_ muscles relax
`
`
`
`Intercostal
`-
`_
`tare rcostal
`
`muscles
`.
`=
`muscles
`contract
`5
`.
`.
`-
`relax
`
`Diaphragm relaxes
`and moves up
`
`
`
`Diaphragm contracts
`and flattens
`
`intercostal muscles and pectoralis minor
`the diaphragm.
`in).
`Figure 1.1 During inspiration.
`muscles contract. causing the lungs to expand and air lo enter the lungs. As [he diaphragm.
`intercostal muscles and pectoraiis minor relax. the lungs contract. causing air to leave the lungs (in,
`which is referred to as expiration (l‘iom 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 phm'ynx. 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
`The trachea is composed of ribbed cartilage which extends 10 cm 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 broncht‘oies. Each bronchiole has
`five or more alveoinr (hurts at the end. which, in turn. end in circular sacs. Each
`alveolar sac contain several alt-'eoli (see figure 1.3). Alveoli are the site of gas
`exchange.
`
`20
`
`20
`
`

`

` 4
`
`Design of pulse oximcters
`
`Bronchioles
`
`
`
`
`Diaphragm
`
`Figure 1.2 Air travels through lhe nasal cav‘ily‘. into the pharynx, trachea, bronchi, and finally the
`lungs. The bronchi. hl'EInCl'llFllCS, alveolar ducts and ulveoli compose the pulmonary tree with its
`branch like system {adapted from Cure! Corporation).
`
`Alveolar sac——>
`
`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,
`decreasingthe amount of airspace. This causes air to flow out of the lungs when
`the pressure inside the lungs is greater than the pressure outside the lungs (see
`figure l.1(b]). Usually only [0% of the total lung volume is exchanged in normal
`breathing. With deeper, more rapid breathing.
`the turbulence of the air flow
`increases, causing greater resistance to airflow.
`
`l
`
`21
`
`21
`
`

`

`Normal oxygen transport
`
`5
`
`1.3 DIFFUSION TO BLOOD
`
`The process of ventilation provides a continuous supply of fresh air in the lungs.
`Alter oxygenated blood has been circulated through the body,
`it is brought back
`to the lungs through arterial capillaries to exchange gases, receiving oxygen and
`ridding itself of carbon dioxide. Blood is reoxygenated and is then recirculated
`through the body.
`Gas exchange occurs through the process of diffusion. Difiitsion is the net
`movement of particles from an area of higher partial pressure to a region of
`lower partial pressure through a process of random motion. The actual gas
`exchange to the blood takes place through the process of diffusion in the alveoli.
`
`1.3.1 The alveoli
`
`The alveoli are surrounded by large pulmonary capillary beds. Since diffusion
`can only occur over a distance of 1 mm. the gas exchange takes