REFLECTANCE-BASED PULSE OXIMETER
`FOR THE CHEST AND WRIST
`
`A Major Qualifying Project Report:
`
`Submitted to the Faculty
`
`Of the
`
`WORCESTER POLYTECHNIC INSTITUTE
`
`In partial fulfillment of the requirements for the
`
`Degree of Bachelor of Science
`
`By
`
`_________________________________________
`Alexandra Fontaine
`
`_________________________________________
`Arben Koshi
`
`_________________________________________
`Danielle Morabito
`
`_________________________________________
`Nicolas Rodriguez
`
`Submitted on:
`
`________________________________________________________________
`Professor Yitzhak Mendelson, Advisor, Dept. of Biomedical Engineering
`
`Approved by:
`
`- 1 -
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`Masimo Ex. 2015
`Apple v. Masimo
`IPR2020-01714
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`

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`Authorship
`
`Section
`
`Introduction
`
`Literature Review
`
`Design Approach
`
`Author
`
`Nicolas Rodriguez, Danielle Morabito, Alex
`Fontaine, Arben Koshi
`Nicolas Rodriguez, Danielle Morabito, Alex
`Fontaine, Arben Koshi
`Alex Fontaine, Danielle Morabito
`
`Device Development
`
`Nicolas Rodriguez, Danielle Morabito
`
`Methods
`
`Final Design
`
`Results
`
`Discussion
`
`Summary
`
`Conclusion
`
`Nicolas Rodriguez, Danielle Morabito
`
`Arben Koshi, Nicolas Rodriguez, Danielle
`Morabito
`Alex Fontaine, Arben Koshi
`
`Arben Koshi, Alex Fontaine
`
`Arben Koshi
`
`Arben Koshi
`
`Future Improvements
`
`Arben Koshi, Nicolas Rodriguez
`
`Appendix A
`
`Appendix B
`
`Appendix C
`
`Appendix D
`
`Danielle Morabito
`
`Arben Koshi
`
`Alex Fontaine
`
`Arben Koshi
`
`i
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`- 2 -
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`Abstract
`
`Reflectance-based pulse oximetry is a technique used for noninvasively monitoring the oxygen saturation
`
`(SpO2) and pulse rate (PR). However, there is little supporting evidence that it can accurately collect
`
`measurements from the chest and wrist. In this project, a reflectance-based pulse oximeter was built and
`
`used to collect measurements while sitting, standing, during self-induced hypoxia, and during self-
`
`induced hyperventilation then compared to the measurements taken by a HOMEDIC Deluxe Pulse
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`Oximeter. The prototype was able to accurately measure within an error of + 1% and ±3% for SpO2 and
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`PR respectively from the wrist while an error of ±1% and +4% for SpO2 and PR respectively from the
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`chest.
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`- 3 -
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`Acknowledgements
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`We would like to thank Professor Mendelson for advising this project. We would also like to
`acknowledge Dr. Adriana Hera and Lisa Wall for their support.
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`iii
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`Executive Summary
`Oxygen saturation (SpO2) is the measurement of oxyhemoglobin (HbO2) in arterial blood. SpO2 is an
`
`important vital measurement because it shows the levels of blood oxygenation. Traditionally, SpO2 is
`
`measured by invasively drawing blood samples. This method, however, is not ideal and it is unable to
`
`provide clinicians with real-time measurements. With the need for a noninvasive way to measure SpO2,
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`pulse oximetry was developed. The use of this technology allows clinicians to determine SpO2 in patients
`
`that are sedated, anesthetized, unconscious, or unable to regulate their own oxygen supply.
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`
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`Reflectance-based pulse oximetry allows measurements to be taken from areas of the body in
`
`which transmittance based pulse oximetry cannot be applied. In reflectance-based pulse oximetry, the
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`incident light is passed through the skin and is reflected off the subcutaneous tissue and bone. To this day,
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`being able to measure signals from the chest and wrist with one single device has not been successfully
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`achieved. Such a device would allow patients to measure SpO2 and pulse rate (PR) without hindering
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`their normal day-to-day activities.
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`The prototype pulse oximeter constructed during this project consists of two hardware
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`components and a programmed LabVIEW Virtual Instrument (VI). The hardware components consist of
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`the sensor and a circuit which produces, collects, and processes photoplethysmographic (PPG) signals.
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`The VI collects the PPG signals produced by the hardware and process them in order to produce
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`numerical results for PR and SpO2 . The optical sensor is made up of two Light Emitting Diodes (LEDs),
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`a red LED, with a peak emission wavelength of 660 nm, and an infrared emitter with a peak emission
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`wavelength of 940 nm. These LEDs are positioned next to each other in the center of a circular Printed
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`Circuit Board (PCB) and surrounded by 8 photodiodes (PD). The circuitry for the sensor consists of an
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`Arduino Duo Microprocessor which is programmed to light up the red and infrared LEDs intermittently at
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`a frequency of 100Hz. The PDs are connected in photovoltaic mode in order to produce a voltage output.
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`Operational amplifiers are utilized to amplify the photodiode output. Once amplified, the red and infrared
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`PPG signals obtained from the photodetectors are sent through two Sample-and-Hold circuits to separate
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`the signals into their respective alternating current (AC) and direct current (DC) components for further
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`filtering and amplification.
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`The four input signals sent to the LabVIEW software : AC red, AC infrared, DC red and DC
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`infrared access the VI via a National Instruments (NI) Data Acquisition (DAQ) system. The AC
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`components of the red and infrared PPGs are measured using a peak-to-peak detection algorithm, while
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`the DC components are measured by finding their respective averages. Once the signals are processed,
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`SpO2 is calculated by obtaining the ratio of the AC and DC components of the red PPG and dividing that
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`by the ratio of the AC and DC components of the infrared PPG. To calculate PR, the frequency of the
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`infrared AC signal is measured using frequency measurement parameters in LabVIEW and then
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`iv
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`- 5 -
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`multiplied by 60 to display PR in beats per minute (bpm). To compare the measurants taken from the
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`pulse oximeter prototype, a transmission-type finger HOMEDICS Deluxe Pulse Oximeter was utilized as
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`reference. The reliability of the Deluxe Pulse Oximeter was tested against a Biopac ECG model 100C
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`module and was concluded that the HOMEDICS Deluxe Pulse Oximeter was provided accurate enough
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`measurements for pulse rate.
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`For testing, the sensor was strapped to the wrist and chest of each subject using a Velcro strap
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`while the HOMEDICS Deluxe Pulse Oximeter was placed on the subject’s index finger. The VI was set
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`up to collect, average and display SpO2 and PR data every 10 seconds throughout a 6 minute timespan
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`accounting for 36 measurements. At this point, a second individual that was monitoring the reference
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`device recorded the corresponding SpO2 and PR values displayed by the HOMEDICS pulse oximeter.
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`Subjects were tested on the chest and wrist while sitting, standing, during self-induced hyperventilation,
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`and during self-induced hypoxia.
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`While collecting data, it was noticed that the PR measurements collected from the chest had
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`significantly larger margins of error compared to those from the wrist. One possible explanation for this
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`discrepancy deals with the LabVIEW algorithm for PR calculation. Instead of doing peak-peak analysis,
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`we opted to use a search tool which graphs a power spectrum of the data and searches for the highest
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`amplitude frequency between 0.75Hz and 2.25Hz. This method is very effective at PR values ranging
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`between 45 and 135 bpm, but loses its accuracy at higher PR values. Measurements above 135 bpm were
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`detected by the reference, but not accurately detected by the prototype.
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`Margins of error obtained from the standing and sitting measurement tests on the wrist included
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`0.6%, and 0.2% for SpO2 and 0.2%, 1.1% for PR respectively. Measurements from the chest displayed
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`errors of 0.4% and 0.3% for SpO2 and 0.1%, and 0.7% for PR while standing and sitting respectively.
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`Based on this data, our prototype for a reflectance-based pulse oximeter for the chest and wrist was
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`successful in measuring PR and SpO2.
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`v
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`Table of Contents
`Authorship.................................................................................................................................................................. i
`
`Abstract ...................................................................................................................................................................... ii
`
`Acknowledgements ............................................................................................................................................... iii
`
`Executive Summary .............................................................................................................................................. iv
`
`Table of Figures .................................................................................................................................................. viii
`
`Table of Tables ........................................................................................................................................................ x
`
`Abbreviations .......................................................................................................................................................... xi
`
`1
`
`2
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`Introduction .................................................................................................................................................. 12
`
`Literature Review ....................................................................................................................................... 13
`
`2.1
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` Oxygen Saturation .............................................................................................................................. 13
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`2.2
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`
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`Pulse Oximetry .................................................................................................................................... 13
`
`2.2.1
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`Principle of a Pulse Oximeter ................................................................................................ 15
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`2.2.2 Methods of Light Detection .................................................................................................... 16
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`2.2.3
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`Photoplethysmogram ................................................................................................................ 17
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`2.2.4 Wavelength Optimization ....................................................................................................... 17
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`2.2.5
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`Limitations and Applications of Pulse Oximetry ............................................................. 17
`
`2.3
`
` Reflectance vs. Transmittance Pulse Oximetry ......................................................................... 18
`
` New Studies for Pulse Oximeters .................................................................................................. 19
`2.4
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`3 Design Approach ........................................................................................................................................ 21
`
`3.1
`
`
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`Initial Client Statement ..................................................................................................................... 21
`
`3.2
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` Clinical Need ....................................................................................................................................... 21
`
`3.3
`
` Design Parameters .............................................................................................................................. 21
`
`3.3.1 Objectives .................................................................................................................................... 21
`
`3.3.2 Constraints ................................................................................................................................... 22
`
`3.3.3
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`Functions ...................................................................................................................................... 22
`
`3.3.4 Design Specifications ............................................................................................................... 23
`
` Revised Client Statement ................................................................................................................. 25
`3.4
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`4 Device Development ................................................................................................................................. 27
`
`4.1
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` Device Alternatives............................................................................................................................ 27
`
`4.1.1
`
`Sensor Design 1.......................................................................................................................... 27
`
`4.1.2
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`Sensor Design 2.......................................................................................................................... 28
`
`
`4.2
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`Software Design .................................................................................................................................. 29
`
`5 Methods ......................................................................................................................................................... 30
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`5.1
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`
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`5.2
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`
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`5.3
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`
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`5.4
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`
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`Photodetection Unit ........................................................................................................................... 30
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`PPG ......................................................................................................................................................... 30
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`Filter Design ........................................................................................................................................ 30
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`Software ................................................................................................................................................ 31
`
`5.4.1
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`Incoming Signals ....................................................................................................................... 32
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`5.4.2
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`5.4.3
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`Filtering ........................................................................................................................................ 34
`
`Frequency of Pulse Rate .......................................................................................................... 35
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`5.4.4
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`Pulse Rate Calculation ............................................................................................................. 36
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`5.4.5
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`Spectral Measurements ............................................................................................................ 37
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`5.4.6
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`SpO2 Calculation ........................................................................................................................ 39
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`5.4.7
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`Preliminary VI Test ................................................................................................................... 44
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`5.5
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` Experimentation/Testing .................................................................................................................. 48
`
`6
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`Final Design ................................................................................................................................................. 51
`
`6.1
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` Device Hardware ................................................................................................................................ 51
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`6.1.1
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`Sensor ............................................................................................................................................ 51
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`6.1.2 Circuitry ........................................................................................................................................ 52
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`6.2
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`
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`Software ................................................................................................................................................ 56
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`7 Results ........................................................................................................................................................... 59
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` Comparison Graphs ........................................................................................................................... 61
`7.1
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`7.2
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` Dynamic Response Plots .................................................................................................................. 73
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`7.3
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` Residual Plots ...................................................................................................................................... 91
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`8 Discussion..................................................................................................................................................... 98
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`10 Summary ..................................................................................................................................................... 101
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`11 Conclusion .................................................................................................................................................. 103
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`12 Future Improvements .............................................................................................................................. 104
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`References ............................................................................................................................................................ 106
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`Appendix A: Description of LabVIEW ....................................................................................................... 107
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`Appendix B: Bill of Materials ........................................................................................................................ 117
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`Appendix C: Data Sets ..................................................................................................................................... 118
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`Appendix D: Filter Bode Plots ....................................................................................................................... 128
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`Table of Figures
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`Figure 2.1: Arterial blood changing over time, PPG [11] .................................................................................................................. 14
`Figure 2.2: Optical absorption spectra of Hb, HbO2, MetHb, and BbCO [11] ........................................................................... 15
`Figure 2.3: Transmittance pulse oximetry ............................................................................................................................................. 18
`Figure 2.4: Reflectance pulse oximetry ................................................................................................................................................... 18
`Figure 2.5: Forehead pulse oximeter ....................................................................................................................................................... 19
`Figure 3.1: Objective tree ............................................................................................................................................................................ 22
`Figure 5.1: Unfiltered output of a PPG signal with an AC component riding on top of a DC component ..................... 30
`Figure 5.2: Configuration of functions to separate incoming signals .......................................................................................... 33
`Figure 5.4: Filter function and graph ...................................................................................................................................................... 35
`Figure 5.5: Tone measurements output frequency .............................................................................................................................. 35
`Figure 5.6: Configure tone measurements parameters ..................................................................................................................... 36
`Figure 5.7: Functions used to calculate pulse rate ............................................................................................................................. 37
`Figure 5.8: Amplitude and level measurements functions with numeric indicators ................................................................ 38
`Figure 5.9: Configure amplitude and level measurements set to mean (DC) ............................................................................ 39
`Figure 5.10: SpO2 calculation .................................................................................................................................................................... 40
`Figure 5.11: The configuration of function that limit the displayed SpO2 .................................................................................. 41
`Figure 5.12: VI Block Diagram ................................................................................................................................................................. 42
`Figure 5.13: VI Front Panel........................................................................................................................................................................ 43
`Figure 5.14: Front panel for the simulated hypoxia test with low amplitude ............................................................................ 45
`Figure 5.15: Front panel for the simulated hypoxia test with high amplitude .......................................................................... 46
`Figure 5.16: Block diagram for the simulated hypoxia test ............................................................................................................. 47
`Figure 5.17: Application of the sensor to the wrist (left) and the chest (right) ......................................................................... 49
`Figure 6.1: Top View of sensor module with photodiodes and LEDs. ......................................................................................... 51
`Figure 6.2: Platform view of sensor module ......................................................................................................................................... 52
`Figure 6.3: Block diagram of the prototype pulse oximeter ............................................................................................................ 53
`Figure 6.4:LED driver timing diagram with 2.5ms time increments ............................................................................................ 54
`Figure 6.5: Circuit design for pulse oximeter device ......................................................................................................................... 55
`Figure 6.6: Block Diagram of Final VI ................................................................................................................................................... 57
`Figure 6.7: Front Panel of Final VI ......................................................................................................................................................... 58
`Figure 7.1: Example of corrupted PPG .................................................................................................................................................. 60
`Figure 7.2: Comparison graph for the chest sitting tests. The solid line represents the regression line and the dashed
`line represents the identity line. ................................................................................................................................................................. 62
`Figure 7.3: Comparison graph for the chest standing tests. The solid line represents the regression line and the
`dashed line represents the identity line. .................................................................................................................................................. 64
`Figure 7.4: Comparison graph for the wrist sitting tests. The solid line represents the regression line and the dashed
`line represents the identity line. ................................................................................................................................................................. 66
`Figure 7.5: Comparison graph for the wrist standing tests. The solid line represents the regression line and the
`dashed line represents the identity line. .................................................................................................................................................. 68
`Figure 7.6: Comparison graph for hyperventilation tests ................................................................................................................ 70
`Figure 7.7: Comparison graph for hypoxia tests. The solid line represents the regression line and the dashed line
`represents the identity line. .......................................................................................................................................................................... 72
`Figure 7.8: Oxygen saturation measurement validation plot for the chest standing test ...................................................... 74
`Figure 7.9: Pulse rate measurement validation plot for the chest standing test ....................................................................... 75
`Figure 7.10: Oxygen saturation measurement validation plot for the chest sitting test ......................................................... 77
`Figure 7.11: Pulse rate measurement validation plot for the chest sitting test ......................................................................... 78
`Figure 7.12: Pulse rate measurement validation plot 1 for hyperventilation test .................................................................... 80
`Figure 7.13: Pulse rate measurement validation plot 2 for hyperventilation test .................................................................... 81
`Figure 7.14: Pulse rate measurement validation plot for the wrist sitting test ......................................................................... 83
`Figure 7.15: Oxygen saturation measurement validation plot for the wrist sitting test ......................................................... 84
`Figure 7.16: Oxygen saturation measurement validation plot for the wrist standing test .................................................... 86
`Figure 7.17: Pulse rate measurement validation plot for the wrist standing test ..................................................................... 87
`Figure 7.18: Oxygen saturation measurement validation plot 1 for hypoxia test .................................................................... 89
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`Figure 7.19: Oxygen saturation measurement validation plot 2 for hypoxia test .................................................................... 90
`Figure 7.20: Residual plot for hyperventilation testing for data set 1 (dotted lines: + 2SD, solid line: average of
`differences) ........................................................................................................................................................................................................ 92
`Figure 7.21: Residual plot for hyperventilation testing for data set 2(dotted lines: + 2SD, solid line: average of
`differences) ........................................................................................................................................................................................................ 93
`Figure 7.22: Residual plot for hyperventilation testing for data set 3 (dotted lines: + 2SD, solid line: average of
`differences) ........................................................................................................................................................................................................ 94
`Figure 7.23: Residual plot for hypoxia testing for data set 1 (dotted lines: + 2SD, solid line: average of differences)
` ............................................................................................................................................................................................................................... 96
`Figure 7.24: Residual plot for hypoxia testing for data set 2 (dotted lines: + 2SD, solid line: average of differences)
` ............................................................................................................................................................................................................................... 97
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`- 10 -
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`Table of Tables
`Table 3.1: Respironic's WristOx Ambulatory Finger Pulse Oximeter Specifications [21] ...................................................... 23
`Table 3.2: Santa Medical's Finger Pulse Oximeter Specifications[7]............................................................................................ 24
`Table 3.3: Crucial Medical Systems Finger Pulse Oximeter Specifications [22]........................................................................ 24
`Table 3.4: Project Specifications ............................................................................................................................................................... 25
`Table 7.1: Calculated accuracy for the average SpO2 and PR for each data set ..................................................................... 59
`Table 7.2: Prototype accuracy based on all data collected ............................................................................................................. 60
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`Abbreviations
`O2: Oxygen
`Hb: Hemoglobin
`HbO2: Oxyhemoglobin
`SpO2: Oxygen saturation
`SaO2: Oxygen saturation measured invasively
`PR: Pulse rate
`COHb: Carboxyhemoglobin
`MetHb: Methyl hemoglobin
`PPG: Photoplethysmogram
`VI: Virtual Instrument
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`1 Introduction
`
`One of the most important elements needed to sustain life is oxygen (O2) because it is used by cells to
`
`turn sugars into useable energy. Oxyhemoglobin (HbO2) is the protein hemoglobin, found in red blood
`
`cells, bounded to O2 that delivers 98% of oxygen to cells. The measurement and calculation of the
`percentage of HbO2 in arterial blood is known as oxygen saturation (SpO2). [1]
`
`Originally, SpO2 was measured by taking samples of blood and measuring O2 levels directly. This
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`method was invasive and was unable to provide real-time measurements. This measuring technique made
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`it impossible for SpO2 to be recognized as an important measure of wellness until a non-invasive method
`of measuring SpO2 in real-time was established. [2]
`
`The need for a non-invasive method of measuring SpO2 in real-time led to the development of
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`pulse oximetry. Pulse oximetry derives SpO2 and pulse rate (PR) from a photoplethysmogram (PPG). The
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`PPG is obtained by measuring changes in light absorbed by the blood. Red and infrared wavelengths are
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`used to obtain the PPG because these wavelengths are easily transmitted through tissues, allowing SpO2
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`to be calculated from the ratio of the absorption of the red and infrared light.
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`The first device used to continuously measure blood oxygen saturation of human blood in vivo
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`(SaO2) was built by Karl Matthes in 1935. [2] However, it was not until 1983 that William New and Mark
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`Yelderman, after recognizing the need of an accurate oximeter in the operating room evaluated and
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`produced the pulse oximeter with aims to make it an intraoperative monitoring device. [2] Pulse oximetry
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`allows for an accurate determination of O2 levels in patients that are sedated, anesthetized, unconscious,
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`and unable to regulate their own oxygen supply as well as provides information needed to avoid
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`irreversible tissue damage. [2]
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`Since the invention of pulse oximetry, the measurement of SpO2 has become an important part of the
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`medical world. Nevertheless, improvements such as the application of the reflectance-based technique to
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`measure SpO2 from multiple locations on the body are still to be developed. This project demonstrates the
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`use of reflectance-based pulse oximetry to obtain measurements for PR and SpO2 from the chest and
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`wrist. This development in pulse oximetry technology will pave the way for the development of new and
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`novel pulse oximeters that can be worn as accessories, are easily concealed under clothing, and more
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`acclimated to use outside of hospital settings.
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`2 Literature Review
`
`This is a thorough literature review covering the necessary background needed to fully understand this
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`project.
`
` Oxygen Saturation 2.1
`
`SpO2 is the amount of O2 that is carried in the blood. In the human body, SpO2 is defined as the ratio of
`HbO2 to the total concentration of Hb (reduced Hb + HbO2) present in the blood [1].
`
` ( )
`
`