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  • “Investigation of Photodetector Optimization in Reducing Power Consumption by a Noninvasive Pulse

    Oximeter Sensor”

    A Thesis

    Submitted to the Faculty

    of the

    WORCESTER POLYTECHNIC INSTITUTE

    in partial fulfillment of the requirements for the

    Degree of Master of Science

    by

    ___________________________

    Chirag J. Pujary Date: Jan 16, 2004

    Approved: Professor Yitzhak Mendelson, Ph.D., Major Advisor Associate Professor, Biomedical Engineering Professor Ross D. Shonat, Ph.D., Committee Member Assistant Professor, Biomedical Engineering Professor Peder C. Pedersen, Ph.D., Committee Member Professor, Electrical and Computer Engineering

  • ii

    ABSTRACT

    Noninvasive pulse oximetry represents an area of potential interest to the army,

    because it could provide cost-effective, safe, fast and real-time physiological assessment

    in a combat injured soldier. Consequently, there is a need to develop a reliable, battery-

    powered, wearable pulse oximeter to acquire and process photoplethysmographic (PPG)

    signals using an optimized sensor configuration. A key requirement in the optimal design

    of a wearable wireless pulse oximeter is low power management without compromising

    signal quality. This research investigated the advantage gained by increasing the area of

    the photodetector and decreasing the light emitting diode (LED) driving currents to

    reduce the overall power requirement of a reflectance mode pulse oximeter sensor. In

    vitro and preliminary in vivo experiments were conducted to evaluate a multiple

    photodetector reflectance sensor setup to simulate a varying detection area. It was

    concluded that a reflection pulse oximeter sensor employing a large area photodetector is

    preferred over a similar transmission type sensor for extending the battery life of a

    wireless pulse oximeter intended for future telemedicine applications.

  • iii

    ACKNOWLEGEMENTS

    I wish to extend my deepest appreciation to the following:

    Prof. Yitzhak Mendelson for his assistance, guidance, and patience throughout the course of this work as well as his confidence in my abilities. Prof. Ross Shonat and Prof. Peder Pedersen for their valuable comments and, for taking time to review this work. Mark Savage, for his valuable contributions throughout this project especially, during initial design and brainstorming sessions. My parents, Jayakar and Chitra Pujary, for their support, care, love and strong upbringing, which has greatly helped me to follow my dream and achieve goals in life. My sister, Apeksha, for keeping me in touch with my parents and, for her words of encouragement and love. Samta and her family, for all the love, and support. Last, but never the least, my friends and room-mates, for their friendship and support. This work is supported by the U.S. Army Medical Research and Materiel Command under Contract No. DAMD17-03-2-0006. The views, opinions and/or findings contained in this report are those of the author and should not be construed as an official Department of the Army position, policy or decision unless so designated by other documentation.

  • iv

    TABLE OF CONTENT

    ABSTRACT....................................................................................................................... ii

    ACKNOWLEGEMENTS ............................................................................................... iii

    TABLE OF CONTENT................................................................................................... iv

    LIST OF FIGURES ........................................................................................................ vii

    LIST OF TABLES ......................................................................................................... xiii

    1. INTRODUCTION......................................................................................................... 1

    1.1 Significance of Oxygen Saturation ..........................................................................1

    1.2 Pulse Oximetry.........................................................................................................2

    1.3 Principle of Pulse Oximetry.....................................................................................3

    1.4 Pulse Oximeter Sensors ...........................................................................................7 1.4.1 Transmission Mode:........................................................................................7 1.4.2 Reflection Mode:.............................................................................................8

    1.5 Advantages and Disadvantages of Reflectance and Transmittance Pulse Oximetry ..................................................................................................................9

    1.6 Types of Pulse Oximeter Probes............................................................................10 1.6.1 Reusable Probes: ..........................................................................................10 1.6.2 Disposable Probes:.......................................................................................10

    1.7 Applications of Pulse Oximetry.............................................................................11 1.7.1 Surgery and Post Anesthetic Care Units: .....................................................12 1.7.2 Fetal and Neonate Care: ..............................................................................12 1.7.3 Future Applications in Combat Casualty Care: ...........................................13

    2. SENSOR COMPONENTS......................................................................................... 14

    2.1 Light Emitting Diodes (LED) ................................................................................14 2.1.1 Principle of Operation:.................................................................................15 2.1.2 LED Characteristics: ....................................................................................16 2.1.3 Light Emitting Diodes vs Solid-State Lasers: ...............................................17 2.1.4 Depth of Light Penetration: ..........................................................................19 2.1.5 LED Power Consumption Concerns:............................................................20

  • v

    2.2 Photodiode .............................................................................................................21 2.2.1 Photocurrent and photovoltaic operation modes: ........................................21 2.2.2 Characteristics of Silicon-Photodiodes (Si-PD):..........................................24 2.2.3 Noise Current: ..............................................................................................25

    2.3 Optimum LED/PD Separation Distance ................................................................29

    3. RESEARCH OBJECTIVES...................................................................................... 31

    4. DESIGN DECISION MATRIX................................................................................. 34

    4.1 Objective Tree........................................................................................................34

    4.2 Pairwise Comparison Technique ...........................................................................38

    4.3 Initial Decision Matrix...........................................................................................40

    4.4 Final Decision Matrix ............................................................................................46

    5. MATERIALS .............................................................................................................. 50

    5.1 Experimental Sensor Setups ..................................................................................50 5.1.1 Prototype Ring Sensor: .................................................................................50 5.1.2 Prototype 6-PD Disc Sensor:........................................................................51 5.1.3 Prototype 12-PD Reflectance Disc Sensor:..................................................52 5.1.4 Central Hub Connector (Multiple DB-9 Female Interfaces): ......................52

    5.2 Pulse Oximetry Instrumentation ............................................................................54 5.2.1 Photoplethysmogram Signal Processing Unit (PSPU): ...............................55 5.2.2 Reference Pulse Oximeter (RPO): ................................................................56 5.2.3 Experimental Pulse Oximeter (EPO): ..........................................................56

    6. METHODOLOGY ..................................................................................................... 58

    6.1 Sensor Mode Studies..............................................................................................58

    6.2 Estimation of Power Consumption in Transmission and Reflection Modes .........60

    6.3 Testing Multiple PD Performance Using a Prototype 6-PD Reflectance Sensor.....................................................................................................................61 6.3.1 Dark Tests:.................................................................