Pulse Oximeter

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<p>PULSE OXIMETRY: IMPROVING OUR WORLD ONE HEART BEAT AT A TIME</p> <p>AN ABSTRACT SUBMITTED ON FIFTH DAY OF MAY 2009 TO THE DEPARTMENT OF BIOMEDICAL ENGINEERING IN PARTIAL FULLFILLMENT OF THE REQUIRMENTS OF THE SCHOOL OF SCIENCE AND ENGINEERING OF TULANE UNIERSITY FOR THE DEGREE OF BACHELOR OF SCIENCE IN BIOMEDICAL ENGIEERING BY</p> <p>_____________________ LUCAS MARSH</p> <p>APPROVED:______________ CEDRIC WALKER, PH.D.</p> <p>1</p> <p>Pulse Oximetry: Improving Our World One Heart Beat at a Time Lucas Marsh Biomedical Engineering Tulane University New Orleans, LA Advisor: Dr. Cedric Walker Engineering World Health was established back in 2001 when two professors, Dr. Malkin and Dr. Kiani, witnessed the condition of hospitals in Nicaragua. They set out to create an organization that utilizes the expertise of college engineering programs around the country to design equipment and products that would better the condition of hospitals in developing nations. Engineering World Health (EWH)expressed a need for a cheap pulse oximeter. Many third world countries need a small cheap battery powered device that provides both visual and auditory feedback of a patients heart rate. The oximeter needs to cost fewer than eight dollars to produce and it must fit into a small container that will be able to withstand a force up to 5 newtons. The oximeter will utilize an infrared Light Emitting Diode (LED) and a detector to compute the heart rate. The LED will shine light through the finger and the detector will output a voltage. This voltage will then be converted to a digital signal and the oximeter will compute the heart rate with this information. The oximeter must be able to display a heart rate varying from 30 up to 180 beats per minute and must provide a beep whenever there is a pulse. The oximeter will be built to the specifications laid out by EWH and will provide the adequate read-outs to measure the pulse. 2</p> <p>Some issues that could arise during the build are that the oximeter might be over cost and also there might be issues with the oximeter being able to survive a fall. Some sources of error could occur because of a lack of power from the batteries or failure to pick up an accurate pulse. The results will be assessed by whether or not they fulfill the requirements specified by Engineering World Health.</p> <p>3</p> <p>PULSE OXIMETRY: IMPROVING OUR WORLD ONE HEART BEAT AT A TIME</p> <p>A THESIS SUBMITTED ON THE FIFTH DAY OF MAY 2009 TO THE DEPARTMENT OF BIOMEDICAL ENGINEERING IN PARTIAL FULLFILLMENT OF THE REQUIRMENTS OF THE SCHOOL OF SCIENCE AND ENGINEERING OF TULANE UNIERSITY FOR THE DEGREE OF BACHELOR OF SCIENCE IN BIOMEDICAL ENGIEERING BY</p> <p>_____________________ LUCAS MARSH</p> <p>APPROVED:______________ CEDRIC WALKER, PH.D. 4</p> <p>ACKNOWLEDGEMENTS First off I would like to thank my parents who birthed me and raised me. They also deserve thanks for putting up with me all the years that I lived at home. And of course I would not be here at Tulane without their financial support. I love you guys. I would also like to thank all my friends here at Tulane who have encouraged me through the good times and the bad. I love all you guys and I will miss you all when I graduate. I would like to thank all the teachers and professors over the years that have taught me and nurtured me. I would not be who I am without your influence. I would specifically like to thank Dr. Cedric Walker for everything he has done for me. You are my favorite professor and I have really enjoyed all of your classes. And your humor just hits the spot every time. High Noon Club Forever. Thanks to Justin Cooper for living in 441 and always being around for me to bother. Finally I would like to thank Tulane for giving me a chance to shine. Thank you for providing me a lab and almost endless resources, without your contributions this research would not have been possible. Shout out to all the BME 09 seniors. Roll Wave Roll.</p> <p>5</p> <p>TABLE OF CONTENTS Section: Abstract Title: Abstract: Thesis Title: Acknowledgements: Table of Contents: List of Tables &amp; Figures: Introduction: Background: Blow Flow History of Pulse Oximetry Principles of Pulse Oximetry Signal Processing Microprocessors Materials and Methods Results Discussion Conclusion References Appendices Biography Page Number: 1 2 3 4 5 6 7 9 9 10 11 14 19 24 30 32 34 35 36 54</p> <p>6</p> <p>LIST OF TABLES &amp; FIGURES Figure: 1: Circulation System 2: Light Absorption from Tissues 3: Location of LED and Detector 4: Isobestic Point 5: High Pass filter 6: Low Pass filter 7: Op-amp &amp; Active Filter 8: Voltage Divider &amp; Follower 9: Progression of Processing Power 10: Basic Code Flow Diagram 11: Configuration of LED and Detector 12: Schematic of Signal Condition Section 13: Pin Diagram for 3 Digit LED Display 14: Pin Diagram for 16f88 15: Detailed Code Flow Diagram 16: Original signal from phototransistor 17: Final Signal after conditioning 18: Displayed Heart vs. Actual Page Number: 10 12 12 13 15 15 16 17 19 20 23 23 24 25 26 27 27 28</p> <p>7</p> <p>INTRODUCTION Engineering World Health expressed a need for a low cost Pulse Oximeter. Pulse oximeters are one of the most commonly requested pieces of medical equipment, nearly every patient in US hospitals uses a pulse oximeter on to monitor their pulse. This is not the case in developing countries; where equipment such as pulse oximeters are difficult to find (EWH). The purpose of this project is to develop a pulse oximeter that has a total cost of $8 or less. The primary function of the oximeter will be to display the heart rate of the patient. Other functions will include an auditory beep on each pulse and a digital display showing the patients heart rate. Calculating oxygen concentration will not be a function of the oximeter Pulse Oximetry is a non-invasive way to measure a patients heart rate. A sensor is placed on a thin part of the patients anatomy, in this case the finger. On one side of the sensor lies an infrared light emitting diode (LED), on the other side lays an infrared detector. The LED shines infrared light onto the finger. While most of this light is absorbed by tissues within the body, a small portion of the light actually passes through the finger and is picked up by the detector. The detector then converts this light into an analog voltage signal. As the heart pumps blood through the body, the amount of blood within vessels will vary. Specifically, during the heart beat, a wave of blood rushes through the vessels. This increased concentration of blood will absorb more infrared light, allowing even less light to pass through to the detector. This change in blood concentration will show up in the analog output of the infrared detector. The output from the detector is then modified to a useful signal, and used to calculate the patients heart rate. The infra detector used is a phototransistor that is calibrated to pick up light in the infrared spectrum. 5 volts are applied to the phototransistor. When the transistor is not picking up any light, all 5 volts flow through the transistor. As more light is detected, the voltage that is allowed to flow through 8</p> <p>decreases, such that in full light almost no voltage passes though. Recall that during each heart beat, more blood is in each vessel, thus allowing less light to pass through. The change in voltage during a heart beat is very minute, around 10 milli-volts, but still a change none the less. This voltage drop is resting on top of a 5 volt DC signal, and is also cluttered with interference. In order to create a usable signal from this output, it must be run through a series of filters. First the signal is directed through a high-pass filter, which eliminates the DC portion of the signal, as well as any voltage drop that occurs at a rate slower than a beating heart. Then the signal is passed through a low-pass filter, which eliminates any part of the signal that occurs more frequently than a beating heart. Not only do these filters eliminate unwanted noise in the signal, they also amplify the output. The final product of the signal processing is a square wave that jumps from 0 to 5 volts when the heart contracts, and then returns to 0 as the heart relaxes. The goal of this project will be to feed the signal into a microchip or Programmable Interface Controller (PIC)which calculates the heart rate and then displays it on a simple LED read-out. This entire ensemble will be housed in a small device that slips over the finger. The device will be constructed from cheap parts available almost anywhere in the world. This will keep the price low and provide hospitals all around the world with a valuable tool.</p> <p>9</p> <p>BACKGROUND Blood Flow The blood flow through the body is called circulation (Human Physiology). The heart connects the two major portions of the circulation's circuit, the systemic circulation and the pulmonary circulation. The blood vessels in the pulmonary circulation carry the blood through the lungs to pick up oxygen and get rid of carbon dioxide, while the blood vessels in the systemic circulation carry the blood throughout the rest of our body. As the heart pumps it generates pressure because the circulation system is closed loop, meaning there is nowhere for pressure to escape to. This blood pressure is defined to be the force exerted by the blood against the vessel wall. It is this pressure caused by the pumping of the heart that keeps your blood circulating. Every blood vessel in the circulatory system has its own blood pressure, which changes continually. The term blood pressure is most commonly used to refer to arterial pressure. Arterial blood pressure rises and falls in a pattern corresponding to the phases of the cycles of the heart, the cardiac cycle. When the ventricles contract, their walls squeeze the blood inside their chambers and force it into the pulmonary artery and aorta. As a result, the pressures in these arteries rise sharply. When the ventricles relax, they begin to fill with blood again to prepare for the next contraction and the arterial pressure drops. The surge of blood entering the arteries during a ventricular contraction causes the elastic walls of the arteries to swell, but the pressure drops almost immediately as the ventricle completes its contraction and the arterial walls recoil. This surge of blood is entitled the pulse and it occurs every time the heart beats (Human Physiology).</p> <p>10</p> <p>Figure 1: Circulation</p> <p>History of Pulse Oximetry In 1864 George Gabriel Stokes discovered that hemoglobin is the carrier of oxygen in blood (Wilson). Then in 1874 Karl von Vierordt uses light to distinguish between blood that is fully saturated in oxygen and blood that is not. The first oximetry measurements were traced back to the 1930s when German scientists used spectrophotometers to research light transmission through human skin (Wilson). In 1939, German researchers reported use of an "ear oxygen meter" that used red and infrared light to compensate for changes in tissue thickness, blood content, light intensities and other variables. However, it was not until World War II that interest in oximetry took hold as there was a need to 11</p> <p>evaluate oxygenation of high altitude pilots. Around that time a British researcher, Millikan, used two wavelengths of light to produce a practical, lightweight aviation ear oxygen meter for which he coined the word "oximeter" (History). In 1964, a San Francisco surgeon developed a self-calibrating, 8wavelength oximeter that was later marketed by Hewlett Packard in the 1970s. This system was used in clinical environments but was very large, weighing approximately 35 pounds and had a bulky clumsy earpiece; it also came with a large price tag of around ten thousand dollars. However, it did allow for continuous noninvasive monitoring of arterial oxygenation and heart beat. In the late 1970s, the Biox Corporation in Colorado made significant advances in pulse oximetry. They first introduced the use of Light Emitting Diodes for the red and infrared light sources (History). Through the 1980s advances were made in size, cost, and various probing sites. By 1987, the standard of care for the administration of a general anesthetic in the US included pulse oximetry. From the operating room, the use of pulse oximetry rapidly spread throughout the hospital, first in the recovery room, and then into the various intensive care units. Pulse oximetry was of particular value in the neonatal unit where the patients do not thrive with inadequate oxygenation, but also can be blinded with too much oxygen. Furthermore, obtaining an arterial blood gas from a neonatal patient is extremely difficult (History). Principles of Pulse Oximetry Pulse Oximetry is based upon the absorption of infrared light by oxygenated hemoglobin. Infrared light is within the spectrum of 850-1000nm, but oxygenated hemoglobin has a peak absorption wavelength of around 900nm (Principles). Pulse oximetry uses LED to shine infrared light through a reasonably translucent site with good blood flow, such as the finger or ear lobe. At the measuring site there light is constantly absorbed by skin, tissue, venous blood, and the arterial blood. This produces DC, or Direct Current, portion of the signal because it remains constant. With each heart beat the heart 12</p> <p>contracts and there is a surge of arterial blood, which momentarily increases arterial blood volume across the measuring site (Principles). This results in more light absorption during the surge. This produces the AC, or Alternate Current, portion of the signal because it alternates with the amount of light. If light signals received at the photo detector are looked at 'as a waveform', there should be peaks with each heartbeat and troughs between heartbeats. If the light absorption at the trough (which should include all the constant absorbers, DC portion) is subtracted from the light absorption at the peak then, the resultants are the absorption characteristics due to added volume of blood only; which is arterial. Since peaks occur with each heartbeat or pulse, the term "pulse oximetry" was coined (Principles).</p> <p>Figure 2: shows the various tissues that absorb light, and what causes the variable absorption levels (Principles).</p> <p>Figure 3: shows the basic layout of LEDs and Detector on a finger (Principles). 13</p> <p>Figure 4 shows the isobestic point, point of maximum absorption by HbO2 Signal Processing A phototransistor is used as the photo detector in this projects pulse oximeter. A transistor is a three terminal, solid state electronic device. One can control electric current or voltage between two of the terminals by applying an electric current or voltage to the third terminal. With a phototransistor the third controlling terminal is replaced by a light sensitive surface. When light hits this surface, the 14</p> <p>photons dislodge electrons, these electrons then form a current. This current regulates the amount of voltage allowed to pass through the transistor. The more lig...</p>