Design of a SpO 2 Pulse Oximeter Prototype

Transcription

1 VIETNAM NATIONAL UNIVERSITY - HCMC International University Biomedical Engineering Department Design of a SpO Pulse Oximeter Prototype by Le Nguyen Trong Nghia A thesis submitted to the Biomedical Engineering Department in partial fulfillment of the requirements for the Degree of Engineer Ho Chi Minh city, Vietnam 0/01

2 DESIGN OF A SpO PULSE OXIMETER PROTOTYPE APPROVED BY: Prof. Vo Van Toi, Ph.D., Chair PhD. Truong Quang Dang Khoa PhD. Nguyen Thanh Hai PhD. Vo Thi Kieu Loan PhD. Nguyen Duc Thang THESIS COMMITTEE

3 ACKNOWLEGMENTS Firstly, I would like to send my deep gratitude to Prof. Vo Van Toi who has given me his constant encouragement and support to help me to finish my project successfully. Secondly, I also want to give my gratefulness to PhD. Nguyen Thanh Hai, Msc. Nguyen Thanh Tam and Msc. Ngo Thanh Hoan for their precious advice, technical help, and support from the beginning of my study. They have made these months a great learning experience. And finally, I am really thankful to my friends because of their enthusiasm and helpful support during the completion of this thesis. 3

4 TABLE OF CONTENTS LIST OF FIGURES... 6 ABSTRACT... 7 KEY WORDS... 8 CHAPTER INTRODUCTION Background Introduction Motivation...11 LITERATURE REVIEW A brief history Principle of pulse oximetry Significance Limitation and Advancement...1 METHODOLOGY Fundamental principle...3. Hardware description Measurement strategy...37 RESULTS Achievement Implementation

5 3. Discussion...49 CONCLUSION...51 REFERENCES...5 APENDICES

6 LIST OF FIGURES Figure Page 1. Red (R) and infrared (IR) scaled alternating current (AC) signals at arterial oxygen saturation (SaO ) of 0%, 85% and 100%. The numeric value of the redto-infrared (R/IR) ratio can be easily converted to SaO The absorption spectra of both HbO and Hb. Hb has a higher absorption than HbO at 660nm (RED). In the infrared region, HbO has a higher absorption Basic Oximeter Sensor Components Blue Trace IR Light / Red Trace RED Light AC and DC component of a SpO signal Calibration curve for pulse oximeter Above shows the results of the differentiator (black) and the peak follower envelope (blue) on the IR waveform (Red) The finished prototype The prompt at the beginning of the test The result of my SpO test

7 ABSTRACT Pulse oximetry is a non-invasive method which allows health care providers to monitor the oxygenation of patient s blood. Oxygen saturation is the percentage of haemoglobin molecules bound with oxygen molecules. The basic principle of a pulse oxymeter is based on the measurement of the red and infrared light absorption. Infrared is absorbed more by oxygenated hemoglobin and red transmitted, by opposition to the deoxygenated cells. With the low cost and availability of Microchips, there is a question why these instruments are so expensive and why they are not available at our local drugstores like glucose meters. Based upon the above principle, a simple prototype of pulse oximeter can be made using a Microchip dspic30f digital signal processor and a small available LCD. Therefore, we can think about a simple, reliable and affordable product that can go into mass-production. In conclusion, with cheap, simple and available components, we have the possibility to successfully build a prototype of pulse oximeter that can be utilized in medical centers, clinics, and nursing homes and do not have to buy the available commercialized products. 7

8 KEY WORDS Pulse oximetry, pulse oximeter, oxygen saturation, hemoglobin (or hemoglobin), absorption, transmit, LTF (Light to Frequency) 8

9 CHAPTER I INTRODUCTION 1. Background The Pulse Oximeter is an instrument that is widely used in today s medical fields. There are many different models available by different companies. Some units such as those utilized in hospitals are integrated into larger systems used to monitor other vital signs. Other smaller more portable units can be found in doctor s offices, clinics, and nursing homes and are also used by home health-care professionals. In the Oct/Nov 000 issue of RT The Journal for Respiratory Care Practitioners, an article titled Recent Developments in Pulse Oximetry is quoted as Pulse Oximetry is often considered the fifth vital sign, after heart rate, blood pressure, temperature and respiratory rate. Pulse Oximetry has served as an important tool for the clinician by providing continuous monitoring of the critically ill patient s arterial oxygen saturation (SaO), by calculating an estimate of the SaO (known as the SpO) via an algorithm and displaying a readout of this estimation. 9

10 . Introduction A pulse oximeter is a medical device that indirectly monitors the oxygen saturation of a patient's blood and changes in blood volume in the skin, producing a photoplethysmograph. It is often attached to a medical monitor so staff can see a patient's oxygenation at all times. Most monitors also display the heart rate. Portable, battery-operated pulse oximeters are also available for home blood-oxygen monitoring. The original oximeter was made by Millikan in the 1940s. The precursor to today's modern pulse oximeter was developed in 197, by Aoyagi at Nihon Kohden using the ratio of red to infrared light absorption of pulsating components at the measuring site. It was commercialized by Biox in The device did not see wide adoption in the United States until the late 1980s. Basically, the principle of the pulse oximetry is based upon the measurement of the red and infrared light absorption. A sensor is placed on a thin part of the patient's body, usually a fingertip or earlobe, or in the case of an infant, across a foot. Light at red (660nm) and infrared (910nm) wavelengths is passed sequentially through the patient to a photodetector. The changing absorbance at each of the two wavelengths is measured, allowing determination of the absorbance due to the pulsing arterial blood alone, excluding venous blood, skin, bone, 10

11 muscle, fat, and fingernail polish. Based upon the ratio of changing absorbance of the red and infrared light caused by the difference in color between oxygen-bound (bright red) and oxygen-unbound (dark red or blue, in severe cases) blood hemoglobin, a measure of oxygenation (the percentage of hemoglobin molecules bound with oxygen molecules) can be made. Using the above methodology, we would be able to build a prototype of pulse oximeter with only some low-cost and available components in the market. 3. Motivation Over the past few years, I have had experience with the nursing home setting since I had to be there for 3 months. It is very common for bed-ridden patients to contract pneumonia during the flu season. A pulse oximeter is normally used to determine if the patient is having difficult time breathing. A particular nursing home I was at had one such instrument. During the winter, I had contracted pneumonia. I asked the nurse if I could be examined. They had to wait for the oximeter because it was being used in another part of the building. Because of the instruments high cost, the nursing home couldn t afford a second or perhaps a third. My question was - does anybody make one? During my 11

12 search, I have read an interesting article of an individual who was on the same question. With the low cost and availability of microchips, I can t understand why these instruments are so expensive and why they are not available at our local drugstores like glucose meters. 1

13 CHAPTER II LITERATURE REVIEW 1. A brief history In 1935, Matthews developed the first -wavelength ear oxygen saturation meter with red and green filters, later switched to red and infrared filters. This was the first device to measure oxyhemoglobin saturation. In 1949 Wood added a pressure capsule to squeeze blood out of an ear to obtain zero setting in an effort to obtain absolute oxygen saturation value when blood was readmitted. The concept is similar to today's conventional pulse oximetry but was hard to implement because of unstable photocells and light sources. This method is not used clinically. In 1964 Shaw assembled the first absolute reading ear oximeter by using eight wavelengths of light. Commercialized by Hewlett Packard, its use was limited to pulmonary functions and sleep laboratories due to cost and size. 13

14 Pulse oximetry was developed in 1974, by Takuo Aoyagi and Michio Kishi, bioengineers, at Nihon Kohden using the ratio of red to infrared light absorption of pulsating components at the measuring site. Susumu Nakajima, a surgeon, and his associates first tested the device in patients, reporting it in It was commercialized by Biox in 1981 and Nellcor in Biox was founded in 1979, and introduced the first pulse oximeter to commercial distribution in Biox initially focused on respiratory care, but when the company discovered that their pulse oximeters were being used in operating rooms to monitor oxygen levels, Biox expanded its marketing resources to focus on operating rooms in late 198. A competitor, Nellcor (now part of Covidien, Ltd.), began to compete with Biox for the US operating room market in Prior to its introduction, a patient's oxygenation could only be determined by arterial blood gas, a single-point measurement that takes a few minutes of processing by a laboratory. In the United State alone, approximately billion dollars was spent annually on this measurement. With the introduction of pulse oximetry, a non-invasive, continuous measure of patient's oxygenation was possible, revolutionizing the practice of anesthesia and greatly improving patient safety. Prior to its introduction, studies in anesthesia journals estimated US patient mortality as a 14

15 consequence of undetected hypoxemia at,000 to 10,000 deaths per year, with no known estimate of patient morbidity. 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 to 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. In 1995, Masimo introduced Signal Extraction Technology (SET) that could measure accurately during patient motion and low perfusion. Some have termed newer generation pulse oximetry technologies as High Resolution Pulse Oximetry (HRPO). One area of particular interest is the use of pulse oximetry in conducting portable and in-home sleep apnea screening and testing. In 009, the world's first Bluetooth-enabled fingertip pulse oximeter was introduced by Nonin Medical, enabling clinicians to remotely monitor patients pulses and oxygen saturation levels. It also allows 15

16 patients to monitor their own health through online patient health records and home telemedicine systems.. Principle of pulse oximetry The principle of pulse oximetry is based on the red and infrared light absorption characteristics of oxygenated and deoxygenated hemoglobin. Oxygenated hemoglobin absorbs more infrared light and allows more red light to pass through. Deoxygenated (or reduced) hemoglobin absorbs more red light and allows more infrared light to pass through. Red light is in the nm wavelength light band. Infrared light is in the nm wavelength light band. Pulse oximetry uses a light emitter with red and infrared LEDs that shines through a reasonably translucent site with good blood flow. Typical adult/pediatric sites are the finger, toe, pinna (top) or lobe of the ear. Infant sites are the foot or palm of the hand and the big toe or thumb. Opposite the emitter is a photodetector that receives the light that passes through the measuring site. There are two methods of sending light through the measuring site: transmission and reflectance. In the transmission method, as shown in the figure on the previous page, the emitter and photodetector are 16

17 opposite of each other with the measuring site in-between. The light can then pass through the site. In the reflectance method, the emitter and photodetector are next to each other on top the measuring site. The light bounces from the emitter to the detector across the site. The transmission method is the most common type used and for this discussion the transmission method will be implied. After the transmitted red (R) and infrared (IR) signals pass through the measuring site and are received at the photo-detector, the R/IR ratio is calculated. The R/IR is compared to a "look-up" table (made up of empirical formula) that convert the ratio to a SpO value. Most manufacturers have their own look-up tables based on calibration curves derived from healthy subjects at various SpO levels. Typically an R/IR ratio of 0.5 equates to approximately 100% SpO, a ratio of 1.0 to approximately 8% SpO, while a ratio of.0 equates to 0% SpO. Fig. 1: Red (R) and infrared (IR) scaled alternating current (AC) signals at arterial oxygen saturation (SaO ) of 0%, 85% and 100%. The numeric value of the red-to-infrared (R/IR) ratio can be easily converted to SaO. 17

18 The major change that occurred from the 8-wavelength Hewlett Packard oximeters of the '70s to the oximeters of today was the inclusion of arterial pulsation to differentiate the light absorption in the measuring site due to skin, tissue and venous blood from that of arterial blood. At the measuring site there are constant light absorbers that are always present. They are skin, tissue, venous blood, and the arterial blood. However, with each heart beat the heart contracts and there is a surge of arterial blood, which momentarily increases arterial blood volume across the measuring site. This results in more light absorption during the surge. If light signals received at the photodetector 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) is subtracted from the light absorption at the peak then, in theory, 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. This solved many problems inherent to oximetry measurements in the past and is the method used today in conventional pulse oximetry. 18

19 Still, conventional pulse oximetry accuracy suffered greatly during motion and low perfusion and made it difficult to depend on when making medical decisions. Arterial blood gas tests have been and continue to be commonly used to supplement or validate pulse oximeter readings. The advent of "Next Generation" pulse oximetry technology has demonstrated significant improvement in the ability to read through motion and low perfusion; thus making pulse oximetry more dependable to base medical decisions on. 3. Significance A pulse oximeter is useful in any setting where a patient's oxygenation is unstable, including intensive care, operating, recovery, emergency and hospital ward settings, pilots in unpressurized aircraft, for assessment of any patient's oxygenation, and determining the effectiveness of or need for supplemental oxygen. Assessing a patient's need for oxygen is the most essential element to life; no human life thrives in the absence of oxygen (cellular or gross). Although a pulse oximeter is used to monitor oxygenation, it cannot determine the metabolism of oxygen, or the amount of oxygen being used by a patient. For this purpose, it is necessary to also measure carbon dioxide levels. It 19

20 is possible that it can also be used to detect abnormalities in ventilation. However, the use of a pulse oximeter to detect hypoventilation is impaired with the use of supplemental oxygen, as it is only when patients breathe room air that abnormalities in respiratory function can be detected reliably with its use. Therefore, the routine administration of supplemental oxygen may be unwarranted if the patient is able to maintain adequate oxygenation in room air, since it can result in hypoventilation going undetected. Because of their simplicity and speed, pulse oximeters are of critical importance in emergency medicine and are also very useful for patients with respiratory or cardiac problems, especially COPD, or for diagnosis of some sleep disorders such as apnea and hypopnea. Portable battery-operated pulse oximeters are useful for pilots operating in a nonpressurized aircraft above 10,000 feet (1,500 feet in the US) where supplemental oxygen is required. Prior to the oximeter's invention, many complicated blood tests needed to be performed. Portable pulse oximeters are also useful for mountain climbers and athletes whose oxygen levels may decrease at high altitudes or with exercise. Some portable pulse oximeters employ software that charts a patient's blood oxygen and pulse, serving as a reminder to check blood oxygen levels. 0

21 4. Limitation and Advancement Pulse oximetry measures solely of oxygenation, not ventilation, and it is not a substitute for blood gases checked in a laboratory because it gives no indication of base deficit, carbon dioxide levels, blood ph, or bicarbonate HCO3 concentration. The metabolism of oxygen can be readily measured by monitoring expired CO. Saturation figures also give no information about blood oxygen content. Most of the oxygen in the blood is carried by hemoglobin. In severe anemia, the blood will carry less total oxygen, despite the hemoglobin being 100% saturated. Falsely low readings may be caused by hypo-perfusion of the extremity being used for monitoring (often due to the part being cold or from vasoconstriction secondary to the use of vasopressive agents); incorrect sensor application; highly calloused skin; and movement (such as shivering), especially during hypo-perfusion. To ensure accuracy, the sensor should return a steady pulse and/or pulse waveform. Falsely high or falsely low readings will occur when hemoglobin is bound to something other than oxygen. In cases of carbon monoxide poisoning, the falsely high reading may delay the recognition of hypoxemia (low blood oxygen level). Methemoglobinemia characteristically causes pulse oximetry readings in the mid of 1980s. Cyanide poisoning can also give a 1

22 high reading because it reduces oxygen extraction from arterial blood (the reading is not false, as arterial blood oxygen is indeed high in early cyanide poisoning). Pulse oximetry only reads the percentage of bound hemoglobin. Hemoglobin can be bound to other gases such as carbon monoxide and still read high even though the patient is hypoxemic. The only noninvasive method that allows the continuous measurement of the dyshemoglobins is a pulse CO-oximeter. Pulse CO-oximetry was invented in 005 by Masimo and currently allows clinicians to measure total hemoglobin levels in addition to carboxyhemoglobin, methemoglobin and PVI, which initial clinical studies have shown may provide clinicians with a new method for noninvasive and automatic assessment of patient fluid volume status. Appropriate fluid levels are vital to reducing postoperative risks and improving patient outcomes as fluid volumes that are too low (under hydration) or too high (over hydration) have been shown to decrease wound healing, increase risk of infection and cardiac complications.

23 CHAPTER III METHODOLOGY 1. Fundamental principle The fundamental basis for Oximetry is that blood has different optical properties at different levels of oxygen saturation. This means that if multiple sources of light at different wavelengths are used to examine the blood, they can be compared algorithmically by a computer to determine the level of oxygen saturation in the blood. The beauty of the concept is that the examination can be made invasively by passing light through an extremity such as a finger, ear lobe, etc. The light is collected by a light sensitive solid-state device such as a photo-detector, phototransistor or more recently a LTF (Light to Frequency) converter. Oximeter probes are designed to have multiple light sources (LEDs) that are switched on/off during the measurement phase of the instrument. The oxygen carrying component of blood is called hemoglobin. It is also the colored substance in your blood. The amount of visible light absorbed changes with each level of oxygenation. On a molecular scale, there are two forms of the hemoglobin molecule. One is called oxidized 3

24 hemoglobin (HbO) and the other is called reduced hemoglobin (Hb). Both of these molecular forms have different optical characteristics. The absorption of both forms of hemoglobin can be seen below. Fig. : The graph shows the absorption spectra of both HbO and Hb. Hb has a higher absorption than HbO at 660nm (RED). In the infrared region, HbO has a higher absorption. The point where the absorption of the HbO and the Hb are equal is called the isobestic point. 4

25 R IR ` Fig. 3: Basic Oximeter Sensor Components LTF The above figure illustrates two LED sources: RED and INFRARED that transmit light through a finger. The light is received by the LTF (Light to Frequency sensor). The output of this sensor has a frequency that is proportional to the intensity of the light for each source. The sources are time multiplexed. Each source is switched on for a brief instant and the frequency is measured. Light is absorbed from each source by the tissue as well as the blood. The absorption is different in the tissue for each source. As blood flows through the finger a pulsatile component is present. The pulsatile component represents the arterial flow of the blood which contains 97% of the oxygen in the body. The constant or DC component that is picked up by the LTF represents the tissue, venous and capillary absorption. The below figure shows the pulsatile and DC components for each signal. The RED trace represents the RED source and the BLUE trace represents the IR trace. The Black line represents the DC value for each of the sources. 5

26 Fig. 4: Blue Trace IR Light / Red Trace RED Light Fig. 5: AC and DC component of a SpO signal 6

27 The system measures both the AC and DC levels obtained from both sources and computes the SpO based on the following equation: R R log R IR log IR rms DC rms DC SpO R R R Fig. 6: Calibration curve for pulse oximeter 7

28 The Heart Rate is determined by measuring the elapsed time between peaks of the IR signal. The heart rate is then calculated using the equation: BPM 60 PeriodinSeconds. Hardware description The heart of the pulse oximeter is a Microchip dspic30f3013 Digital Signal Controller. DIGITAL SIGNAL CONTROLLER (dspic30f3013) The Microchip dspic30f3013 digital signal controller was a nice choice for this project because of its excellent computational abilities, available hardware, its low cost and its ease of field programmability. SYSTEM CLOCK The 16-bit 8-pin development board comes with an internal 8MHz crystal. This worked out perfect for the application. The on-chip PLL was used to boost the operating frequency 16x. With the instruction clock set 8

29 at ¼ input clock frequency, the operating frequency of the clock for the system is: Fcy = 3MHz. TIMER T1 T1 is a 16-bit timer that acts as a time-base generator with interrupt to implement a simple state machine used to control the sequencing of events. Timer T1 also sets the sample rate for all signal processing. T1 is a type-a timer which uses the MHz internal clock as its input. A prescaler is used to divide the frequency by 56 to drop the TMR1 module input to Hz. The Period Register PR1 is set to 576 so the timer provides an interrupt period of 5ms. TIMER T T is also a 16-bit timer that is setup to be free-running with a 9.419MHz input to the TMR module. Timer T is a type-b timer but does not utilize an interrupt in its operation. It is used to measure the period of the output from the LTF device. The LTF output period is inversely proportional to the intensity of the light striking its surface. 9

30 INPUT CAPTURE 1 It is connected to the output of the LTF sensor. It is setup to trigger on each rising edge of its input. When the IC1 event occurs, an interrupt is produce and the value of timer T is copied. An interrupt handler saves this value. On the second input capture event, the period of the LTF can be determined by taking the difference between this value and the saved value. The period is used to compute a frequency which is proportional to the intensity of the light measured. SPI MODULE The SPI module is enabled for use in communicating with the Microchip MCP48 serial digital to Analog converter IC. The SPI is setup in master mode to have a 16 bit serial output at a clock rate of 8MHz. Since the SPIDAC device has 1 bit resolution, the other 4 bits are used for control. The SPI module sends LED intensity commands from the digital controller to the DAC IC. UART MODULE A serial output from the system sends data at a rate of 115,00 bits per second to the development computer with an 8 bit data size, 1 stop bit, 30

31 no-parity and no flow control. Since the data comes out of the system in serial form, a special scope program was written and used to analyze system operation during development. Using the SW1 input on the development board, one of three diagnostic screens can be selected. DIGITAL I/O Ten pins were used as digital output and a single pin for a digital input. They are identified below: DIGITAL OUTPUTS RB0 VIRon Signal to the H-Bridge to control the IR light source. A high state turns IR LED on, a low state turns IR LED off. RB1 VRon Signal to the H-Bridge to control the RED light source. A high state turns the RED LED on while a low state turns the RED LED off. RB3 R/*W Controls Read/Write input on the LCD display. A high state sets display into READ mode while a low state sets display to WRITE. RB4 D4 LCD Data input line Bit 4 RB5 D5 LCD Data input line Bit 5 31

32 RB6 D6 LCD Data input line Bit 6 RB7 D7 LCD Data input line Bit 7 RB9 PULSE RF4 E RF5 RS Diagnostic Bit used during development Enable Line input for the LCD display. Register Select line for the LCD display. High state selects the internal data register while a low state selects the command register. DIGITAL INPUT RD8 SW1 Input connected to a momentary switch with pull-up resistor on the development board. This switch is used to toggle one of three data outputs from the serial port. This was used during development and may be used for diagnostics. 3

33 LED DRIVER CIRCUIT H-BRIDGE In reference to the schematic located in Appendix 6, the LED module is driven by an H-Bridge. This circuit uses a 9V decoupled supply to deliver pulsed power to the LED's at variable drive currents. BUFFER AMPLIFIERS A Microchip MCP600 dual op-amp package is used to drive the bottom half of the H-Bridge circuit and isolate it from the SPIDAC. These amplifiers along with the current feedback resistors R1 and R are used to set the current in each branch of the H-Bridge circuit thus setting the LED intensities. Element U4B controls the IR LED and U4A controls the RED element. SPI-DAC A Microchip MCP48, Digital to Analog Converter with SPI interface and internal reference connects the dspic30f controller to the buffer amplifiers. This device takes a 1 bit command over the SPI link. The SPI is setup for a 16 bit word transfer using an 8MHz clock. The bottom 1 bits 33

34 of the word are data and the upper nibble are control bits. One of these control bits is used to select one of the two DAC circuits in the device. DAC output A (pin 8) controls the intensity of the IR Led while DAC output B controls (pin 6), the RED emitter. DIGITAL CONTROL BITS Two bits from the dspic30f device are used to switch the LED's on and off. The upper half of the H-Bridge is controlled by output pins RBO and RB1. These pins control transistor drivers Q and Q1 which drive the PNP transistors in U. Pin RB0 will turn on the IR Led while pin RB1 will turn on the RED Led. The software will switch only one light source on at a time. Below are the code segments for SPI initialization and DAC Output. LIQUID CRYSTAL DISPLAY The system uses a line by 16 character display to provide the SPO data as well as the heartbeat information to the user. A HD44780U Liquid Crystal Display was chosen to do the job because it is inexpensive, robust and easy to work with. To reduce the number of I/O pins required by the microcontroller, a 4 bit data bus was chosen. There 34

35 are three additional control lines that the micro has to supply to communicate with the display. The 8 bit data is multiplexed into the display from the micro using the upper nibble of the lower byte of PORT B. One nibble is loaded at a time on the upper four pins of the display bus (the display bus is an 8 bit bus but has both modes of operation). See the attached code segment on the next page for details on how the display is initialized and how data is written to the display. R/*W INPUT This is an input to the display that allows the software to select either a read or a write operation. This application uses only writes to the display and does not necessitate the use of the read function. RS INPUT This input to the display allows writing to either the DATA register or the CONTROL register. When the input is low, Instructions can be written to the control register. When the input is high, ASCII data can be written to the display. 35

36 ENABLE By applying a pulse on this input of about 50us, the data and control bytes can be written to either the data or control registers respectively. COMMUNICATION PORT The communications port uses only a single direction of the UART MODULE. It was used to collect data during the development of the software and can also be used to monitor the heartbeat waveform from both the IR and RED sensors if one wishes. Input RD9 which is connected to SW1 on the demo board is used to toggle between three different data output modes. More will be explained on this later. The UART pin of the controller is connected to a MAX3 (EIA- 3 Driver/Receiver) manufactured by Texas Instrument. The IC converts the output voltage level of the controller UART to those necessary to connect to an RS-3 device. The output goes to the 9-pin DB9 connector which may be wired to the RS-3 port of a computer. The software sets up the UART MODULE to have the following parameters: Baud Rate: Data Size: 8 bits 36

37 Parity: Stop Bits: Flow Control: None One None 3. Measurement strategy As discussed in the HARDWARE section of this paper, Timer T1 sets up a 5ms interrupt. The interrupt handler sets up a state machine with two states and an initialization state that takes place in the main program. The state machine has the following flow diagram: STATE = -1 Initialization state Executed in main program Turn OFF both LEDs Set DAC values Initialize Timer T1 STATE = 0 The system waits for the next T1 interrupt 5ms later. During this time, the IC1 interrupt gets center stage. MEASURE is the state counter for the IC1 interrupt. Interrupt T1 occurs Set PULSE = 0 (Diagnostic) Set MEASURE = 0 Initialize Timer T Initialized IC1 STATE = 1 STATE = 1 After 5ms, the second T1 event Occurs. The measurements have Been taken and it is time to process The data. This occurs during this STATE Interrupt T1 occurs Set PULSE = 0 (Diagnostic) Perform LP filtering Perform HP filtering Perform DC tracking Set PULSE = 1 (Diagnostic) Calculate AC signal Values STATE = 0 37

38 In the main program, at a presetable number of samples, the program runs the auto_adj() procedure which tunes the IR LED to match the intensity of the RED LED. After this routine is executed, the STATE counter is set to 1, and the beats goes on. During STATE = 0, the T1 interrupt handler sets up the entry point for the IC1 interrupt. The state counter MEASURE = 0. The RED DAC is loaded and the RED LED is turned on. The IC1 interrupt will occur on the rising edge of each and every LTF output pulse until the IC1 interrupt is disabled. The interrupt occurs twice for each emitter measuring the PERIOD from the TLF device and calculating the subsequent INTENSITY. IC1 Interrupt Handler MEASURE=0 Read IC1 buffer and save T0 = IC1BUF MEASURE = 1 IC1 Interrupt Handler MEASURE=1 Read IC1 buffer Calculate RED period (the difference) Compute Intensity for RED Turn OFF RED Led Turn ON IR Led MEASAURE = 38

39 IC1 Interrupt Handler MEASURE= Read IC1 buffer and save T0 = IC1BUF MEASURE = 3 MEASURE=3 Read IC1 buffer Calculate IC1 IR Interrupt period ((the Handler difference) Compute Intensity for IR Turn OFF IR Led MEASURE = 0 Disable IC1 Disable T The STATE=1 of the T1 interrupt handler process the Low Pass Filter, High Pass Filter and DC Tracking filter algorithms for both the RED and IR sources[1]. LOW PASS FILTER The 4 th order LP algorithm in Z-domain is: Fc = Low Pass Cutoff Frequency Ts = Sample Rate Wc Fc C 1 WcTs tan M N D D D D D C 4 4C 6C 4C C 4 MC 4 4 MC NC 4 MC 3 MC 3 NC 3 MC NC MC 1 MC 4 MC 1 39

40 N N N N N ) ( ) ( ) ( z X z Y Z D D Z Z D D Z D Z N Z N Z N Z N N z H L The coefficients of the above function are determined in the main program while the algorithm is executed after each set of Red and IR samples are acquired in the interrupt handler for the T1 timer. This occurs every 10ms [1]. DIFFERENTIATOR (HIGH PASS FILTER) The differentiator is used by the peak follower algorithm to detect an edge for timing of the heart wave [1]. Fh = High Pass Cutoff Frequency Ts = Sample Rate C K C C K 1 1 tan WcTs C Wc Fh

41 Y ( z) K 1 1 H H ( z) 1 X ( z) 1 K Z 1 Z DC TRACKING FILTER Averages the Low Pass output and removes the AC signal. The output of this algorithm is the DC level for each color LED [1]. K 1 56 H T Y ( z) K ( z) 1 X ( z) 1 Z (1 K ) PEAK FOLLOWER The peak follower algorithm uses the differentiated Low-pass filter output sequence from the IR sensor as its input. The idea is to allow the Peak variable to follow these differentiated peaks and decay at a very slow rate. It works much like a filter capacitor in a power supply. It attempts to follow the peaks and decays very slowly producing an envelope that can be used to time the heart-wave that is produced by the IR signal. This IR signal was chosen because its amplitude is greater 41

42 than that of the RED source and it doesn t vary as much as the RED signal does [1]. HEARTBEAT/SpO CALCULATION The peak follower is used to time the period of the heart wave and also set up a period for the RMS calculation of the AC portion of the signal. In the T1 handler, the AC signal is calculated by taking the difference between the Filtered Sensor output and the DC level (from the DC tracking algorithm)[1]. For example, the AC signal from the RED sensor is: Rdiff = Rfilt R_DC; The RMS value is calculated for each sample of the Rdiff and accumulated using the statement: Rsum = RDiff*RDiff + Rsum; After the period is detected, the RMS value is calculated and Rsum set back to 0: Rms = sqrt(rsum/period); Rsum = 0; Once the RMS value is calculated, the SPO is calculated by finding the ratio R; R = Rrms/RDC / Irrms/IRDC Then, SpO s 10.00R R 6 / 817 R

43 SIMPLE MOVING AVERAGE FILTER A simple moving average filter is used to smooth the heart-rate and the SpO calculations over 10 samples. It follows the general form: Y k Y K 1 Where, X K X N K 1 N = 10 Fig. 7: Above shows the results of the differentiator (black) and the peak follower envelope (blue) on the IR waveform (Red) 43

44 CHAPTER IV RESULTS 1. Achievement Base on the above methodology, a prototype of pulse oximeter was successfully built. The prototype was designed with fully functional characteristics of a pulse oximeter and worked well. The hardware is contained of a printed circuit board (PCB), a 16x LCD, a simple amplifier circuit and a Nellcor ds-100a SpO sensor. In order to implement this circuit, an algorithm for the dspic30f3013 microcontroller was also designed. Schematic Diagram 44

45 4 8 Fig. 8: The schematic diagram VCC R1 10K P1 U1A P TL R3 100K C1 100nF TO BOARD-M R 100K TO SENSOR-FM Fig. 9: The schematic and layout of amplifier circuit 45

46 PCB Layout Fig. 10: The top layer of PCB Fig. 11: The bottom layer of PCB 46

47 Fig. 1: The finished prototype. Implementation When the system is powered up, there will be a brief prompt on the display as that shown below in figure 10. Fig.13: The prompt at the beginning of the test 47

48 After the unit has gone through its initial power up, it will display the prompt "WAITING...". Place one of our index fingers into the sensor. Figure 11 shows the sensor that has already been fitted to my friend s finger. Once the controller detects the placement of the finger, it will begin taking measurements. After about 10 seconds, we will have a stable reading of both our heart-rate (BPM) and our blood oxygen saturation (SPO). Below is a photograph of the system measuring my heart-rate at 80 beats-per-minute and my SpO at 99%. The heart-rate was up slightly because of the success of the project. Fig. 14: The result of my SpO test 48

49 3. Discussion After several months of working with my teacher and my friends, I have consequently finished my project to complete my graduate thesis. A prototype of pulse oximeter was successfully built with its merits and I am really happy to see it function well. However, nothing is perfect and my product is the same, and there are still shortcomings in my design. The displayed values of heart-rate and SpO are not as exact as the commercialized products. There are some gradients in the result but the differences are still small and it will not strongly effect the diagnosis of the doctor. My prototype would be better if I made the sensor myself. At the beginning of the project, I have intended to build the sensor with a LED module and a LTF photo-detector. However, I have failed to finish my intended plan because of limited time for the thesis and my difficulties in ordering the needed components from the USA. I have really been disappointed about this incompletion. In the future, there are several things needed to finish my project. The most important thing is that we need to design a hand-made sensor that can be compatible with the PCB I have designed so that we do not have to buy the Nellcor sensor and reduce the cost for the product. 49

50 Furthermore, if we can build an interface between the oximeter and the computer, it is easier for the doctor to manage the status of patients. Bluetooth and wireless communication are the most popular solutions that we can consider. 50

51 CHAPTER VI CONCLUSION In conclusion, the pulse oximeter is a medical device that is used to measure oxygen saturation (Sp0) and heart-rate non-invasively. The basic principle of a pulse oxymeter is based on the measurement of red and infrared light absorption. Infrared is absorbed more by oxygenated hemoglobin and red transmitted, by opposition to the deoxygenated cells. With the availability of Microchip dspic30f microcontroller and supporting components, we would be able to successfully build a prototype of pulse oximeter that functions well. Therefore, we can think about a simple, reliable and affordable product that can go into massproduction and is going to be utilized in medical centers, clinics and nursing homes while we do not have to buy the available commercialized products. 51

52 REFERENCES 1. Jubran A: Pulse oximetry. In: Tobin MJ (ed). Principles and Practice of Intensive Care Monitoring. New York: McGraw Hill, Inc.; pp Hill Aileen, RRT : Recent Developments in Pulse Oximetry, RT The Journal for Respiratory Care Practitioners, Oct/Nov 000, p1 3. Tremper KK, Barker SJ: Pulse oximetry. Anesthesiology 1989, 70:98 108, 5: Wukitisch MW, Peterson MT, Tobler DR, Pologe JA: Pulse oximetry: analysis of theory, technology, and practice. J Clin Monit 1988, 4: Townsend, Neil: Medical Electronics, Michaelmas Term, Emergency Care Research Institute: Pulse oximeters. Health Devices 1989, 18: Alexander CM, Teller LE, Gross JB: Principles of pulse oximetry: theoretical and practical considerations. Anesth Analg 1989, 68: Jeff B, Light-to-Frequency Conversion. Circuit Cellar 004, pp Kamat Vijaylakshmi : Pulse Oximetry, Indian Journal of Anesthesia, Aug 00, p Oximeter.org, Principles of Pulse Oximetry Technology, 00, TAOS, Inc., Pulse Oximetry, 1. Microchip.com, PO Box - Utilizing a Microchip dspic30f01 digital signal controller, 007, on%0notes&showfield=no 5

53 APENDICES APPENDIX 1 TRANSMITTANCE Materials can absorb electromagnetic radiation such as light. The definition of transmittance through a material with concentration C is: Io Concentration I T I Io where: I = Output Intensity Io = Input Intensity ABSORBANCE The absorbance by the material is defined as the log of the inverse of transmittance: A log 1 T log( 1) log( T) log(t) APPENDIX I A log Io 53

54 BEER-LAMBERT LAW A Io IiIiz Iz Iz I Iz-dI I I Z dz, di B The Beer-Lambert model can be derived for a material of absorbing species. First consider the absorption di, of an infinitesimal cross-sectional slab of material of thickness dz. The total length of the path of the material is B with a total cross-section of A. The intensity is Io at Z=0 and I at the full length of the material. The intensity at Z is Iz. The intensity after the absorption di is (Iz di). Consider a molecular cross section of (cm ). If there is a concentration of N (mol/cm ) absorbers in this molecular cross section. Then the total absorbed light on this infinite cross-sectional slab would be: * N * A * dz (cm ) * (# mol/cm 3 ) * (cm ) * (cm) = #mol*cm The fraction of photons absorbed across this surface area A is: 54

55 * N * A * dz / A = * N * dz (#mol) The fraction of the photons absorbed across dz becomes, - * N * dz = di/iz Integrating both sides, I 0 1 di I N dz ln( I) ln( Io) NB I ln NB Io B 0 Remember that absorbance is equal to the left side of the equation, and then the equation becomes: A = NB Since N (molecules/cm 3) *(1 mol/6.03x10 3 molecules) * 1000 cm 3 /liter = c (mol/liter) and.303 * log(x) = ln(x) then, log I *(6.03x10 Io 0 /.303)* c* b or, A bc 55

56 where, = * (6.03 x 10 0 /.303) = * (.61 * 10 0 ) So, A log I Io bc Beer-Lambert Law 56

57 APPENDIX 3 OXYGEN SATURATION Hemoglobin is the primary component that carries oxygen from the lungs to the rest of the body via passages called arteries, veins and capillaries. It is a protein that is bound to the red blood cells. Oxygen is chemically combined with the hemoglobin inside of the red blood cells and makes up nearly all the oxygen present in the blood. The absorption of visible light at different frequencies by hemoglobin varies with oxygenation as can be seen in Figure 1. Two forms of the hemoglobin molecule are oxidized (HbO) and reduced hemoglobin (Hb). The oxygen saturation is defined, as SaO (SpO ) and s a function of the concentration of the two forms of hemoglobin in the blood: Ko SaO Ko Kr Where, Ko = Concentration of HbO Kr = Concentration of Hb 57

58 Arterial SpO is a parameter measured with Oximetry and is normally expressed as a percentage. Under normal conditions, arterial blood is 97% saturated while venous blood is 75% saturated. The wavelength range 600nm 1000nm is the range for which there occurs the least amount of attenuation of light by body tissue because tissue and pigmentation absorb blue, green and yellow light and water absorbs the longer infrared wavelength. The Oxygen Saturation (SPO ) of the arterial blood may be determined by measuring the transmitted light through the fingertip or earlobe at two different wavelengths and then compared ratiometrically. The two wavelengths that best suit this type of measurement are RED (660nm) and INFRARED (940nm). According to the Beer-Lambert law there is a linear relationship between the absorbance and concentration of each component of the blood at that particular frequency of light. Also, the intensity of the light will decrease logarithmically with the path length. 58

59 Suppose we had a source of length Z with incident light intensity Io and transmitted intensity of I. If there were two species of absorbers called HbO (RED) and Hb (Blue) with the concentrations Ko and Kr respectively. These are analogous to oxygenated hemoglobin and reduced hemoglobin. Now supposed we examine the specimen using two light sources 1 and. Using Beer-Lambert law, at each wavelength, we get: At ( 1 ) At ( ) I I 1 ( K O1 Where: K 0 = concentration of hemoglobin K 1 = concentration of reduced hemoglobin o1 = absorption coefficient of HbO at 1 r1 = absorption coefficient of Hb at 1 o = absorption coefficient of HbO at r = absorption coefficient of Hb at Writing both equations in log form: I I r 1 We can express both functions as the ratio (R). O ( K O O K r I 1 log ( O 1K O r1k r ) Z I 01 I log ( O K O r K r ) Z I 0 r K ) Z r ) Z 59

60 60 Then solving for K r, Substituting this into the equation for SaO yields: where: r r O O r r O O K K K K I I I I R log log r r O O r r O O K K K K R r r O O O r R R K K ) ( ) ( O r O r r r R R SaO log log I I I I R

61 Below is a graph of the above equation showing the theoretical values. The second trace shows an actual calibration using arterial blood samples. This is the curve for the HP M1190A probe. The difference has been found to be caused mainly by scattering effects and non-ideal light sources. Using Matlab, analyzing the graphical data above, data from the real calibration curve shown above was extracted into vector form. The data was input into Microsoft excel and fit to a 3 rd order equation. It is this equation which expresses SpO = f (R) that was used in this project. The graph of this data and the fit equation are shown below. 61

62 6

63 APPENDIX 4 BLOCK DIAGRAM 63