DESIGN AND DEVELOPMENT OF PULSE OXIMETER R.C. Chpta, S.S. Ahluwalia and S.S. Randhawa Central Scientific Instruments Organisation, CHANDIGARH - 160 020, INDIA ABSTMCT Pulse Oximeter is a medical electronic Instru- ment that measures the oxygen saturation (SaOz) of arterial blood and pulse rate by non-invasive techni- ques. It enables prompt recognition of hypoxemia. In transmittance t e Pulse Oximeter that CSIO has dewloped, the %sorption of light by oxygenated and reduced hemoglobin is measured at two wavelengths - 660 nni (redj and 940 nm (Infrared). .4t each wave- lengths, the light detected by the photodiode consists of a cardiac synchronous A.C Signal arising from arterial blood volume pulsations, superimposed on a D.C. level. The D.C. level depends on LED intensity, tissue absor tion, path length, and detector sensitivity. The tissue ,a g sorption and path length may vary widely between individuals and probe site. Thr ratio of Red and Infrared Sipals after normalisation is calculated andisrelated\~tharterialoxygensaIuration.TheSaO1_ is finally calculated using the well ltnown Mendelson and Kent equation which is derived based on Beer - Lambert law, in CSIO's Pulse Chimeter, a Nellcor finger clip probe is employed. The !,ystem operates at 400 Hz which is locked with power Pine frequency. Constant current feedback circuits are employed for driving the LED's in a particular sequence. The detec- tor output is given to sample Sr hold circuits for deniul- tiplexing the three signals namely Red, ambient & Infrared. The effect of ambient light is subtracted from Red Sr Inlrared signals Sr the signals are given to microprocessor through a 12 bit ADC. The digital design is based around SOS5 microprocessor with 4 K bytes of EPROM for monitor pro mms, 2 K bytes of RAM for temporary data storage, keyboard s( display interfaces. Our oximeter has provision for high and low alarm settin s of Sa02 from 50% to 100% and Pulse rate setting, fom 30 BPM to 250 BPlLl. It also gives the alarm conditions if probe is accidentally disconnected from finger. The Sa02 is correctly measured in the range of 65 to 1GO% within 2 2% accuracy and pulse rate within L 3 bpm. Keywords: Pulse Oximetry, Hypoxemia, Car- diac Synchronous, Arterial Oxygen Saturation, Microprocessor INTR ODUCTIOIV: A Pulse Oximeter is essentially a portable, non- invasive monitor of oxygen saturation which enables prompt recognition of hypoxemia. Pulse oximetry basi- cally measures ox $en saturation (SnOz), the percent- age of hem0 lo in saturated wvilh oxygen. Pulse Oximetry has gb een recommended as a standard for care of every general anesthetic. Recently, the use of pulse oximeters has been reportcd in the clinical management of pediatric patients and critically ill new- Proceedincis RC IEEE-EMBS & 14th BMESI - 1995 born infants with cardiac and pulmonary Idisease. The device provides valuable data regarding blood oxygenation and this information is obtained easily, continuously, and noninvasively. PRINCIPLE Oxymetry works by transilluminating the tissue containing an artcrial bed and evahating absorption of red and infrared light.The Pulse Oximeter is based upon two physical principles. First, the light absor- bance of oxysenated hemo' lobiri HbOz) is different oximeter's two wavelengths is shown in Fig. (1). from that of reduced VI emogobin(Hb) at the i I I 0, L----+-t-.----- 2:~ 500 555 GCO G50 7CO 730 OOIJ 050 930 950 loco i'lJ"elLnQ' I ((17) 1 licnoglob~n cx' iC'h0). C 2NCJ Second, the absorbances at both wavelengths have pulsatile (AC) component, which is the rcsult of tluc- tuating volume of arterial blood between the source and detector[l].The output light at each wavelength consists of two components. The first component varies with pulsation of the blood. The second is large, cunbtant light output level; this is the light that passed through the tissues without being absorbed or scat- tered. These are referred to as the AC and DC com- ponents respectively are shown in Fig.(2). 1.13The amplitude of both the DC and AC levels are directly dependent on the incident light intensity. Dividing the AC level by DC level (at each wavelength) gives a corrected AC level that is no longer a function of the incident intensity. This corrected AC level is a function only of the combined extinction of the two s ecies of hemoglobin and the path length of arterial byood through which the light has passed. The pre- viously described ratio uses the corrected change in AC light level. Thus : WR = (ACUDC1)/(AC9JDC?) - (1) In this equation, 1 is the red wavelength and 2 is the infrared wavelength[2]. If the Beer-Lambert law is deemed to hold, [hen Mendelson and Kent have shown that it is possible to derive the following relationship bet\veen Sa02 and RAR ratio: EIR (Hb) (FUR) ~ ER (Ifb) Sa02 % [ 111 (Ilb) - EIR (Ilboz)] (WR) + [ER(13boz -ER (IIb where E(Hb) and E(Hbo2) are the millimolar extinction coefficients of reduced and oxyhemoglobin respectively[3]. We have used this equation for oxygen saturation calculation in our design. If we plot SaOz% versus R/IR as per e uation(2), we will get a curve of nature shown in Fig231 for some specified extinction coefficient and this curve is termcci as the calibration curve. ?J) :r i \ Now our problems reduces to finding out a means to measure the R/IR ratio, because if we have R/IR ratio, then we can readily solve the equation(2) to get the oxygen saturation level. SYSTEM DESIGN The basic block diagram layout has been shown in Fig. (4). It consists of following modules: THE PROBE: In CSIO Pulse Oximeter, Nellcor Fmger Probe DS-100 has been used as sensor. It consists of two narrow band LED's as sources of light. One is Red LED of 660 nm wavelength and other one is Infrared LED of 940 nm placed on one side and a photo detec- tor placed on opposite side of the probe. In our design of the Pulse Oximeter, light at wavelength 660 nm (Red) & 940 nm (Infrared) alternatively wth no lights in between are passed through the vascular bed. A signal detected from the detector consists of a cardiac synchronous AC signal from arterial blood superimposed on DC level. The DC level depends on LED intensity; tissue absorption, path length and detector sensitivity. This detected signal is sampled at different times to separate out two different signals, one the red