IMTC 2006 – Instrumentation and Measurement Technology Conference Sorrento, Italy 24-27 April 2006 Virtual Instrument for the Measurement of Haemo-dynamic Parameters Using Photoplethysmograph K. Ashoka Reddy, J. Rezuana Bai, Boby George, N. Madhu Mohan and V. Jagadeesh Kumar Department of Electrical Engineering, IIT Madras, Chennai-600 036, India Phone: +91-44-22574406, Fax: +91-44-22574402, Email: vjk@iitm.ac.in Abstract – This paper presents the design and development of a virtual instrument for the measurement of haemo-dynamic parameters namely, pulse rate and oxygen saturation in arterial blood based on the popular photoplethysmographic (PPG) principle. A clip-on sensor, housing red and infrared (IR) light emitting diodes and suitable photo detectors is developed. The sensor is interfaced to a PC utilizing the audio channel of the sound card, thus dispensing with expensive analog to digital converter hardware. Since the frequency response of the audio channel is not suitable for the PPG waveforms of red and IR, FM modulation and demodulation are employed. An empirical relationship is developed for the computation of the oxygen saturation in arterial blood using the red and IR PPG data and the well-known and well-established extinction coefficients of haemoglobin with and without oxygen. Data acquisition and processing are accomplished under LabVIEW virtual environment. Keywords – PC audio channel, Pulse oximeter, Oxygen saturation, photoplethysmograph, Noninvasive measurement, Biomedical signal processing, Virtual instrument. I. INTRODUCTION Haemo-dynamic parameters such as blood pressure, pulse rate, oxygen content in arterial blood as well as venous refilling time help a physician diagnose several ailments of the human body [1]. It has been established that using photoplethysmographic (PPG) principle, parameters such as pulse rate, oxygen saturation in arterial blood and venous refilling time can be ascertained [2-5]. Pulse oximetry, first introduced by Takuo Aoyagi et al. [5], provides continuous measurement of blood oxygen saturation and is an important medical technique for emergency, critical care and for everyday medical checkups. The values of PPG at systole and diastole are traditionally used for oxygen saturation measurements in a typical pulse oximeter [6-11]. We now present a virtual instrument that employs red (R) and infrared (IR) sources and detectors to obtain two PPGs, at these wavelengths. Negative feedback compensation is applied so that the red and IR PPGs obtained are normalized, thus enabling computation of oxygen saturation from these PPGs to be free of patient dependent parameters. II. OXYGEN SATURATION IN ARTERIAL BLOOD Oxygen required for the functioning of the body is transported through the arterial blood with the help of red blood cells (haemoglobin, Hb). The amount of oxygen in arterial blood is an important parameter for a physician to diagnose cardio-pulmonary defects. Under normal physiological conditions, about 98% of oxygen present in the blood is combined with haemoglobin. One molecule of haemoglobin can carry up to four molecules of oxygen, which is then 100% saturated with oxygen. The amount of O 2 bound to haemoglobin in a given quantity of arterial blood is usually represented by % SaO 2 . When the SaO 2 is measured with a pulse oximeter, the abbreviation is SpO 2 . Functional %SaO 2 : It is defined as the ratio of oxy- haemoglobin concentration to the total concentration of arterial haemoglobin [1],[2]. Functional % 2 2 2 [HbO ] SaO = ×100 [Hb]+[HbO ] (1) where [ ] denotes concentration and it is assumed that dysfunctional forms of haemoglobin, namely, carboxy- haemoglobin (COHb) and methaemoglobin (MetHb) are not present. Fractional SaO 2 : When quantities of all four species of haemoglobin are known, a fractional saturation can be calculated as: Fractional 2 2 2 [HbO ] %SaO = ×100 [Hb]+[HbO ]+[COHb]+[MetHb] (2) Levels of COHb and MetHb should be negligible in a normal nonsmoking patient. As such, a measure of the functional oxygen saturation is more than adequate for normal clinical applications. However, if one needs to measure the fractional oxygen saturation then only in-vitro multi-wavelength laboratory CO-oximeter can be used. III. PHOTOPLETHYSMOGRAPH If we illuminate a part of the body and obtain either the reflected or transmitted light, the resulting signal is called photoplethysmograph (PPG). To obtain a PPG, we need a light source and a photo detector. If the PPG is obtained from the reflected light, the source and the detector will be housed on the same plane as indicated in Fig. 1(a). Earlier works have concluded that the optimum distance between the source and detector in the sensor of Fig. 1(a) lies in the range of 4 to 5 mm. This type of sensor can be used over the skin on any part of the body. On the other hand, if one has to obtain a PPG using the transmitted light, the source and sensor need to be arranged on two different but parallel planes, as indicated in Fig. 1(b). It is obvious that sensors of this type can be 1167 0-7803-9360-0/06/$20.00 ©2006 IEEE