Spectroscopic Properties of Blood for Pulse Oximeter Design

Abstract

Pulse oximeter is an important medical device as it is used for emergency situations in hospitals for monitoring patient's peripheral oxygen saturation (%SpO2). For more than 30 years many research studies have focused on designing a wearable compact pulse oximeter device for accurate %SpO2 measurements. Most of these devices provide values that are either 2% above or below the desired %SpO2 values, calculated using arterial blood gas (ABG). Pulse oximeter provides %SpO2 readings of arterial blood and the pulse rate through convenient placement of the sensor on the finger. In the transmittance type pulse oximeter, the absorbance of light by oxy hemoglobin and de-oxy hemoglobin is measured at two wavelengths one each from the Red band and the Infrared (IR) band. The two band's combined wavelength ranges from 600nm to 1000nm on the electromagnetic spectrum. At each wavelength, the light is detected after placing a finger between the light source and the detector of spectrophotometer. The detected signal consists of a cardiac synchronous AC signal which is due to the changes in arterial blood volume, and the DC level which is due to bone, tissue and non-pulsatile blood. The ratio of signals corresponding to Red and IR bands is calculated and is related to arterial oxygen saturation. In this paper, we investigate the spectral properties of blood through spectrophotometer-based readings from finger in the wavelength range of 600nm to 1000nm and determine the optimum wavelength combination for designing the transmittance type pulse oximeter. For this purpose, a comparison of all combinations of Red and IR band wavelengths was carried out. The results of our study indicate that more than one combination of wavelengths can be used for designing pulse oximeters based on the absorbance values observed in the IR and Red bands