INVESTIGATION OF PHOTOPLETHYSMMOGRAPHIC SIGNALS AND ARTERIAL BLOOD OXYGEN SATURATION VALUES (SpO2) DURING BLOOD PRESSURE CUFF-INDUCED HYPOPERFUSION

Photoplethysmography is a non-invasive electro-optical technique widely used in the study and monitoring of the pulsations associated with changes in blood volume in a peripheral vascular bed. Photoplethysmography is used in the estimation of arterial oxygen saturation (SpO2) by pulse oximetry. A reflectance finger photoplethysmographic (PPG) probe and a multiplexed data acquisition system operating simultaneously at two wavelengths and incorporating an external lead II electrocardiogram (ECG) reference channel, and a commercial finger pulse oximeter has been developed. The aim of this study is to investigate the morphology and amplitude of PPG signals and its effect on pulse oximetry during blood pressure cuff-induced hypoperfusion. PPG signals and SpO2s and standard ECG traces were obtained from 14 healthy volunteers and displayed on a personal computer. Measurable PPG signals at both infrared and red wavelengths were obtained from all induced pressures prior to full brachial occlusion. There are statistically significant differences between the ac PPGs in the low pressures (0 to 80 mmHg) than those in the upper pressures (90 to 150 mmHg) at both wavelengths. Both pulse oximeters showed gradual decrease of saturations during induced hypoperfusion which demonstrate the direct relation between blood volumes (PPG amplitudes), arterial vessel stenosis and blood oxygen saturation. The custom made pulse oximeter was found to be more sensitive to SpO2 changes than the commercial pulse oximeter especially at high occluding pressures.