Development of a Ring-Shaped Pulse Oximeter to Analyse Angle Dependency in SpO2 Estimation

Photoplethysmography (PPG) is a non-invasive, optoelectric method widely used to estimate vital parameters such as heart rate and blood oxygen saturation (SpO2).Typically, the determination of SpO 2 relies on a devicespecific calibration curve, which correlates the ratio-ofmodulation (R) with SpO2 values and is commonly established using linear regression analysis.This work examines the dependency of SpO2 estimation on the angle between light-emitting diodes (LED) and photodiodes (PD) using a ring-shaped PPG sensor.The developed sensor, including an opto-electric system, collected PPG signals from PDs at four angles (45, 90, 135, 290).A hypoxia study involving five healthy subjects was performed, collecting PPG data from the developed sensor and a reference pulse oximeter.Subsequently, the collected data was analysed to estimate the respective angles' R-value and to compare them to the reference values.The comparison showed high inter-individual signal quality variability.However, results from goodquality signals showed a clear angle dependency.