Pulse oximetry based on photoplethysmography imaging with red and green light : Calibratability and challenges

Abstract

Remotely measuring the arterial blood oxygen saturation (SpO₂) in visible light (Vis) involves different probing depths, which may compromise calibratibility. This paper assesses the feasibility of calibrating camera-based SpO₂ (SpO_2,cam) using red and green light. Camera-based photoplethysmographic (PPG) signals were measured at 46 healthy adults at center wavelengths of 580 nm (green), 675 nm (red), and 840 nm (near-infrared; NIR). Subjects had their faces recorded during normoxia and hypoxia and under gradual cooling. SpO_2,cam estimates in Vis were based on the normalized ratio of camera-based PPG amplitudes in red over green light (RoG). SpO_2,cam in Vis was validated against contact SpO₂ (reference) and compared with SpO_2,cam estimated using red-NIR wavelengths. An RoG-based calibration curve for SpO₂ was determined based on data with a SpO₂ range of 85-100%. We found an [Formula: see text] error of 2.9% (higher than the [Formula: see text] for SpO_2,cam in red-NIR). Additional measurements on normoxic subjects under temperature cooling (from [Formula: see text] to [Formula: see text]) evidenced a significant bias of - 1.7, CI [- 2.7, - 0.7]%. It was also noted that SpO[Formula: see text] estimated at the cheeks was significantly biased (- 3.6, CI [- 5.7, - 1.5]%) with respect to forehead estimations. Under controlled conditions, SpO[Formula: see text] can be calibrated with red and green light but the accuracy is less than that of SpO[Formula: see text] estimated in the usual red-NIR window.

Keywords: Calibration; PPG imaging; Pulse oximetry; Video-health monitoring.