Near-infrared Reflectance Pulse Oximetry: Proof of Concept Study

BACKGROUND: Transmission mode pulse oximeters are widely used for non-in-vasive blood oxygen saturation (SaO2) measurements (Nitzan et al., 2014). They commonly exploit photoplethysmography (PPG) signals at red-NIR wavelength pairs (e.g. 660/940 nm) with different penetration depths in skin. In reflectance mode, differences in photon path lengths in skin at both wavelengths cause calibration and accuracy problems. OBJECTIVE: To investigate whether arterial oxygen saturation (SaO2) can be reliably measured using a pair of selected near-infrared (NIR) wavelengths with similar penetration depths and photon path lengths in skin. METHODS: We performed photoplethysmography (PPG) measurements at promising from this point 808 nm and 1064 nm wavelengths of lasers and LEDs. The back-scattered PPG pulses were recorded using an originally developed measurement setup, focusing on the AC signal amplitudes and the AC-DC component amplitude ratios. RESULTS: Detectability of reflectance PPG signals from fingers and other body sites by Si-photodiodes was demonstrated at both working wavelengths, so confirming their suitability for pulse oximetry. To achieve controllable reduction of SaO2, arterial cuffs and inhaling of a hypoxic gas mixture with reduced oxygen concentration were applied. As expected, the SaO2 level decreased with growing cuff pressure and with lower oxygen content in the inhaled gas. CONCLUSIONS: Our results supported potential of using the 808/1064 nm or close to them NIR wavelength pairs (e.g. emitted by double-wavelengths semiconductor mi-cro-lasers) for exploitation in future reflectance pulse oximeters. If compared with the “classical” 660nm/940nm wavelength pair, the differences of oxy-and deoxy-hemoglo-bin absorption in this NIR spectral range are several times lower, so special attention in further studies should be paid to proper NIR-PPG signal acquisition and processing.