Multiwavelength photoplethysmography signal analysis as a function of varied wrist contact pressure

Photoplethysmography (PPG) sensors, commonly used today in wearable biomedical devices, are an easily deployable low-cost technology to estimate heart rate, heart rate variability, and, when coupled with multiple wavelengths, blood oxygenation. PPG sensors can measure these parameters by measuring volumetric changes in the blood due to light absorption. While the technology is established, its utility is hindered due to several factors, including skin contact pressure. Thus, to advance fitness trackers toward regulated medical devices, confounding factors related to contact pressure and its effect on the PPG wave morphology, intensity, and signal-to-noise ratio (SNR) need to be better understood. Toward that end, this study evaluates the effects of contact pressure between the PPG sensor and the wrist, particularly on the palmar side of the wrist along the radial artery. The amplitude of the AC and DC components of the PPG signal, the perfusion index, and the signal morphology were determined for varying contact pressures utilizing three wavelengths of light, green, red, and infrared (537, 660, and 880 nm, respectively). As pressure was applied and varied, the compression effects on the pulsatile and non-pulsatile components were observed, along with changes in the perfusion index and prominent features of the PPG signal. The results showed significant differences in morphology and intensity, which varied with each PPG wavelength. These varying effects of PPG signal morphology, intensity, and SNR as a function of contact pressure suggest that pressure must be considered with wearable sensing technology and when developing signal analysis methods in order to allow the advancement of fitness trackers toward regulated medical devices.