The effect of contacting force on photoplethysmographic signals

Photoplethysmography (PPG) has been used in oxygen saturation measurement, heart rate monitoring, and the assessment of peripheral circulation and large artery compliance. However, the waveform of the photoplethysmographic signal may be affected by the contacting force between the sensor and the measurement site. The aim of this study is to investigate the change in pulse amplitude (AC), DC amplitude, ratio of AC/DC and normalized pulse area of the reflective photoplethysmographic signals with increasing contacting force, from 0.2 N to 1.8 N. Signals were recorded from the fingers of fifteen healthy subjects. With increasing contacting force, the DC amplitude increased and the normalized pulse area decreased, whereas the pulse amplitude and the ratio of AC/DC increased first and then decreased. For different subjects, the pulse amplitude and the ratio of AC/DC peaked at different contacting forces, from 0.2 N to 1.0 N, and most of the subjects achieved their maximum pulse amplitude within 0.2-0.4 N. Over the range of contacting force between 0.2 N and 0.8 N, the DC amplitude and the normalized pulse area had significant changes (p < 0.001). The results suggest that the effects of contacting force should be carefully examined in the design of photoplethysmography-based health care devices.