A correlation study on the features of photoplethysmographic signals

In order to study the underlying physiological process of photo-electrical activities and understand its physical generation mechanism, the photoplethysmogphic (PPC) signals at the fingertip were recorded simultaneously with the corresponding ECG signals from normal human subjects under the following conditions: a) at the rest or relaxed state; and (b) at different levels of contacting forces applied on the sensors. A significantly high correlation (r = 0.9621 plusmn 0.029, p < 0.0001 ) was found between the RR interval of ECG and the YG interval (the falling time) of PPG under all the conditions. The FY interval (rising time) of PPG also exhibits a high correlation (r = -0.94, p < 0.0001 ) with the pulse transit time (PTT) estimated as the time delay from ECG to PPG signals only at the relatively higher levels of contacting force. The correlation between the PTT and the RR intervals, on the other hand, is not significant. The results of this study indicated that the YG interval (besides FF intervals) of PPG could reflect to a large extent the relevant rhythm information associated with the duration of a heart cycle, while the FY interval might provide valuable information about the cardiac dynamics and the properties of the arterial elasticity under certain conditions