Group Delay Effect Analysis Between Arterial Blood Pressure and Photoplethysmography Waveforms

Photoplethysmography (PPG) is a promising tool for ambulatory and personalized health applications. Thus, various cardiovascular parameters are assessed from waveform interpretation and analysis. The relevant parameters in this domain are: left ventricular ejection time (LVET), isovolumetric contraction time (IVCT) and relaxation time (IVRT). Being time dependent markers, propagation time delay is the main limitation during PPG signal analysis. This effect originates from the traveling mechanical blood pressure wave, between arterial central site to the peripheral ones. Since the waveform contains various frequency components and it is subject to various arterial impedance zones, group delay is present. In the current study, this effect is investigated by considering common signal segments between invasive arterial blood pressure and PPG. Around 7800 cardiac cycles were analyzed from 56 individual patients during the surgery procedure. Depending on vasomotor state, the defined signal segments scored an average delay time between 5 ms and 25 ms. The segment defined around maximum systolic value reflects a minimum delay during current cardiac cycles, while the segment around diastolic value shows the opposite behavior. Therefore, during systolic and diastolic phases, arterial impedance is changing dynamically leading to various rates in delay time. These findings highlight the need of developing signal compensation techniques during PPG waveform assessment. In the absence of such measures, the extracted cardiovascular markers are biased.