Effects of filtering on multi-site photoplethysmography pulse waveform characteristics

Multisite photoplethysmography (PPG) allows pulse waveforms collected simultaneously from different peripheral body sites to be investigated. PPG waveforms are site-dependent, comprising a pulsatile ('AC') component synchronized to each heart beat, superimposed on a slowly varying ('DC') baseline. Careful high pass filtering of the pulses is needed to reduce the dominant lower frequencies but without distorting pulse shape, and sufficient computer quantization levels to reliably reproduce the pulse. In this study pulses were measured from the right ear, thumb and great toe of 10 healthy adult subjects. Pulses were then filtered offline at cut-off frequencies between 0.05 to 1 Hz using a digital single pole high pass filter. The relationship between filter cut-off frequency on lower to higher frequency amplitudes, AC pulse amplitude relative to system noise, and visible pulse shape distortion were investigated. The ratio of low to high frequency amplitudes decreased with increasing cut-off frequency (highest at toe, lowest at ear). The AC pulse amplitude relative to system noise increased with increasing cut-off frequency (highest at thumb, lowest at toe). A qualitative visual inspection of the waveforms showed that pulse shape distortion was likely at cut-off frequencies greater than 0.2 Hz. A single pole high pass filter cut-off frequency of 0.15 Hz for multisite PPG measurements enables faithful pulse shape reproduction for subsequent cardiovascular modelling.