Observations on the photoplethysmograph pulse derived from a laser Doppler flowmeter

A photoplethysmograph signal has been derived from a laser Doppler flowmeter which uses optical fibres separated by 1 mm. When compared with a conventional photoplethysmograph, the trace contains a higher proportion of random signal superimposed on the pulse. The bandlimited laser photoplethysmograph (0.5-16 Hz) was recorded from the finger in six subjects. At low blood flows or during arterial occlusion the random component was more evident, producing a significant signal power increase in the range 8-16 Hz. It is proposed that this noise associated with laser photoplethysmographs is partly intrinsic signal due to low frequency Doppler shifts, which predominate at low blood flows, or intensity fluctuations as slow moving blood cells traverse the laser speckle pattern. Increasing the fibre separation above 2-3 mm reduces the relative amplitude of the random component producing a much improved photoplethysmograph signal. Multiple scattering at this separation tends to reduce the proportion of low frequency Doppler shifts. Signal averaging may be used to smooth the photoplethysmograph from the 1 mm fibre separation to give a pulse essentially identical to the improved signal obtained at higher separations. Photoplethysmograph amplitude demonstrated a non-linear relationship with the laser Doppler value from the same site during finger re-warming following cold stress.