The time-variable photoplethysmographic signal; dependence of the heart synchronous signal on wavelength and sample volume.

The photoplethysmorgraphic signal (PPG) from forearm skin and the ECG were recorded simultaneously from healthy subjects. The optical signal was derived with a fibre-optic probe which consisted of 61 fibre pairs. The peak-to-peak averaged a.c.-signal was calculated. The dependency of this signal and the signal-to-noise ratio (SNR) on the light wavelength and on sample volume was studied. The light intensities used at 560 nm were 0.015 and 0.029 mWmm-2 and at 940 nm we used 0.029, 1.37 and 2.77 mWmm-2. A theoretical model for calculation of the sample volumes was also developed. The amplitude and SNR of the a.c.-signal at 560 nm is larger than at 940 nm at the same intensity (0.029 mWmm-2). At 940 nm using 1.37 and 2.77 mWmm-2 the a.c.-signal increased with the number of fibre pairs (volume) while the SNR was little affected. At 560 nm it is suggested that the major contribution to the a.c.-signal is pulsations in the ascending arterioles and at 940 nm from pulsations in the larger vessels deeper in the tissue volume under study.