Power spectrum analysis of spontaneous fluctuations in the photoplethysmographic signal.

: Direct measurement of variability in the peripheral circulation was performed by means of photoplethysmography, which provides assessment of blood volume in tissue and systolic blood volume increase. Both parameters showed oscillations at a relatively low frequency, about 0.02 Hz, and a relatively high frequency, about 0.3 Hz, which is related to respiration. These frequencies are similar to those found in measurements of heart rate variability--which is also obtained by measuring the photoplethysmographic period variability--but the powers of the different frequencies are different. The low frequency oscillation, which is attributed to the activity of the sympathetic nervous system, is more pronounced in the measurements of tissue blood volume and of systolic blood volume increase than in the measurement of heart rate.