The variability of the photoplethysmographic signal - a potential method for the evaluation of the autonomic nervous system

TLDR

Heart‑rate variability reflects sympathetic and parasympathetic modulation, and the baseline and amplitude of the photoplethysmographic signal also oscillate at these low‑ and high‑frequency bands. The study recorded finger‑PPG from healthy and diabetic subjects, extracted baseline, amplitude, and period for each pulse, and quantified variability as the coefficient of variation (SD/mean). Diabetic patients showed reduced low‑frequency variability in PPG amplitude and heart‑period, lower inter‑hand correlation of baseline and amplitude, indicating diminished autonomic activity and suggesting that PPG variability may serve as a non‑invasive autonomic assessment tool.

Abstract

The heart rate variability is composed of low- and high-frequency fluctuations, which are mediated by the sympathetic and the parasympathetic nervous systems. The baseline and the amplitude of the photoplethysmographic (PPG) signal also show fluctuations in the same frequencies. In the current study, PPG examinations were performed on the fingers of normal subjects and diabetic patients, and three parameters were derived from each PPG pulse: the baseline of the pulse, its amplitude and its period (which is equal to the heart period). The level of the variability of each PPG pulse parameter was measured by the ratio of the standard deviation of the parameter to its mean value. The level of the low-frequency fluctuations for the PPG amplitude and for the heart cycle period did not differ between males and females, but was lower for diabetic patients, indicating lower activity of the autonomic nervous system. The curves of the baseline and the amplitude of the PPG signal for the non-diabetic subjects showed high correlation between the left and the right hands. For most of the diabetic patients the right-left correlation coefficients were significantly lower than those for the non-diabetic subjects. Our initial results have shown that the variability of the PPG parameters shows promise for the assessment of the function of the autonomic nervous system.