The Effects of Sympathectomy on Finger Photoplethysmography and Temperature Measurements in Healthy Subjects

Abstract

Photoplethysmography uses light transmission to measure changes in tissue volume. The resulting photoplethysmogram is composed of AC and DC components. Limited data are available on the effects of vasodilation on the AC and the DC components of the photoplethysmograph signal. The aims of our study were (1) to investigate the effects of sympathectomy on different components of the photoplethysmogram, and (2) to compare sympathectomy-induced changes in the photoplethysmogram with changes in peripheral temperature. In 10 healthy subjects, sympathectomy-induced peripheral vasodilation was achieved using an axillary brachial plexus block. The nonblocked arm served as control. We obtained measurements of bilateral continuous measurements of finger blood volume (by photoplethysmography) and finger temperature. We separated the finger photoplethysmogram into its AC and DC components. In addition, we calculated the ratio of AC to DC (AC/DC). All data were recorded until 30 minutes after the end of brachial plexus block. Repeated-measures analysis of variance followed by the Dunnett post hoc test determined the effect of brachial plexus block on the finger photoplethysmogram and finger temperature. The DC component of the finger photoplethysmogram decreased (vasodilation) significantly (P < 0.0001) after brachial plexus block in the blocked arm starting 2.7 minutes after the block. Average decrease in DC values was -51% ± 19% (95% confidence interval: -61% to -42%) at 30 minutes after the block. None of the other photoplethysmogram components changed significantly from preblock baseline values. On average, the finger temperature increased significantly (P < 0.0001) starting 5.7 minutes after brachial plexus block in the blocked arm. Average increase in temperature was 7.1°C ± 3.8°C (95% confidence interval: 5.1°C-9.0°C) 30 minutes after the block. The DC component of the photoplethysmogram had the highest sensitivity and specificity to predict a successful block. This study characterizes sympathectomy-induced changes in the AC and DC components of the finger photoplethysmogram. In this experimental model, we found the DC component to be most sensitive in detecting peripheral vasodilation.