The position of the dichrotic notch in digital photoplethysmography is not affected by local vasodilation

Purpose The vertical position of the dichrotic notch of finger photoplethysmography (PPG) is affected by systemic vasodilation. The aim of this study was to test whether a local upper limb vasodilation would alter the position of the dichrotic notch. Methods Patients undergoing upper limb vasodilation caused by brachial plexus block (n = 8) and video-assisted bilateral sympathectomy (n = 8 with 2 measurements each) were analyzed. Continuous PPG recordings and brachial Doppler sonography were recorded before and five minutes after regional anesthesia and sympathetic ablation. The dichrotic notch of PPG was expressed as % of the PPG waveform amplitude. Results Local vasodilation after the interventions was evidenced by a decrease in Doppler’s resistive index from 0.88(0.2) to 0.79(0.2) (p = 0.021) in the regional blocks and from 0.83(0.3) to 0.80(0.3) (p = 0.003) in the sympathectomies. PPG amplitude increased from 46(24) to 59(12)% (p = 0.008) in the regional anesthesia-group and from 34(29) to 44(35)% (p = 0.010) in the sympathectomy-group. The position of the dichrotic notch was neither affected in the regional anesthesia-group before [15(12)%] and after [15(10)%;p = 0.916] nor in the sympathectomy-group before [14(21)%] and after [13(19)%;p = 0.849]. Conclusions Local vasodilation in the arm does not influence the position of the dichrotic notch in finger PPG despite increases in its amplitude. The dichrotic notch is thus a systemic phenomenon that depends on pulse wave reflections.