Syncopal attack alters the burst properties of muscle sympathetic nerve activity in humans

This study aimed at examining whether the properties of microneurographically recorded muscle sympathetic nerve activity (MSNA) were altered during hypotensive attacks. A retrospective study was performed on 74 subjects who participated in tilt studies when vasodepressive syncope was induced incidentally in six subjects. The specific features of MSNA that distinguish this activity from skin sympathetic nerve activity are (1) rhythmic pulse synchronous burst discharge, (2) a duration of approximately 150-300 ms, and (3) no response to arousal stimuli were abolished during the syncopal attack. The altered features observed during the syncopal attack in these six subjects were (1) scattered reflex latencies of MSNA peak from the ECG R-wave, (2) elongated burst duration twice to five times as long as that in conscious state, and (3) response to arousal stimuli. The reduced input from the baroreceptors due to suppression on the central sympathetic volley proximal to the nucleus tractus solitarius might be attributed to the lost features characteristic of MSNA.