Sympathetic outflow to muscles during vasovagal syncope.

Sympathetic vasoconstrictor impulses were recorded with microelectrodes in peroneal muscle nerve fascicles during vasovagal syncope in two subjects. After a preceding period of increased nerve activity, onset of syncope was associated with bradycardia and sudden cessation of sympathetic outflow. The findings provide direct evidence that neurally mediated muscular vasodilatation occurring during syncope is due to inhibition of vasoconstrictor impulses.