Persistence of muscle sympathetic nerve activity during vasovagal syncope

To determine whether sympathetic nerve firing to skeletal muscle vasculature is withdrawn during vasovagal syncope (VVS) precipitated by passive graded head-up tilt (HUT) table testing.

We performed passive graded HUT table testing in 18 patients with a history of recurrent postural VVS whom we evaluated during the syncopal event. All patients developed typical VVS during testing. Blood pressure was measured continuously via intra-brachial arterial line. Muscle sympathetic nerve activity (MSNA) was measured using an electrode in the peroneal nerve. Passive graded HUT was then applied. No pharmacological agents were used to provoke syncope. The recording site was maintained through the syncopal event in 10 of 18 patients and we were able to demonstrate persistence of MSNA during syncope in 9. The predominant haemodynamic pattern of syncope in this cohort was mixed--hypotension and bradycardia, with heart rate not falling <40 b.p.m. (n = 10).

Our data challenge the established view that the final trigger for human orthostatic vasovagal reactions is sympathetic nervous system inhibition. Efferent sympathetic nerve traffic to the skeletal muscle vasculature was nearly always maintained through the faint. This finding supports an alternative viewpoint, that vasodilator mechanisms underlie the blood pressure fall in VVS.