Cerebral vasoconstriction during head-upright tilt-induced vasovagal syncope. A paradoxic and unexpected response.

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Abstract
BACKGROUNDTo determine the effect of vasovagally mediated syncope on the cerebral circulation, transcranial Doppler sonography was used to assess changes in cerebral blood flow velocity during head-upright tilt-induced syncope.METHODS AND RESULTSThirty patients (17 men and 13 women; mean age, 43 +/- 22 years) with recurrent unexplained syncope were evaluated by use of an upright tilt-table test for 30 minutes, with or without an infusion of intravenous isoproterenol (1-4 micrograms/min), in an attempt to provoke bradycardia, hypotension, or both. Transcranial Doppler sonography was used to assess middle cerebral artery systolic velocity (Vs), diastolic velocity (Vd), ratio of systolic to diastolic velocities, pulsatility index (PI = Vs-Vd/Vmean), and resistance index (RI = Vs-Vd/Vs) before, during, and after tilt. Syncope occurred in six patients (20%) during the baseline tilt and 14 (46%) during isoproterenol infusion (total positives, 66%). In the tilt-positive patients, concomitant with the development...