Identical pattern of cerebral hypoperfusion during different types of syncope

Syncope is caused by insufficient oxygen supply to the brain. There have been attempts to classify syncope on the basis of defects in the venous system, arterial system (that is impaired systemic vascular resistance) or a combination of the two (that is mixed). We examined the hypothesis that a comparable decrease in cerebral perfusion would be evident at pre-syncope irrespective of the category of dysfunction. Young healthy volunteers (N=37) participated. The protocol consisted of 15 min supine rest, followed by 60 degrees head-up tilt and lower body suction in increments of -10 mm Hg for 5 min each until pre-syncope. Beat-to-beat blood pressure (BP) (Finometer or intra-arterial), cardiac output (Finometer), middle cerebral artery blood velocity (MCAv), end-tidal CO(2) and cerebral oxygenation were monitored continuously. At pre-syncope, mixed dysfunction was common (21 out of 37 participants), followed by venular dysfunction (15 out of 37 participants). In the venular and mixed groups, comparable orthostatic tolerance and declines in BP (-37 vs -43% from baseline, respectively), end-tidal PCO(2), MCAv (-35 vs -38%) and cerebral oxygenation (-5 vs -7%) were evident despite distinct mechanisms purportedly being responsible for the hypotension. Although different determinants of hypotension do exist, cerebral hypoperfusion occurs to a similar extent.