Cerebral autoregulation during orthostatic stress in healthy controls and in patients with posturally related syncope

Posturally related syncope (PRS) is a common and distressing problem, which frequently occurs in people with no apparent clinical disorder and is ultimately caused by a reduction in blood supply to the brain. The aim of this study was to compare cerebrovascular responses to orthostatic stress in otherwise healthy patients suffering from PRS, and who were shown to have a poor orthostatic tolerance (n=28), with those in healthy control subjects with good orthostatic tolerance (n=11). Responses of heart rate, arterial blood pressure, end tidal carbon dioxide and middle cerebral artery (MCA) blood flow velocity were determined during a progressive orthostatic stress test of combined head-up tilting and lower body suction, which was continued until presyncope. We assessed the efficiency of autoregulation of cerebral blood flow from the relationship between values of MCA velocity and pressure obtained over the expected range for autoregulation (> 55 mmHg). All patients with PRS had a significant correlation between MCA velocity and pressure, but this was seen in only two of the controls. Furthermore, the values of the correlation coefficients were significantly higher in patients than controls, (p<0.001). We interpret these data as indicating that autoregulation in patients with PRS is less effective than in controls and suggest that this provides evidence for a link between abnormalities of regulation of the cerebral circulation and predisposition to syncope.