Autoregulatory cerebral vasodilation occurs during orthostatic hypotension in patients with primary autonomic failure

It is unclear whether patients with autonomic failure autoregulate cerebral blood flow during hypotension. The objective in this study was to examine cerebral autoregulatory capacity in patients with autonomic failure by studying changes in middle cerebral artery blood flow velocity using transcranial Doppler ultrasonography before, during, and after tilt-induced hypotension. Nine patients with primary autonomic failure were evaluated. Mean arterial pressure and middle cerebral artery blood flow velocity were simultaneously recorded while the patients were in the supine position, during 60^o head-up tilt, and after they were returned to the horizontal position. The results were as follows: during tilt-induced hypotension, mean arterial pressure decreased significantly more than middle cerebral artery mean blood flow velocity (58% versus 36%, p<0.0002). After return to the horizontal position, mean arterial pressure returned to baseline, and middle cerebral artery blood flow velocity transiently increased above pretilt value (p<0.02). It is concluded that cerebral autoregulatory vasodilation occurs in patients with autonomic failure. This was demonstrated by a more pronounced decline in mean arterial pressure than in middle cerebral artery blood flow velocity during hypotension and by a transient increase in middle cerebral artery blood flow velocity (ie, hyperemic response) after blood pressure was restored.