Orthostasis as a test for cerebral autoregulation in normal persons and patients with carotid artery disease

Orthostasis reduces mean flow velocity (FVmean) in cerebral arteries. This might be used as an alternative provocation test for cerebral hemodynamics in patients with carotid artery disease (CAD). In 21 unilateral CAD patient s and 21 controls, FVmean in both middle cerebral arteries (MCA) was measur ed by transcranial Doppler, together with blood pressure (BP) and heart rat e (HR) during a tilt table test. Cerebrovascular reserve (CVR) was measured by an acetazolamide test. In all cases, FVmean dropped to a lower level (c ontrols: 81.9 +/- 9.4% of baseline; patients: 84.3 +/- 7.9% symptomatic sid e, 85.6 +/- 9.0% contralateral). Impaired CVR patients showed a smaller (p< 0.01) decrease (90.6 +/- 3.3%) compared to contralateral (84.9 +/- 6.0%), t o normal CVR patients (81.1 +/- 7.8%) and to controls. Heart rate increased in both groups (controls: +16.6 +/- 9.9%, patients +10.3 +/- 9.9%; p<0.01) ; BP showed no change. Orthostasis induces a decrease of MCA Fvmean as alre ady previously described. This decrease is significantly smaller in patient s with impaired CVR. Since BP does not change, some authors explain the low er MCA Fvmean during orthostasis as caused by sympathetic induced vasoconst riction of cerebral resistance vessels. The authors speculate that in impai red CVR-patients autoregulative protection against ischemia might limit vas oconstriction. In combination with standard tests for measurement of CVR, t his test might be useful for evaluation of cerebral autoregulation.