AUTOREGULATION OF CEREBRAL BLOOD-FLOW IN ORTHOSTATIC HYPOTENSION

Background and Purpose-We sought to evaluate cerebral autoregulation i n patients with orthostatic hypotension (OH), Methods-We studied 31 pa tients (aged 52 to 78 years) with neurogenic OH during 80 degrees head -up tilt. Blood flow velocities (BFV) from the middle cerebral artery were continuously monitored with transcranial Doppler sonography, as w ere heart rate, blood pressure (BP), cardiac output, stroke volume, CO 2, total peripheral resistance, and cerebrovascular resistance. Result s-All OH patients had lower BP (P<.0001), BFV diastolic (P<.05). CVR ( P<.007), and TPR (P<.02) during head-up tilt than control subjects, In control subjects, no correlations between BFV and BP were found durin g head-up tilt, suggesting normal autoregulation. OH patients could be separated into those with normal or expanded autoregulation (OH_NA; n =16) and those with autoregulatory failure (OH_AF; n=5). The OH_NA gro up showed either no correlation between BFV and BP (n=8) or had a posi tive BFV/BP correlation (R-2>.75) but with a nat slope. An expansion o f the ''autoregulated'' range was seen in some patients. The OH_AF gro up was characterized by a profound Fall in BFV in response to a small reduction in BP (mean Delta BP <40 mm Hg; R-2>.75). Conclusions-The mo st common patterns of cerebra response to OH are autoregulatory failur e with a flat flow-pressure relationship or intact autoregulation with an expanded autoregulated range. The least common pattern is autoregu latory failure with a steep now-pressure relationship. Patients with p atterns 1 and 2 have an enhanced capacity to cope with OH, while those with pattern 3 have reduced capacity.