Autoregulation of cerebral blood flow in orthostatic hypotension

Background and Purpose-We sought to evaluate cerebral autoregulation in patients with orthostatic hypotension (OH), Methods-We studied 31 patients (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 were heart rate, blood pressure (BP), cardiac output, stroke volume, CO2, total peripheral resistance, and cerebrovascular resistance. Results-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 during 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 group showed either no correlation between BFV and BP (n=8) or had a positive BFV/BP correlation (R-2>.75) but with a nat slope. An expansion of the "autoregulated" range was seen in some patients. The OH_AF group 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 most common patterns of cerebra response to OH are autoregulatory failure with a flat flow-pressure relationship or intact autoregulation with an expanded autoregulated range. The least common pattern is autoregulatory failure with a steep now-pressure relationship. Patients with patterns 1 and 2 have an enhanced capacity to cope with OH, while those with pattern 3 have reduced capacity.