Cerebrovascular and cardiovascular responses to graded tilt in patients with autonomic failure

Background and Purpose Patients with autonomic nervous system failure often experience symptoms of orthostatic intolerance while standing. It is not known whether these episodes are caused primarily by a reduced ability to regulate arterial blood pressure or whether changes in cerebral autoregulation may also be implicated. Methods Eleven patients and eight healthy age-and sex-matched control subjects were studied during a graded-tilt protocol. Changes in their steady state middle cerebral artery mean flow velocities (MFV), measured by transcranial Doppler, brain-level mean arterial blood pressures (MABP(brain)), and the relationship between the two were assessed. Results Significant differences between patients and control subjects (P<.05) were found in both their MFV and MABP(brain) responses to tilt. Patients' MFV dropped from 60+/-10.2 cm/s in the supine position to 44+/-14.0 cm/s at 60 degrees head-up tilt, whereas MABP(brain) fell from 109+/-11.7 to 42+/-16.9 mm Hg. By comparison, controls' MFV dropped from 54+/-7.8 cm/s supine to 51+/-8.8 cm/s at 60 degrees, whereas MABP(brain) went from 90+/-11.2 to 67+/-8.2 mm Hg. Linear regression showed no significant difference in the MFV-MABP(brain) relationship between patients and control subjects, with slopes of 0.228+/-0.09 cm . s(-1) . mm Hg-1 for patients and 0.136+/-0.16 cm . s(-1) . mm Hg-1 for control subjects. Conclusions The present study found significant differences between patients and control subjects in their MFV and MABP(brain) responses to tilt but no difference in the autoregulatory MFV-MABP(brain) relationship. These results suggest that patients' decreased orthostatic tolerance may primarily be the result of impaired blood pressure regulation rather than a deficiency in cerebral autoregulation.