Transfer Function Analysis of Cerebral Autoregulation Dynamics in Autonomic Failure Patients

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Abstract
Background and Purpose Autonomic nervous system diseases affect systemic blood pressure regulation. Patients with autonomic nervous system diseases have consistently larger drops in blood pressure associated with standing than the normal population. Autonomic dysfunction and/or these changes in blood pressure may affect dynamic cerebral autoregulation. Methods Heart rate, mean blood flow velocity (MBFV) of the middle cerebral artery via transcranial Doppler ultrasound, mean arterial blood pressure adjusted to brain level (MABP brain ) via Finapres, and end tidal CO 2 were measured continuously during graded tilt (after 5 minutes in supine position as baseline, −10°, +10°, +30°, +60°, −10°, and supine recovery) in autonomic failure patients and their age- and sex-matched control subjects. The dynamic response of MBFV to spontaneous variations in MABP brain was investigated by cross-spectral analysis. The transfer gain and phase relationships between MBFV and MABP brain were determined from the final 256 beats of each 5-minute–tilt segment. The transfer gain was normalized to mean MABP brain and MBFV and then converted to decibels (dB). Results MBFV variation (0.03 to 0.14 Hz) preceded MABP brain by similar phase angles in patients and control subjects and in all tilt conditions (patients: 31±5°; control subjects: 30±5°; mean±SEM). Patients had a higher supine gain than control subjects ( P <.05). Both patients and control subjects showed a significant decrease in gain with tilt and by 60° the patients were not different from the control subjects (supine to 60°: patients=5.23±0.77 to −1.65±0.89 dB; control subjects=1.74±0.82 to −1.80±0.62 dB). Conclusions These data indicate an altered, yet present, autoregulatory response with autonomic failure.