TRANSFER-FUNCTION ANALYSIS OF CEREBRAL AUTOREGULATION DYNAMICS IN AUTONOMIC FAILURE PATIENTS

Background and Purpose Autonomic nervous system diseases affect system ic blood pressure regulation. Patients with autonomic nervous system d iseases have consistently larger drops in blood pressure associated wi th standing than the normal population. Autonomic dysfunction and/or t hese changes in blood pressure may affect dynamic cerebral autoregulat ion. Method Heart rate, mean blood flow velocity (MBFV) of the middle cerebral artery via transcranial Doppler ultrasound, mean arterial blo od pressure adjusted to brain level (MABP(brain)) via Finapres, and en d tidal CO2 were measured continuously during graded tilt (after 5 min utes in supine position as baseline, -10 degrees, +10 degrees, +30 deg rees, +60 degrees, -10 degrees, and supine recovery) in autonomic fail ure patients and their age-and sex-matched control subjects. The dynam ic response of MBFV to spontaneous variations in MABP(brain) was inves tigated by cross-spectral analysis. The transfer gain and phase relati onships between MBFV and MABP(brain) were determined from the final 25 6 beats of each 5-minute-tilt segment. The transfer gain was normalize d to mean MABP(brain) and MBFV and then converted to decibels (dB). Re sults MBFV variation (0.03 to 0.14 Hz) preceded MABP(brain) by similar phase angles in patients and control subjects and in all tilt conditi ons (patients: 31+/-5 degrees; control subjects: 30+/-5 degrees; 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 degrees the patients were not different from the control subjects (supine to 60 degrees: patients=5.23+/-0.77 to - 1.65+/-0.89 dB; control subjects=1.74+/-0.82 to -1.80+/-0.62 dB). Conc lusions These data indicate an altered, yet present, autoregulatory re sponse with autonomic failure.