DETERIORATION OF CEREBRAL AUTOREGULATION DURING ORTHOSTATIC STRESS - INSIGHTS FROM THE FREQUENCY-DOMAIN

To determine whether dynamic cerebral autoregulation is impaired durin g orthostatic stress, cerebral blood flow (CBF) velocity in the middle cerebral artery (transcranial Doppler) and mean arterial pressure (MA P; Finapres) were measured continuously in 12 healthy subjects during ramped maximal lower body negative pressure (LBNP) to presyncope. Velo city and pressure were averaged over B-min periods of stable data at r est and during LBNP to examine steady-state cerebral hemodynamics. Bea t-to-beat variability of velocity and pressure were quantified by a '' variation index'' (oscillatory amplitude/steady-state mean value) and by power spectral analysis. The dynamic relationship between changes i n pressure and velocity was evaluated by the estimates of transfer and coherence function. The results of the study were as follows. Steady- state MAP remained relatively constant during LBNP, whereas CBF veloci ty decreased progressively by 6, 15, and 21% at -30, -40, and -50 mmHg LBNP, respectively (P < 0.05 compared with baseline). At the maximal level of LBNP (30 s before presyncope) MAP decreased by 9.4% in associ ation with a prominent reduction in velocity by 24% (P < 0.05 compared with baseline). The variation index of pressure increased significant ly from 3.8 +/- 0.3% at baseline to 4.5 +/- 0.6% at -50 mmHg LBNP in a ssociation with an increase in the variation index of velocity from 6. 0 +/- 0.6 to 8.4 +/- 0.7% (P < 0.05). Consistently, the low- (0.07-0.2 0 Hz) and high-frequency (0.20-0.30 Hz) power of variations in pressur e and velocity increased significantly at high levels of LBNP (P < 0.0 5) in association with an increase in transfer function gain (24% at - 50 mmHg, P < 0.05). We conclude that the damping effects of autoregula tion on variations in CBF velocity are diminished during orthostatic s tress in association with substantial falls in steady-state CBF veloci ty. We suggest that these changes may contribute in part to the develo pment of presyncope.