Critical Analysis of Cerebrovascular Autoregulation During Repeated Head-Up Tilt

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Abstract
Background and Purpose — Cerebrovascular autoregulation has been described with a phase lead of cerebral blood flow preceding changes in cerebral perfusion pressure (CPP), but there has been less focus on the effect of CPP on cerebral vascular resistance. We investigated these relations during spontaneous fluctuations (control) and repeated head-up tilt. Methods — Eight healthy adults were studied in supine rest and repeated tilt with 10-second supine, 10 seconds at 45° head-up tilt for a total of 12 cycles. Cerebral blood flow was estimated from mean flow velocity (MFV) by transcranial Doppler ultrasound, CPP was estimated from corrected finger pressure (CPP F ), and cerebrovascular resistance index (CVRi) was calculated in the supine position from CPP F /MFV. Gain and phase relations were assessed by cross-spectral analysis. Results — In the supine position, MFV preceded CPP F , but changes in CVRi followed CPP F . Gain and phase relations for CPP F as input and MFV as output were similar in supine and repeated tilt experiments. Thus, changes in cerebrovascular resistance must have had a similar pattern in the supine and tilt experiments. Conclusions — Cerebrovascular autoregulation is achieved by changes in resistance in response to modulations in perfusion pressure whether spontaneous or induced by repeated tilt. The phase lead of MFV before CPP F is a mathematical and physiological consequence of the relation the input variable (CPP F ) and the manipulated variable (cerebrovascular resistance) that should not be taken as an indication of independent control of cerebral blood flow.