Critical analysis of cerebrovascular autoregulation during repeated head-up tilt

Background and Purpose-Cerebrovascular autoregulation has been described wi th a phase lead of cerebral blood flow preceding changes in cerebral perfus ion pressure (CPP), but there has been less focus on the effect of CPP on c erebral vascular resistance. We investigated these relations during spontan eous 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 degrees 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 finge r pressure (CPPF), and cerebrovascular resistance index (CVRi) was calculat ed in the supine position from CPPF/MFV. Gain and phase relations were asse ssed by cross-spectral analysis. Results-In the supine position, MFV preceded CPPF, but changes in CVRi foll owed CPPF. Gain and phase relations for CPPF as input and MFV as output wer e similar in supine and repeated tilt experiments. Thus, changes in cerebro vascular resistance must have had a similar pattern in the supine and tilt experiments. Conclusions-Cerebrovascular autoregulation is achieved by changes in resist ance in response to modulations in perfusion pressure whether spontaneous o r induced by repeated tilt. The phase lead of MFV before CPPF is a mathemat ical and physiological consequence of the relation the input variable (CPPF ) and the manipulated variable (cerebrovascular resistance) that should not be taken as an indication of independent control of cerebral blood flow.