Regulation of cerebrovascular resistance below the lower limit of cerebral autoregulation during induced hypotension: an observational study.

Background: To maintain adequate perfusion, cerebral blood flow (CBF) is preserved by changes in cerebrovascular resistance (CVR) inversely related to fluctuations in mean arterial blood pressure (MAP). It has been hypothesised that during progressive hypotension, a lower limit of cerebral autoregulation (LLCA) is reached beyond which cerebrovascular dilation becomes exhausted and CBF starts to decrease together with BP. We tested this hypothesis by assessing CVR above and below the LLCA.