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. Methods: Radial arterial pressure, thermodilution cardiac output (CO), and mean middle cerebral artery blood velocity (MCAV(mean)) were recorded during sustained intraoperative hypotension clinically needed for off-pump aortic root aneurysm surgery. For each participant, the individual LLCA was determined. Systemic vascular resistance (SVR) and CVR were calculated, and changes below and above the LLCA were assessed with a generalised linear effect models. Results: For 50 participants undergoing aortic root surgery who met inclusion criteria, LLCA was located at 58 (12) mm Hg, with a corresponding MCAV(mean) of 32 (8) cm s(-1) and CO of 5.1 (1.2) L min(-1). Above the LLCA, the decline in CVR and SVR were similar, both with 19% per 10 mm Hg decrease in MAP (P<0.001). Below the LLCA, CVR declined at a lower rate (7% per 10 mm Hg), whereas the decrease in SVR was 13% per 10 mm Hg decrease in MAP (both P<0.001). Conclusions: The continuing decline of CVR below the LLCA indicated that brain vasculature is still able to react on changing BP. This implies that LLCA should not be regarded as a fixed point but rather a transitional zone between exhausted and normally functioning autoregulation.