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Cerebral hemodynamics during sustained intra-operative hypotension.

Background Static cerebral autoregulation (CA) maintains cerebral blood flow (CBF) relatively constant above a mean arterial blood pressure (BPmean) of 60-65 mmHg. Below this lower limit of CA (LLCA), CBF declines along with BPmean. Data are lacking describing how CA reacts to sustained hypotension, since hypotension is usually avoided. In this study, we took advantage of a procedure requiring sustained hypotension. We assessed static CA for LLCA determination, and a more continuous CA, which counter short-term blood pressure variations. With these data, we analyzed CA during longstanding hypotension. Methods Continuous arterial blood pressure and middle cerebral artery blood flow velocity (MCAVmean) were monitored in 23 patients that required deep intra-operative hypotension. The LLCA was determined for every patient, and BPmean below this LLCA was classified as the patient specific hypotension. With the mean flow index (Mxa) continuous CA (Mxa-CA) was quantified. Mxa was calculated and averaged after induction of general anesthesia (baseline), every 15 minutes during, and 15 minutes after one-hour of hypotension. Functioning CA was defined as Mxa <0.4. Data are expressed as median (25th-75th percentile). Results The LLCA was located at 56 (47-74) mmHg. At baseline, Mxa was 0.21 (0.14-0.32) and 0.61 (0.48-0.78) during hypotension (p<0.01), with no appreciable change over time, n=12. After blood pressure restoration, Mxa improved, 0.25 (0.06-0.35, n=9). Conclusions Mxa-CA became and remained disturbed during the one-hour of hypotension, and improved after blood pressure restoration. This completely reversible situation suggests no ischemic hyperemia occurs and renders an adaptation mechanism during sustained hypotension unlikely.