Limits of cerebrovascular autoregulation during and after major non-cardiac surgery

Abstract

Background: Cerebrovascular Autoregulation (CVA) is a vital mechanism that ensures a constant cerebral blood flow (CBF) even when perfusion pressure fluctuates. This process is especially critical during surgical and anesthetic procedures. The perioperative assessment of CVA helps to identify the lower limit of cerebrovascular autoregulation (LLA) and the upper limit of cerebrovascular autoregulation (ULA), which are the blood pressure thresholds beyond which cerebral hypoperfusion or hyperperfusion may occur. CBF tends to decrease when blood pressure falls below the LLA and increase when blood pressure exceeds the ULA. Aims: The objective of this study was to evaluate and compare the intra- and postoperative LLA and ULA, along with the Time-Weighted Average (TWA) Mean Arterial Blood Pressure (MAP) below the LLA and above the ULA. Additionally, the study sought to identify factors related to these parameters during the perioperative period. Methods: This was a sub-study within an ongoing single-center, prospective cohort study conducted from August 2021 to September 2023. It included adult patients scheduled for non-cardiac surgery exceeding 120 minutes, under general anesthesia, with invasive blood pressure monitoring, pre-existing anemia, or anticipated blood loss exceeding 500ml. CVA was measured through the correlation between MAP and regional cerebral oxygen saturation (rSO2) as detected by near-infrared spectroscopy (NIRS). Patients were assessed both intraoperatively and postoperatively. The LLA and ULA were determined using an automated curve-fitting algorithm as the MAP at which the cerebral oximetry index (COx) decreased from ≥ 0.3 to < 0.3 or increased from < 0.3 to ≥ 0.3, respectively, as MAP increased. The TWA-MAP below the LLA and above the ULA were calculated as areas between the MAP and the respective limit curves, normalized to the duration of CVA assessment. Multivariable linear regression models were used to analyze factors associated with the perioperative LLA, ULA, TWA-MAP below the LLA and TWA-MAP above the ULA. Results: The final analysis included 423 patients. The median intraoperative LLA and ULA were 69.40 (63.94, 76.21) and 86.06 (79.42, 93.64) mmHg, respectively. The median postoperative LLA and ULA were 71.09 (62.94, 79.52) and 87.42 (77.07, 97.69) mmHg, respectively. The median intraoperative TWA-MAP below the LLA and above the ULA were 0.43 (0.11, 1.36) and 1.19 (0.43, 2.32) mmHg, respectively. The median postoperative TWA-MAP below the LLA and above the ULA were 0.14 (0.00, 0.74) and 0.293 (0.03, 0.86) mmHg, respectively. The differences in LLA and ULA between intra- and postoperative periods were not statistically significant (p=0.343 for LLA, p=0.322 for ULA). However, the intraoperative TWA-MAP below the LLA and above the ULA were significantly higher than their postoperative counterparts (p < 0.001 for both). Sex, the hemoglobin level before the surgery, and the occurrence of a blood transfusion during surgery were found to have a correlation with the intraoperative LLA; sex, BMI, the ASA physical status classification, and the use of sevoflurane anesthesia were determined to affect the intraoperative ULA; the change in hemoglobin levels from the preoperative period to the postoperative period was linked to the the postoperative LLA; sex and the duration of the surgery showed a relationship with the postoperative ULA; sex, Obstructive Sleep Apnea Syndrome (OSAS) status, blood transfusion during surgery, and preoperative hemoglobin levels were associated with the intraoperative TWA-MAP below the LLA; no variables were related to the intraoperative TWA-MAP above the ULA; age and OSAS status were correlated with the postoperative TWA-MAP below the LLA; sex, arterial hypertension, blood loss, and the amount of noradrenalin administered were in association with the postoperative TWA-MAP above the ULA. Conclusion: Patients tend to have a higher TWA-MAP below the LLA and above the ULA during surgery than after surgery, indicating the need for tailored blood pressure management strategies in different perioperative phases. Multiple factors influence the perioperative limits of CVA and the TWA-MAP, with sex, hemoglobin level, and OSAS status being key determinants.