Additional predictors of the lower limit of cerebral autoregulation during cardiac surgery

Objectives: The lower limit of autoregulation (LLA) of cerebral blood flow was previously shown to vary directly with the Ambulatory Arterial Stiffness Index (AASI) redefined as 1-regression slope of DBP-versus-SBP readings invasively measured from the radial artery before the bypass. We aimed expanding the predictive capacity of the LLA with AASI by combining it with additional predictors and provide new indications whether mean arterial pressure (MAP) is above/below the LLA. Design and method: In 181 patients undergoing cardiac surgery, mean (SD) age 71 (8) years), we identified from the demographic, preoperative and intraoperative characteristics independent and statistically significant 'single predictors' of the LLA (including AASI). This was achieved using multivariate linear regression with a backward-elimination technique. The single predictors combined with 1-AASI generated new multiplicative and additive composite predictors of the LLA. Indicators for the MAP-to-LLA difference (DIF) were determined using DIF-versus-predictor plots. The odds ratio (OR) for the DIF sign (Outcome = 1 for DIF <= 0) and predictor-minus-median sign (Exposure = 1 for Predictor <= Median) were calculated using logistic regression. Results: BMI, 1-AASI and systolic coefficient of variation were identified single predictors that correlated similarly with the LLA (r = -0.26 to -0.27, P < 0.001). The multiplicative and additive composite predictors displayed higher correlation with LLA (r = -0.41 and r = -0.43, respectively, P < 0.001) and improved LLA estimation. The adjusted OR for the composite predictors was nearly twice that of the single predictors. Conclusion: The novel composite predictors may enhance the LLA estimation and the ability to maintain MAP in the cerebral autoregulatory range during cardiac surgery.