Cerebral Pressure-Flow Relationship during Cardiopulmonary Bypass in the Dog at Normothermia and Moderate Hypothermia

Abstract

We studied cerebral autoregulation by analyzing cerebral pressure–flow curves during cardiopulmonary bypass (CPB) with alpha-stat (α-stat) acid–base management at 28 ( n = 9) and 37°C ( n = 9) in two groups of dogs. Cerebral blood flow (CBF) and cerebral metabolic rate for oxygen (CMRO₂) were determined multiple times in each animal over an extensive range of cerebral perfusion pressure (CPP). The CPP was altered by changing perfusion flow rate. The dependence of CBF on CPP during normothermic and moderate hypothermic CPB was assessed using a block design analysis of covariance with CPP as the covariate. We anticipated maximal statistical power with this analysis to define if cerebral autoregulation was intact. This method of statistical analysis was compared with the conventional interpretation by linear regression analysis. Animals were administered sodium thiopental until an isoelectric electroencephalogram was obtained to assure stable depth of anesthesia independently of temperature effects. The animals were randomly assigned to either temperature group. The CBF was determined by injection of radioactive microspheres at each of five target CPPs randomly allocated (50, 60, 70, 80, and 90 mm Hg). The brain oxygen content difference was defined as arterial minus superior sagittal sinus (SSS) oxygen content. No difference in CPP, hemoglobin, arterial carbon dioxide tension, or pH was seen between groups at any time period. In both groups, total CBF (tCBF) increased significantly with increasing CPP ( p = 0.012 and 0.017 for normothermic and hypothermic CPB, respectively; CPP as covariate). The between-group difference in slopes (CPP × temperature effect) approached statistical significance ( p = 0.059). For cerebral hemispheric CBF (hCBF), autoregulation was also impaired ( p = 0.007 and 0.015, respectively). There was a significant between-group difference in slopes with a steeper slope for normothermia ( p = 0.032). An inverse relation for arterial minus SSS oxygen content versus CPP was found ( p = 0.0001 for both groups). The Q₁₀ (the ratio of cerebral metabolic rates for oxygen for a 10°C change in temperature) was 3.8. In this study, using a block design analysis of covariance, tCBF was dependent on CPP during normothermia and moderate hypothermia with deep barbiturate anesthesia. We conclude that during CPB with α-stat acid–base management, the autoregulatory plateau was not horizontal for either temperature. In addition, for supratentorial structures (cerebral hemispheres), autoregulation was significantly more impaired for normothermia than for moderate hypothermia.