CEREBRAL PRESSURE-FLOW RELATIONSHIP DURING CARDIOPULMONARY BYPASS IN THE DOG AT NORMOTHERMIA AND MODERATE HYPOTHERMIA

We studied cerebral autoregulation by analyzing cerebral pressure-flow curves during cardiopulmonary bypass (CPB) with alpha-stat (alpha-sta t) acid-base management at 28 (n = 9) and 37 degrees C (n = 9) in two groups of dogs. Cerebral blood flow (CBF) and cerebral metabolic rate for oxygen (CMRO(2)) were determined multiple times in each animal ove r an extensive range of cerebral perfusion pressure (CPP). The CPP was altered by changing perfusion now 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 anti cipated maximal statistical power with this analysis to define if cere bral autoregulation was intact. This method of statistical analysis wa s compared with the conventional interpretation by linear regression a nalysis. Animals were administered sodium thiopental until an isoelect ric electroencephalogram was obtained to assure stable depth of anesth esia independently of temperature effects. The animals were randomly a ssigned to either temperature group. The CBF was determined by injecti on of radioactive microspheres at each of five target CPPs randomly al located (50, 60, 70, 80, and 90 mm Hg). The brain oxygen content diffe rence was defined as arterial minus superior sagittal sinus (SSS) oxyg en content. No difference in CPP, hemoglobin, arterial carbon dioxide tension, or pH was seen between groups at any time period. In both gro ups, total CBF (tCBF) increased significantly with increasing CPP (p = 0.012 and 0.017 for normothermic and hypothermic CPB, respectively; C PP as covariate). The between-group difference in slopes (CPP x temper ature effect) approached statistical significance (p = 0.059). For cer ebral hemispheric CBF (hCBF), autoregulation was also impaired (p = 0. 007 and 0.015, respectively). There was a significant between-group di fference in slopes with a steeper slope for normothermia (p = 0.032). An inverse relation for arterial minus SSS oxygen content versus CPP w as found (p = 0.0001 for both groups). The Q(10) (the ratio of cerebra l metabolic rates for oxygen for a 10 degrees 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 al pha-stat acid-base management, the autoregulatory plateau was not hori zontal for either temperature. In addition, for supratentorial structu res (cerebral hemispheres), autoregulation was significantly more impa ired for normothermia than for moderate hypothermia.