MINIMUM HEMATOCRIT AT DIFFERING CARDIOPULMONARY BYPASS TEMPERATURES IN DOGS

Background-The purpose of this study was to determine the minimum hema tocrit (Hct) supporting cerebral oxygenation over the range clinically relevant cardiopulmonary bypass (CPB) temperatures in dogs. The effec t of hemodilution on cerebral blood flow (CBF), cerebral metabolic rat e (CMRO2), and cerebral oxygen delivery (CDO2) was determined over a r ange of Hcts during CPB at 38 degrees C, 28 degrees C, and 18 degrees C. Methods and Results-Measurements were obtained at target CPB temper ature and after progressive normovolemic hemodilution in 3 groups of 8 anesthetized animals. Dextran 70 (6%) was used as a diluent. CBF was measured by use of the sagittal sinus outflow technique. CMRO2 and CDO 2 were calculated by the use of standard formulae. In each temperature group, hemodilution was associated with a reciprocal rise in CBF. As Hct was reduced to 0.10+/-0.02 at 38 degrees C, 28 degrees C, and 18 d egrees C, CBF increased 260%, 220%, and 160% of the control nonhemodil uted value. Increases in CBF helped compensate for decreased arterial oxygen content and maintain CDO2. With progressive temperature reducti on, these compensatory flow increases were reduced and CDO2 was decrea sed at lesser degrees of hemodilution. Statistical analysis indicated that cerebral oxygen demand was maintained to an Hct of 0.14, 0.11, an d less than or equal to 0.10 in the 38 degrees C, 28 degrees C, and 18 degrees C groups; however, physiologically important changes in cereb ral oxygen supply occur at Hcts of approximate to 0.18, 0.15, and 0.12 , respectively, at those temperatures. Conclusions-This investigation systematically characterizes the critical Hct supporting cerebral oxyg enation at differing CPB temperatures. Over a range of CPB temperature s, the curve describing the relationship between Hct and cerebral oxyg en balance has a broad plateau, indicating cerebral tolerance for a wi de range of Hcts. The minimum Hct that supports cerebral oxygenation i s shifted leftward as temperature is reduced, but the reduction in cri tical Hct is not proportional, to the reduction in CMRO2. Although we do not advocate hemodilution to these extreme values, we find that the se data provide a physiological foundation for our hemodilution practi ce and may provide some guidance for management of Hct as body tempera ture changes during CPB.