Minimum hematocrit at differing cardiopulmonary bypass temperatures in dogs

Background - The purpose of this study was to determine the minimum hematocrit (Hct) supporting cerebral oxygenation over the range clinically relevant cardiopulmonary bypass (CPB) temperatures in dogs. The effect of hemodilution on cerebral blood flow (CBF), cerebral metabolic rate (CMRO₂), and cerebral oxygen delivery (CDO₂) was determined over a range of Hcts during CPB at 38°C, 28°C, and 18°C. Methods and Results - Measurements were obtained at target CPB temperature 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 CDO₂ were calculated by the use of standard formulae. In each temperature group, hemodilution was associated with a reciprocal rise in CBF. As Hot was reduced to 0.10±0.02 at 38°C, 28°C, and 18°C, CBF increased 260%, 220%, and 160% of the control nonhemodiluted value. Increases in CBF helped compensate for decreased arterial oxygen content and maintain CDO₂. With progressive temperature reduction, these compensatory flow increases were reduced and CDO₂ was decreased at lesser degrees of hemodilution. Statistical analysis indicated that cerebral oxygen demand was maintained to an Hct of 0.14, 0.11, and ≤0.10 in the 38°C, 28°C, and 18°C groups; however, physiologically important changes in cerebral oxygen supply occur at Hcts of ≃0.18, 0.15, and 0.12, respectively, at those temperatures. Conclusions - This investigation systematically characterizes the critical Hct supporting cerebral oxygenation at differing CPB temperatures. Over a range of CPB temperatures, the curve describing the relationship between Hct and cerebral oxygen balance has a broad plateau, indicating cerebral tolerance for a wide range of Hcts. The minimum Hct that supports cerebral oxygenation is shifted leftward as temperature is reduced, but the reduction in critical Hct is not proportional to the reduction in CMRO₂. Although we do not advocate hemodilution to these extreme values, we find that these data provide a physiological foundation for our hemodilution practice and may provide some guidance for management of Her as body temperature changes during CPB.