Minimum hematocrit for normothermic cardiopulmonary bypass in dogs

Background: The purpose of this study was to determine the minimum hematocrit supporting cerebral oxygenation during normothermic cardiopulmonary bypass (CPB) 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 hematocrits. Methods and Results: Measurements were obtained during 37.5°C CPB with progressive normovolemic hemodilution (hematocrit 0.39 to 0.9) in eight anesthetized animals. Dextran 70 (6%) was used as a diluent. CBF was measured using the sagittal sinus outflow technique. CMRO₂ and CDO₂ were calculated using standard formula. Hemodilution was associated with a reciprocal rise in CBF. CBF at a hematocrit of 0.09 was 240% of the CBF when the hematocrit was 0.39. Increases in CBF compensated for decreased arterial oxygen content and CDo₂ was maintained to a mean hematocrit of 0.14±0.02. At a hematocrit of 0.09±0.02, CDO₂ and CMRO₂ declined. Intracranial pressure remained stable throughout. Thus, the critical hematocrit for brain (the hematocrit at which metabolism becomes delivery-dependent) was between 0.09 and 0.14 during normothermic CPB in dogs. Conclusions: This is the first systematic attempt to determine the critical hematocrit supporting cerebral oxygenation during warm CPB. The curve describing the relationship between hematocrit and cerebral oxygen balance has a broad plateau and a genu near a hematocrit value of 0.15. While we do not advocate hemodilution to a hematocrit of 15% during 'warm' CPB, these data provide a physiological foundation for our hemodilution practice and offer an explanation why low hematocrits are tolerated in certain patient populations.