• D J Cook, T A Orszulak, R C Daly, and I MacVeigh.
• Department of Anesthesiology, Mayo Foundation and Mayo Clinic, Rochester, Minn 55905, USA. cook.david@Mayo.edu
• Circulation. 1997 Nov 4; 96 (9 Suppl): II-200-4.

BackgroundThe 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 ResultsMeasurements were obtained during 37.5 degrees 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.ConclusionsThis 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.

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