Cerebral response to hemodilution during hypothermic cardiopulmonary bypass in adults

We examined the cerebral response to changing hematocrit during hypothermic cardiopulmonary bypass (CPB) in 18 adults. Cerebral blood flow (CBF), cerebral metabolic rate for oxygen (CMRO₂), and cerebral oxygen delivery (CDO₂) were determined using the nitrous oxide saturation technique. Measurements were obtained before CPB at 36°C, and twice during 27°C CPB: first with a hemoglobin (Hgb) of 6.2 ± 1.2 g/dL and then with a Hgb of 8.5 ± 1.2 g/dL. During hypothermia, appropriate reductions in CMRO₂ were demonstrated, but hemodilution-associated increases in CBF offset the reduction in CBF seen with hypothermia. At 27°C CPB, as the Hgb concentration was increased from 6.2 to 8.5 g/dL, CBF decreased. CDO₂ and CMRO₂ were no different whether the Hgb was 6.2 or 8.5 g/dL. In eight patients in whom the Hgb was less than 6 g/dL, CDO₂ remained more than twice CMRO₂. Implications: This study suggests that cerebral oxygen balance during cardiopulmonary bypass is well maintained at more pronounced levels of hemodilution than are typically practiced, because changes in cerebral blood flow compensate for changes in hemoglobin concentration.