Effect of hematocrit on cerebral blood flow

Cerebral blood flow (CBF) falls as hematocrit (Hct) rises. Investigators have differed on the relative importance of the increases in arterial O₂ content (Ca(O₂) and red blood cell concentration in mediating the fall. Our experimental protocol attempted to determine the independent effects of these two variables. In 13 unanesthetized lambs (<7 days old) we measured arterial and sagittal sinus blood gases, and O₂ contents, and CBF (microsphere technique) at oxyhemoblobin Hcts of approximately 20 and 40% and after an isovolumic exchange transfusion with a mixture of normal and pure methemoglobin (MHb) containing red cells. Following MHb exchange, Hct rose (19.7 ± 0.3 vs. 38.2 ± 0.4%, X̄ ± SEM) with little change in Ca(O₂) (9.3 ± 0.2 vs. 10.0 ± 0.3 vol%). Arterial PCO₂, pH, mean arterial blood pressure, and cerebral O₂ consumption (CMR(O₂)) did not change. However, CBF fell (153 ± 11 vs. 110 ± 7 ml.100 g^-1.min^-1). CBF declined further when Ca(O₂) rose (17.3 ± 0.5 vol%) at the higher oxyhemoglobin Hct (36.9 ± 0.8%). We calculated that the increase in red cell concentration accounted for 56% of the decrease in CBF that ordinarily occurs as Hct rises from 20 to 40%. The effect of red cell concentration on CBF varied among individual animals. It correlated closely (r = -0.77) with the initial cerebral fractional O₂ extraction [E = CMR(O₂)/(CBF x Ca(O₂))]. Animals with the most luxuriant O₂ supply (CBF x Ca(O₂)) relative to demand (CMR(O₂)) had the greatest decrements in CBF as red blood cell concentration rose.