Abstract
Cerebral blood flow (CBF) falls as hematocrit (Hct) rises. Investigators have differed on the relative importance of the increases in arterial oxygen content (CaO2) and blood viscosity (η) in mediating the fall in CBF. In a previous abstract (Pediatr. Res. 18:377A, 1984) we reported preliminary work in newborn lambs and concluded that the increase in η as Hct rose from 20% to 40% accounted for 45% of the decrease in CBF. Subsequent studies have confirmed this finding. In 18 lambs we performed an isovolemic exchange transfusion with pure methemoglobin containing adult sheep red cells and raised Hct (19.8±0.3 vs 39.0±0.5%; X±SEM) and η (2.6±0.1 vs 4.3±0.2 centipoise: shear rate=230/sec) with minimal increase in CaO2 (9.3±0.2 vs 10.2±0.2 vol%). Other variables known to influence CBF (PaCO2, pH, P50, mean arterial blood pressure) did not change. Similarly, cerebral O2 consumption (CMRO2) was constant. The fall in CBF (microsphere technique: 141±8 vs 103±5 ml/100g/min) must be attributed to the increase in η alone. On the basis of 18 lambs we calculate that the increase in η accounts for 64% of the decrease in CBF as Hct rises from 20% to 40%. The effect of η varied widely among animals, but correlated closely (r = 0.81) with the baseline cerebral fractional O2 extraction E = [CMRO2/(CBFxCaO2)]. Animals with the most luxuriant oxygen supply (CBFxCaO2) relative to demand (CMRO2) had the greater decrements in CBF.
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Hudak, M., Koehler, R., Rosenberg, A. et al. 1416 VISCOSITY REGULATES CEREBRAL BLOOD FLOW INDEPENDENTLY OF ARTERIAL OXYGEN CONTENT. Pediatr Res 19, 346 (1985). https://doi.org/10.1203/00006450-198504000-01440
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DOI: https://doi.org/10.1203/00006450-198504000-01440