EFFECT OF HEMATOCRIT ON CEREBRAL BLOOD FLOW (CBF) IN LAMBS

Abstract

In anemia and polycythemia the relative contributions of changes in arterial O₂ content (CaO₂) and blood viscosity are unclear. We studied this question in 9 unanesthetized lambs (<7 days of age). After initial measurements of arterial and sagittal sinus PO₂, PCO₂, CaO₂, CBF (microsphere technique), cerebral O₂ consumption (CMRO₂), cerebral fractional oxygen extraction (E= CMRO₂/CBFxCaO₂) and the position of the oxyhemoglobin dissociation curve (P₅₀) were made (State 1), we performed an exchange transfusion with methemoglobin-containing adult sheep red cells. This changed hematocrit without changing CaO₂. After exchange, the hematocrit increased from 20 to 40% while CaO₂ changed from 8.8±0.2 (X±SEM) to 10.1±0.2 ml/100 ml. Repeat measurements were made in 1 hour (State 2). PCO₂ and P₅₀ did not change (paired t test). CBF fell (138.8±14.1 vs 102.4±5.4 ml/100g/min, p<0.02), and E rose in each experiment (0.51±0.01 vs 0.60±0.01, p<.001). CMRO₂ was constant (6.2±0.5 vs 6.2±0.4 ml/100g/min). Viscosity rose from 1.7 to 3.7 centipoise (measured at a shear rate of 230/sec.). Four of these animals were given methylene blue to reduce methemoglobin and CaO₂ rose to the value appropriate for the hematocrit. Measurements were repeated (State 3). The ratio (CBF₁-CBF₂)/(CBF₁-CBF₃) represents the proportion of the change in flow resulting from a change in viscosity. This ratio is 0.45. Thus changes in viscosity account for about half of the changes in CBF.

(NIH Grant HD 13830)

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