The circulatory effects of acute hypervolemia and hemodilution in conscious rabbits.

Our goal was to distinguish the circulatory changes due to acute hypervolemia caused by whole blood and colloid solution from those due to acute hemodilution caused by colloid solution in the conscious rabbit. In six rabbits, homologous whole blood, dextran solution, or modified gelatin solution, equal to 32% of blood volume, was infused over 22 minutes. In six rabbits, 42% of blood volume was exchanged with dextran solution over 34 minutes, at constant left atrial pressure. A similar exchange was performed over 28 minutes in a further six arterial baroreceptor-denervated rabbits. Withdrawn blood later was infused over 10-12 minutes. In all rabbits, the baroreceptor-heart rate reflex was characterized by infusion of phenylephrine and sodium nitroprusside. Whole blood hypervolemia caused stroke volume and blood pressure to rise, without a rise in heart rate. Colloid-solution hypervolemia caused stroke volume and heart rate to rise, with little change in blood pressure. The effects of hemodilution were the same as those of dextran-solution hypervolemia, and the fall in hematocrit was similar, but baroreceptor denervation eliminated the rise in heart rate. We conclude that in the conscious rabbit: (1) acute rise in central venous pressure causes tachycardia only when there is concomitant hemodilution; (2) the tachycardia of hemodilution with unchanged central venous pressure is largely baroreceptor-dependent; (3) the effects of hemodilution on systemic vascular conductance appear to be due to reduction in oxygen-carriage rather than in viscosity; (4) the rabbit can closely match increase in conductance by increase in output, which is largely baroreceptor-independent. Circ Res 48: 825-834, 1981