Acute hypervolemia, cardiac performance, and aerobic power during exercise.

The relative importance of blood volume (BV) for the maximum aerobic power (VO2 max) was evaluated in healthy subjects by sequential measurements without intervention under two conditions: 1) after hemodilution with a plasma expander, thus increasing BV but keeping red cell mass constant and lowering hemoglobin concentration [Hb], and 2) after whole blood withdrawal, which restored BV to control conditions but reduced red cell mass and [Hb] to equal conditions under 1. After BV expansion (avg 700 ml), we found an unchanged VO2 max compared with control data despite lowered [Hb]. Cardiac output (Q) was increased after BV expansion at rest and during all exercise levels (maximum 27.4 and 29.5 l . min-1, respectively). Peak stroke volume was increased from 144 to 173 ml. Arterial blood pressures were either unchanged or lowered. In contrast, after blood letting to a similar [Hb], we found a significantly reduced VO2 max. These findings indicate a significant influence of the size of the blood volume on cardiac performance. The increased Qmax is discussed in relation to preload, inotropic state, heart rate, and afterload. Plasma volume expansion causes increased preload which may explain this primary effect on the central circulation (Frank-Starling effect).