Circulatory Effects of Acute Expansion of Blood Volume:: Studies During Maximal Exercise and at Rest

The maximum cardiac output that can be achieved during exercise might be limited either by extracardiac factors influencing ventricular filling, or by the heart itself. In order to investigate this problem, the effect on the cardiac response to maximum exertion of an acute expansion of blood volume was studied in six men with essentially normal cardiovascular systems. Augmentation of blood volume produced by infusion of 1000 to 1200 ml of the subject's own blood resulted in a small increase in central venous pressure at rest (avg = + 1.9 mm Hg) and a substantial increase in cardiac output (avg = + 1.47 liters/min). During exercise, however, the expansion of blood volume caused a large increase in central venous pressure (avg = + 7.4 mm Hg), but no significant increase in cardiac output or maximum O2 uptake. This finding suggests that the maximum cardiac output is not restricted by extracardiac factors and that the upper limit must therefore be determined by the heart itself.