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Theoretical analysis of the noncardiac limits to maximum exercise.

When right atrial pressure (Pra) is greater than zero (atmospheric pressure), cardiac output is determined by the intersection of two functions, cardiac function and return function, which is used here to mean the determinants of venous return. When Pra < or = 0, flow is only determined by circuit function. The objective of this analysis was to determine the potential changes in return function that need to occur to allow the maximum cardiac output during exercise when Pra < or = 0 or is constant. The analysis expands on the model of Green and Jackman and includes the effects of changes in circuit parameters, including venous resistance, changes in capacitance, and muscle contractions. The analysis is based on the model of the circulation proposed by Permutt and co-workers, which assumes that the systemic circulation has two lumped compliant regions in parallel with independent inflow and outflow resistances. Changes in total flow in this model can come about by changes in the distribution of flow between the regions, recruitment of unstressed vascular volume, and changes in the regional venous resistances. The data for the analysis are from previous animal studies and are normalized to a 70-kg man. The major conclusions are that, to achieve the high cardiac output that occurs at peak exercise, there need to be marked changes in the distribution of blood flow, recruitment of unstressed volume, and the venous resistance draining vascular beds. A consequence of the increase in peripheral flow is a marked increase in pressure in the veins of the working muscle. Muscle contractions are potentially a very important mechanism for transiently decreasing this pressure and preventing excessive filtration of plasma during exercise.