Postural effects on the baselines of ventricular performance.

In both dogs and human subjects, fully relaxed in the horizontal position, the ventricular dimensions and stroke volume are generally at or near maximal levels. During exertion the stroke volume does not progressively increase, indeed it rarely exceeds recumbent control values. On sitting or standing the ventricular size and stroke volume diminish to a new lower control level. When control values are measured during quiet standing, exertion produces a slight but consistent increase in stroke volume approximately to the level found in recumbency. This difference in the baseline for control values must be considered in evaluating the cardiovascular response to exercise.