Invariant total heart volume in the intact thorax

Fast multisliced computerized tomography (Dynamic Spatial Reconstructor or DSR) was used to study the change in total heart volume (content of the pericardial sac) between end diastole (ED) and end systole (ES) with lungs held at 0 and 15 cmH₂O airway inflation pressure (P(aw)). Nine dogs were anesthetized and scanned in the DSR. At 0 cmH₂O airway pressure, mean total heart volume changed, on the average, only 2.7 ± 0.6% (SE) of its ED volume comparing ED with ES. With lungs inflated to 15 cmH₂O P(aw), total heart volume decreased 12 ± 0.5%. However, at this new heart volume, the change in total heart volume between ED and ES was only 1.7 ± 0.5%. The data indicate that the epicardial apex of the heart remains relatively stationary while the atrioventricular groove moves toward the apex during systole and away from the apex during diastole. Thus the atria and ventricles empty and fill reciprocally even when the pericardial contents do not fully distend the pericardial sac. The invariant total heart volume observed in these species would minimize the work of the heart by maximizing the percentage of work expended to move blood and minimizing the work expended to move tissue (i.e., lung).