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Effects of work loads imposed at various times in the cardiac cycle were studied in isolated left ventricle preparations of dog hearts. In six preparations, resistance loads were applied briefly at times in ventricular systole; in six, volume loads were applied briefly at times in ventricular diastole; in six, experimental aortic regurgitation was produced. Simultaneous recorc1s of left ventricular and aortic pressures and electrocardiograms were used to cletermilie the subdivisions of the cardiac cycle and to measure the rates of isometric contraction and relaxation for evaluation of changes in ventricular contractility. The results of many observations indicated that the rates of isometric ventricular contraction and relaxation were not fixed by prior events, that contraction and relaxation could be altered independently, and that aortic regurgitation loaded the left ventricle at a time critical for its relaxation. Accommodation to aortic regurgitation included maintenance of contractility without diastolic left ventricular pressure rise; failure of accommodation was characterized by loss of contractility at high diastolic ventricular pressure.

Effects of Work Loads Applied at Various Times in the Cardiac Cycle of the Isolated Dog Heart