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

Abstract

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.

Studies on the First Derivative of the Ventricular Pressure Pulse in Man

“…On the other hand, an acutely induced increase in the peak ventricular pressure or increase in heart rate tended to augment the negative PD (Table III) (18). The observations of these and other investigators (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11) are all compatible with the view that the rate at which ventricular pressure is developed reflects a fundamental property of contracting myocardium. Accordingly, it was thought that measurement of this parameter would provide an approach to the study of ventricular contractility in intact, unanesthetized man.…”

“…However, Zeig, Buckley and Porter have recently shown that contraction and relaxation can be altered independently by acutely changing ventricular work loads (11). SUM …”

“…Conversely, when myocardial ischemia was induced, the slope of the ventricular pressure rise decreased in the face of a rising end-diastolic pressure (6). More recently, although further observations have been made in the dog on the determinants of the slope of the ventricular pressure pulse (7)(8)(9)(10)(11), there have been no systematic studies of the effects of physiologic stresses, pharmacologic agents, and various types of heart disease on the slope of the ventricular pressure pulse in man.…”