Each cardiac cycle can be characterized by a pressure-volume loop that graphically depicts the external work of the ventricle. The pressure-volume loop is determined to a large extent by end-diastolic and end-systolic conditions, each of which reflects the properties of the heart and circulation at these two critical times in the cardiac cycle. The pressure and volume at end-diastole and end-systole, in turn, are determined by interactions between the heart and the circulation.

End-diastolic pressure and volume reflect preload (venous return) and the lusitropic (relaxation) state of the ventricular walls, and end-systolic pressure and volume are determined by the afterload (peripheral and pulmonary resistance) and the inotropic (contractile) state of the myocardium. Alterations in inotropic and lusitropic state lead to predictable changes in the pressure-volume loop that, when combined with knowledge of preload and afterload, can facilitate understanding of the pharmacologic and pathophysiologic responses of the heart.