The characteristics of cardiac muscle contraction and relaxation have been extensively investigated using isolated heart muscle preparations. In such preparations, the effects of changes in preload (Starling’s law of the heart) and in contractility have been clearly defined, and many indices of contractility described. These indices, extensively validated in isolated heart muscle preparations, have been used to assess the inotropic state of the intact heart. However, the intact heart is not a band of muscle lifting a weight, it is a muscle pump ejecting a viscous fluid (the blood) into viscoelastic systems (the systemic and pulmonary circulations). In order to characterize the behaviour of the cardiac pump, the dynamic relationships between pressure and dimensions may be more appropriate than the analysis of pressure alone.