Ventricular pressure-volume relations in vivo.

A number of fundamental mechanical properties of cardiac muscle as well as of the total ventricle are discussed. These include: Starling's law, shortening deactivation, homeometric autoregulation, and so-called hyperactivation associated with a small amount of ejection. Whenever appropriate, muscle properties are related to those of the intact ventricle. The phenomenon of load-dependence of the end-systolic pressure-volume relationship (ESPVR) receives particular attention and is placed within the framework of shortening deactivation and homeometric autoregulation. However, this effect also leads to a more basic definition of myocardial contractility, based on the observed, rather parallel shift of the ESPVR with different afterload conditions. Using this definition, the conclusion is drawn that an increase in afterload is, in fact, met by the left ventricle increasing its inotropic state. Finally, within this approach, it is proposed that the value of end-systolic volume at a chosen constant end-systolic pressure (e.g. 13 kPa or 100 mmHg) be used to characterize changes in myocardial contractility in patients after an intervention. The way of obtaining this parameter is explained.