Cardiac work and efficiency

Extract

See article by Kiriazis and Gibbs [1] (pages 111–119) in this issue.

1 Work output and efficiency of the heart

1.1 Pressure–volume relation and oxygen consumption

Many researchers have tried to relate cardiac energy metabolism in terms of oxygen consumption or heat production to mechanical performance. One now popular and accepted mechanical parameter is based on the pressure–volume relations found in the intact heart where pressure is plotted as a function of volume over the cardiac cycle [2]. The curve that is followed during the heartbeat consists of the following four parts (Fig. 1, top). Starting at end-diastole (right bottom corner) ventricular pressure increases while volume remains constant (isovolumic contraction), during the ejection phase volume decreases but pressure changes little, and, after closure of the aortic valves isovolumic relaxation ensues, which is followed by the filling phase. The area circumscribed is external mechanical work per beat. The triangular area between the systolic and diastolic pressure–volume relations and bounded by isovolumic relaxation is ‘potential energy’. The sum of the two areas, shaded in Fig. 1 top, is called the pressure–volume area (PVA).