Editorial Cardiac work and efficiency

See article by Kiriazis and Gibbs [1] (pages 111 –119) two parts, activation and basal oxygen consumption. The in this issue. first depends on excitation–contraction coupling and is proportional to cardiac contractility. For the quiescent muscle the PVA and the oxygen cost of activation both are zero. The remaining part results from all other processes 1. Work output and efficiency of the heart requiring oxygen such as ion pumps. With increased cardiac contractility the oxygen cost of activation (Fig. 1, 1.1. Pressure–volume relation and oxygen consumption bottom) is increased in a proportional manner but the slope of the relation between PVA and oxygen consumption is Many researchers have tried to relate cardiac energy not affected. metabolism in terms of oxygen consumption or heat The relation between PVA and V holds for all types of O2 production to mechanical performance. One now popular contractions, from isovolumic to isobaric (‘unloaded’) and accepted mechanical parameter is based on the prescontractions. Isovolumic contractions correspond to the sure–volume relations found in the intact heart where largest PVA and thus the largest oxygen consumption [3]. pressure is plotted as a function of volume over the cardiac We see that external work is not a good measure of oxygen cycle [2]. The curve that is followed during the heartbeat consumption. consists of the following four parts (Fig. 1, top). Starting at Many other relations between oxygen consumption and end-diastole (right bottom corner) ventricular pressure mechanical or hemodynamic parameters have been proincreases while volume remains constant (isovolumic posed. The two best known are the rate pressure product contraction), during the ejection phase volume decreases (RPP) and the tension time index (TTI). but pressure changes little, and, after closure of the aortic The RPP is the product of systolic pressure (P ) and s valves isovolumic relaxation ensues, which is followed by heart rate and is used as a measure of oxygen consumption the filling phase. The area circumscribed is external in many biochemical studies. When these studies are mechanical work per beat. The triangular area between the performed in isolated hearts in Langendorff perfusion, i.e. systolic and diastolic pressure–volume relations and no external work is performed, the PVA(0.5P (V 2 s ed 2 bounded by isovolumic relaxation is ‘potential energy’. V )(0.5P /E , with V is end-diastolic volume, and V 0 s s ed 0 The sum of the two areas, shaded in Fig. 1 top, is called and E are the volume axis intercept and the slope of the s the pressure–volume area (PVA). systolic the pressure–volume relation (Fig. 1, top). In other It was shown that the oxygen consumption per beat words, for constant contractility (constant E ) the P relates s s (V ) is linearly related to the PVA (Fig. 1, bottom). To in a quadratic way to the PVA. With an increase in O2 derive oxygen consumption per minute, V may be contractility but constant filling (constant V ) the PVA O ed 2 multiplied by heart rate. The intercept with the V axis is increases in a linear way with P but the intercept of O s 2 the ‘unloaded’ oxygen consumption, found when the heart relation between PVA and oxygen consumption is altered. is not allowed to develop pressure so that the PVA is Thus the RPP, as a measure of cardiac metabolism should negligible. The ‘unloaded’ oxygen consumption consists of be used with care. The use of the TTI [4], the averaged pressure over the ejection period, is also limited because for an isovolumic *Tel.: 131-20-444-8111; fax: 131-20-444-8255. E-mail address: [email protected] (N. Westerhof). contraction the TTI is negligible (ejection time negligible).