Myocardial oxygen consumption during isotonic and isolumic contractions in the intact heart.

BURNS, JOHN W., AND JAMES W. COVELL. Myocardial oxygen consumption during isotonic and isovolumic contractions in the intact heart. Am. J. Physiol. 223(6): 1491-1497. 1972.-There is now substantial evidence that the oxygen cost of performing external cardiac work is less than that associated with the development of active tension. However, because of the complex geometry of the contracting heart, it has not been possible to measure the oxygen cost of shortening against a load and to directly compare it with that of tension development. In the present study, an isolated supported heart preparation was utilized and left ventricular (LV) wall stress (a> was calculated with an analog computer using a thick-walled spherical model for the LV. Isotonic contractions were produced by rapidly altering LV volume to maintain a constant level of wall c during each beat. In seven experiments myocardial oxygen consumption (Mvo,) during isovolumic contractions was compared to Mqoz during isotonic contractions at matched levels of peak LV wall CT ranging from 14.8 to 105.3 g/cm2. MVoz during isotonic contractions averaged 48.3 =t 3.4 (SEM) pi/beat per 100 g LV and was increased to an average of 52.1 & 3.9 pi/beat per 100 g LV during isotonic contractions at comparable levels of wall C. The increment in MVo2 above that associated with tension development was 5.9 =t 1.1 y0 of the total MVo2, and there was a significant positive correlation between this increment in Mvo2 and the external work performed. When this relationship is extrapolated to the intact heart performing higher levels of external work and shortening to a greater extent, external work might comprise an average of as much as 17 y0 and explain the relatively low oxygen cost of augmenting stroke work by increasing stroke volume.