How to measure cardiac energy expenditure

Abstract

Total energy expenditure of the heart can be directly determined by cardiac oxygen consumption (Vo2) according to the energy equivalence of oxygen in aerobic metabolism (1 ml O2 = 19-21 J). However, Vo2 determination of an in situ heart is invasive and not always possible, particularly in clinical settings. To circumvent this problem, various methods to predict Vo2 have been developed over many years. They include external work, total contractile work, systolic pressure, active tension, tension time integral, Vmax, etc. They have high correlations with directly measured Vo2 under limited conditions. Both myocardial tension and contractility have been generally accepted as the primary determinants of Vo2. However, based on detailed mechanoenergetic studies of canine cardiac contractions, we have recently proposed that the total mechanical energy generated by ventricular contraction can be quantitatively assessed by the ventricular pressure-volume area (PVA) which is a specific area bounded by the end-systolic and end-diastolic pressure-volume relations and the systolic pressure-volume trajectory of the ventricle. Myocardial force-length area (FLA) is a muscle version of PVA. Cardiac Vo2 has been shown to linearly correlate with PVA and FLA. This linear relation ascends or descends in proportion to contractility (Emax). These results have shown that PVA is a physiologically sound and reliable predictor of the energy expenditure of the heart.