Myocardial energetics and efficiency: current status of the noninvasive approach.

The heart is an aerobic organ that relies almost exclusively on the aerobic oxidation of substrates for generation of energy. Consequently, there is close coupling between myocardial oxygen consumption (MV̇O2) and the main determinants of systolic function: heart rate, contractile state, and wall stress.1 As in any mechanical pump, only part of the energy invested is converted to external power. In the case of the heart, the ratio of useful energy produced (ie, stroke work [SW]) to oxygen consumed is defined as mechanical efficiency, as originally proposed by Bing et al.2 Under normal conditions this ratio is 25%, and the residual energy mainly dissipates as heat.3 In pathophysiological disease states, such as heart failure, mechanical efficiency is reduced, and it has been hypothesized that the increased energy expenditure relative to work contributes to progression of the disease.4,5 Moreover, therapeutic interventions that enhance mechanical efficiency have proven to be beneficial with respect to outcome.6 It is therefore desirable to quantify efficiency of the heart to study disease processes and monitor interventions. Both cardiac oxidative metabolism and mechanical work, and thus efficiency, can be quantified through invasive measurements. Although these measurements are accurate and currently considered the gold standard, in clinical practice they are limited because of the need for dual-sided heart catheterization and selective catheterization of the coronary sinus. Recent advances in imaging techniques, however, offer the possibility to noninvasively estimate MV̇O2 and mechanical work by positron emission tomography and echocardiography or by magnetic resonance imaging, respectively. This review discusses the principles of mechanical efficiency, together with its invasive and noninvasive assessment, as well as their strengths and pitfalls. Finally, results from clinical pathophysiological studies are discussed.