T he 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 (MV O 2 ) 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 Ϸ25%, 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 MV O 2 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.
Invasive Measurement of Mechanical EfficiencyTo calculate the efficiency of the heart, input and output energy must be obtained. The first can be derived from MV O 2 (mL O2 · min Ϫ1 ) measurements according to the Fickprinciple by multiplying coronary sinus blood flow (mL · min Ϫ1 ) by the arteriovenous oxygen content difference. 7Blood flow can be estimated with the use of thermodilution or Doppler (electromagnetic flowmeter) methods after accessing the coronary sinus through right-sided heart catheterization. As oxygen dissolved in blood is negligible and hemoglobin concentrations in arterial and venous blood are similar, arteriovenous oxygen content difference can be obtained by determination of the differences in oxygen saturation levels between arterial and coronary sinus blood. This method to determine oxygen utilization is currently considered the gold standard, although it should be noted that it is limited by its invasive nature, susceptibility to sampling errors, and the fact that only global MV O 2 can be assessed, which also includes oxidative metabolism of the right ventricl...
