A critical look at currently used indirect indices of myocardial oxygen consumption.

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Myocardial oxygen consumption indices that are frequently applied to man such as tension-time index (TTI), pressure-rate product (P·HR) and triple product (TP) have not been fully validated so far. These easily obtainable indices and a modified TTI\((\overline P \cdot \sqrt {HR} )\), therefore, were examined in 10 closed-chest dogs with very broad variations of hemodynamics and oxygen consumption (3–36 ml/min·100 g) analyzing 162 steady states. Myocardial blood flow was directly measured by a differential pressure coronary sinus catheter.\(M\dot VO_2 \) was varied by administration of catecholamines and other inotropic drugs, atropine, beta-blocking agents and hypo- and hypervolemia. Over a wide range of hemodynamic states, correlations with directly measured\(M\dot VO_2 \) of TTI (r=0.63), P·HR (r=0.87), TP (r=0.65) and\(\overline P \cdot \sqrt {HR} \) (r=0.80) are not satisfactory due to neglect of contractility and cardiac volumes by these terms. Better correlations are obtained when relating these indices to\(M\dot VO_2 \) under different inotropic states. At normal and moderately increased contractility, correlations with\(M\dot VO_2 \) rose as follows: TTI (r=0.96), P·HR (r=0.91), TP (r=0.96) and\(\overline P \cdot \sqrt {HR} \) (r=0.94). Significant rises in correlation are due to the close relationship between peak pressure and dP/dtmax at only moderately increased contraction velocity. Correlation differences within this inotropic range must be related to incorporation or neglect of ejection time as a partial determinant of\(M\dot VO_2 \). At markedly increased contractility, results for these indices, however, are in part very poor: TTI (r=0.40), P·HR (r=0.81), TP (r=0.38) and\(\overline P \cdot \sqrt {HR} \) (r=0.76). Within this inotropic state neglect of dP/dtmax as a major determinant of\(M\dot VO_2 \) and the inverse relationship between ejection time and dP/dtmax mainly account for these correlation shifts. It is concluded that non-invasively obtainable indices, currently in use, are no reliable predictors of actual overall\(M\dot VO_2 \) of the left ventricle if the contractile state of the myocardium is not checked invasively before. The broad variability of the relation of the energy demand of velocity of tension development to maintenance of systolic wall tension is not sufficiently considered by these terms. Appropriate caution, therefore, is necessary when applying those indirect indices of\(M\dot VO_2 \) to humans.