Influence of Nitroprusside and Enoximone

Cardiotonic agents influence myocardial energy consumption by vasodilation, which may reduce energy demand, and by inotropism, which may increase it. To distinguish between the two effects, myocardial oxygen consumption must be analyzed in relation to its hemodynamic determinants. The coupling of myocardial oxygen consumption with its determinants was investigated in 22 patients with idiopathic dilated cardiomyopathy (NYHA Class II and IH). Predicted myocardial oxygen consumption by the pressure-work index, the Bretschneider index, and the pressurevolume area correlated moderately with measured myocardial oxygen consumption (r=0.57, p<O.OOl; r=0.52, p<0.005; and r=0.63, p<0.001). Multiple regression analysis, including left ventricular peak systolic wall stress, systolic stress-time integral, pressure-volume work, maximum rate of left ventricular pressure rise, and mean velocity of circumferential fiber shortening indicated that systolic stress-time integral is the major determinant of myocardial oxygen consumption (r=0.75,p<0.001) in these patients. Enoximone, a phosphodiesterase inhibitor, has an inotropic and a vasodilating effect. To investigate the inotropic portion of the energy cost of this phosphodiesterase inhibitor, the influence of enoximone on myocardial oxygen consumption and systolic stress-time integral was compared with the effects of nitroprusside, which is a vasodilator only. Nitroprusside (10 patients) and enoximone (12 patients) reduced left ventricular systolic stress-time integral from 109±22 to 71±21 (p<0.005) and from 104±23 to 42±+10 (p<0.001) 103 dynes * sec/cm2, respectively. Myocardial oxygen consumption decreased from 159±44 to 112±23 (p<0.005) and from 134±28 to 109±21 (p<0.001) ,ul/beat/100 g, respectively. In both groups, there was a significant correlation between the decrease in myocardial oxygen consumption and the decrease in systolic stress-time integral. The slopes of the respective linear regression lines were significantly different (1.27 for nitroprusside and 0.51 nl * cm2/100 g * dynes * sec for enoximone,pcO.05), indicating a smaller decrease of myocardial oxygen consumption for a given decrease of stress-time integral with enoximone. Applying the pressure-work index or the pressure-volume area instead of systolic stress-time integral yielded comparable results. Thus, vasodilation reduces myocardial oxygen consumption in proportion to the reduction of stress-time integral. With enoximone, the energy-saving effect of vasodilation is counteracted in part by the increased energy demand of inotropic stimulation. (Circulation 1989;80:51-64)