Myocardial energetics: experimental and clinical studies to address its determinants and aerobic limit

Summary

The major determinants of myocardial oxygen consumption (M$\dot V$O₂) were examined in the isolated, servo-regulated, canine heart in which coronary perfusion pressure, heart rate, ventricular volume and pressure could be individually monitored and controlled. We found that the integral of systolic wall force and the time derivative of systolic force development were major determinants of M$\dot V$O₂. Net M$\dot V$O₂, seen in response to increments in contractility (dobutamine) and heart rate, were the result of the relative increments in each of these determinants. Additional studies were performed to assess the heart’s metabolic reserve and aerobic limit (i.e., before the onset of lactate production). We found that with increments in left ventricular work, mediated by increments in filling volume, heart rate, and contractility (dobutamine), myocardial lactate production could be induced, but was dependent on the level of coronary perfusion pressure. When the aerobic limit of the myocardium was exceeded, its performance declined and pulsus alternans appeared.

In patients with cardiomegaly and advanced heart failure given the phosphodiesterase inhibitors enoximone and piroximone we did not observe a rise in M$\dot V$O₂ or the appearance of lactate production in the majority of patients. When patients with documented idiopathic (dilated) cardiomyopathy and marked heart failure received hemodynamically significant doses of dobutamine alone or in combination with amrinone, there again was no evidence of lactate production or a rise in M$\dot V$O₂, while a marked improvement in ventricular function was noted. Thus, these agents served to improve the efficiency of the dilated failing heart. Hence we would conclude that in most cases, the dilated failing heart has an adequate metabolic reserve. Its performance, and indeed its efficiency, can be improved with pharmacologic agents having positive inotropic properties without adversely altering myocardial energetics.