Effect of exhaustive exercise on myocardial performance.

Several investigators have reported ultrastructural changes in hearts of animals exercised to exhaustion. The present study was designed to determine whether functional changes occur in the intact heart at exhaustion. Adult mongrel dogs (n = 8) were chronically instrumented to measure cardiac output, coronary blood flow, aortic blood pressure, left ventricular pressure, +dP/dtmax, and -dP/dtmax. After recovery, the dogs were run to exhaustion at a constant work load, eliciting approximately 70% of maximum heart rate. The exhaustive bouts were terminated when the animals either refused or were unable to continue running, at which time their rectal temperatures approaches 42.2 degree C. The mean exhaustion time was 76.7 +/- 11.8 min. All parameters increased from rest to steady state with the exception of stroke volume (23.2 +/- 4.9 vs. 20.5 +/- 1.6 ml), which remained constant. In the transition from steady state to exhaustion, only +dP/dtmax (6,652 +/- 291 vs. 7,689 +/- 479 Torr/s) and -dP/dtmax (4,110 +/- 227 vs. 4,890 +/- 215 Torr/s) increased significantly; all other values exhibited no significant change. Similarly, when maximum cardiovascular parameters were measured before and after exhaustion, no significant changes were found. These data show that cardiac contractile function is not depressed in dogs as a result of exhaustive exercise.