Myocardial oxygen consumption during atrial pacing at various inotropic levels.

In anaesthetized open-chest pigs (n = 15) we examined whether myocardial oxygen consumption (MVO2) per min increased in proportion to heart rate during right atrial pacing at control, high and low inotropy. By modulating aortic constriction and the circulating blood volume, left ventricular (LV) systolic blood pressure, stroke volume and LV dimensions were kept constant. Examinations at control inotropy (n = 7) showed a linear relationship between increments in MVO2 beat-1 and LV dP/dt when heart rate was increased in four steps, each of 10 beats min-1 from 130 +/- 3 beats min-1 (r = 0.76 +/- 0.08). In a second series (n = 8) heart rate was increased by 36-37 beats min-1 in control experiments, during intracoronary isoproterenol infusion (high inotropy) and after propranolol administration (low inotropy). The increments in MVO2 min-1 during pacing tachycardia were not significantly different at control, high or low inotropy. At high inotropy MVO2 beat-1 and LV dP/dt did not rise significantly during pacing tachycardia. Myocardial oxygen consumption beat-1 increased more at control (6.3 +/- 2.0%) than at high inotropy (diff: P less than 0.02). At low inotropy MVO2 beat-1 increased even more (17.4 +/- 2.8%) than at control inotropy (diff: P less than 0.05). Thus, the increase in MVO2 beat-1 during pacing tachycardia is related to the increase in LVdP/dt and is dependent on the level of inotropy; great increments during tachycardia after propranolol administration and no changes during intracoronary isoproterenol infusion.