Coronary hyperperfusion augments myocardial oxygen consumption

This study was performed to test the hypothesis that increases in myocardial oxygen consumption (MV̇O₂) and myocardial contractile function during exercise are flow limited. Studies were performed in 15 chronically instrumented normal dogs. MV̇O₂ and regional percent systolic wall thickening were measured during control conditions and during maximal vasodilation produced by infusion of adenosine (20-75 μg·kg^-1·min^-1) or adenosine combined with nitroglycerin (0.4 μg·kg^-1·min^-1; TNG) into the left anterior descending coronary artery during a three-stage graded treadmill exercise protocol. Adenosine and adenosine plus TNG significantly increased coronary blood flow by 298 ± 26 and 306 ± 24%, respectively, at rest and by 134 ± 7 and 145 ± 9%, respectively, during the heaviest level of exercise (each P < 0.01). Adenosine and adenosine plus TNG increased MV̇O₂ at rest, but this was associated with a parallel increase in heart rate, so that MV̇O₂ per beat was not significantly changed. Systolic wall thickening was also not changed by hyperperfusion during resting conditions. However, MV̇O₂ per beat was increased by 12 ± 4% with adenosine and by 13 ± 5% with adenosine plus TNG during moderate exercise and by 23 ± 5% with adenosine and by 27 ± 4% with adenosine plus TNG during the heaviest level of exercise (each P < 0.05). Systolic thickening of the full left ventricular wall did not change during hyperperfusion, but thickening in the subepicardial layer was increased by 14 ± 3% with adenosine and 18 ± 3% with adenosine plus TNG during the heaviest level of exercise (each P < 0.05). There was no difference in wall thickening between adenosine and adenosine plus TNG. These findings imply that the increases in MV̇O₂ which occur during exercise are limited by coronary blood flow.