[Mechanism of increase in exercise tolerance in patients with acute myocardial infarction]

The contribution of cardiac output reserve and the skeletal muscle to exercise capacity was investigated in 24 patients with acute myocardial infarction. Symptom-limited exercise tests with a cycle ergometer were performed at 1 week, 3 weeks, and 3 months after the onset of the first infarction. Ventilatory gas was analyzed throughout the testing, and peak oxygen uptake (peak VO2) and anaerobic threshold (AT) were determined. During the test, the cardiac index (CI) was measured by the dye dilution method and the change in CI during exercise (delta CI) was calculated as an index of cardiac output reserve. The cross-sectional area of the thigh muscles (CSA) at the level of 10 cm above the patella was measured using computed tomography. Peak VO2 and AT increased significantly from 1 week to 3 months after the onset of infarction. delta CI increased significantly from 1 week to 3 weeks, and CSA increased significantly from 3 weeks to 3 months. Peak VO2 correlated significantly with both delta CI and CSA at each measurement point, as was AT with delta CI and CSA. Change in peak VO2 correlated with change of delta CI from 1 week to 3 weeks, and also with both delta CI and CSA from 3 weeks to 3 months. These results suggest that both cardiac output reserve and peripheral factors contribute to the exercise capacity up to 3 months after the onset of myocardial infarction. In particular, peripheral factors such as muscle volume are important to improve exercise capacity from 3 weeks to 3 months.