Aerobic responses to low level exercise testing following an acute myocardial infarction

Thirty-four patients who had recently sustained an acute myocardial infarction performed low-level exercise testing with analysis of expired gas 7.1 +/- 2.6 days after the event. They were classified as finishers (F) and nonfinishers (NF) of the low-level protocol. The ejection fraction in the NF was 39 +/- 14% vs 56 +/- 17% in the F (p less than 0.01), and the NF had 2.6 +/- 0.8 vessels stenosed vs 1.8 +/- 0.9 vessels stenosed in the F (p less than .05). Ten normal subjects also performed the exercise test. At the same workload, patients with recent myocardial infarction had significantly lower oxygen consumption (NF less than F), significantly higher minute ventilation (NF greater than F), ventilatory equivalent for oxygen (NF greater than F), and higher respiratory exchange ratio (NF greater than F) than did normal subjects. The heart rate responses were higher in the post infarction patients than in normal subjects. The oxygen pulse was significantly lower in the cardiac patients compared to normals. These findings suggest that during the early recovery phase from an acute myocardial infarction, patients, particularly the NF, utilize less oxygen at submaximal work loads than do normal subjects. This suggests that in these patients part of the energy requirements for exercise are met anaerobically. This could be due to abnormal extraction of oxygen by the working muscles or as a result of poor delivery of oxygen due to abnormal left ventricular function.