Mechanism of increase in anaerobic threshold during recovery phase in patients with acute myocardial infarction.

The contribution of venous return to the increase in anaerobic threshold (AT) in patients with acute myocardial infarction (AMI) was investigated. Twenty-seven patients with uncomplicated AMI underwent supine and sitting cardiopulmonary exercise testing, and 10 out of these 27 patients performed constant-workload tests in which cardiac output before and after a phase II program was measured. Data from 8 patients were eliminated because of restenosis after direct percutaneous transluminal coronary angioplasty. The increase in AT was significantly greater in the sitting position than in the supine position, 15.0+/-9.9% and 6.3+/-6.0% respectively. Average O2 pulse at AT changed non-significantly from 8.6+/-1.5 to 8.7+/-1.2 ml min(-1) beat(-1) in the supine position, but it showed a significant increase from 8.1+/-1.3 to 9.2+/-1.3 ml min(-1) beat(-1) when measured in the sitting position. In the constant-workload studies, stroke index showed a significant increase during both supine and sitting exercise. Percent increase in the stroke index from rest to exercise did not differ significantly in the supine position, but did differ significantly in the sitting position. These results strongly suggest that increased venous return as well as improvement in cardiac pump function play a major role in the mechanism of AT increase in an upright position throughout the recovery phase in AMI patients.