Anaerobic threshold (AT) from cardiopulmonary exercise tests (CPX) is the standard for measuring exercise intensity among patients with cardiovascular disease in Japan. However, it remains controversial whether AT represents the safety limit for exercise intensity in patients with cardiovascular disease. The purpose of this study was to investigate cardiac rehabilitation (CR) efficacy and safety with exercise intensities above the AT and at a traditional AT in a randomized trial. The participants included 57 patients who were admitted to the outpatient CR unit with a diagnosis of acute myocardial infarction. The participants were randomly divided as follows: 25 patients in the AT group, who performed aerobic exercises with an intensity at the AT; and 32 patients in the "Over AT" group, who performed exercises at an intensity higher than the AT. The following components were measured: maximum oxygen uptake (peak VO2), oxygen uptake at the AT (AT VO2), increase in oxygen uptake during exercise (ΔVO2/ΔWR) during the CPX, vascular endothelial function test (%FMD: the percentage of flow-mediated dilation), and isometric knee extension strength. The measurements were obtained at the start of the exercise therapy and after 2, 3, and 4 months. They were compared within and between groups, and the correlation between the rates of improvement was investigated. Peak VO2, AT VO2, ΔVO2/ΔWR, and %FMD had significantly improved after 3 months in both groups. The isometric knee extension strength had improved in the "Over AT" group after 2 months. Interactions were observed with peak VO2, ΔVO2/ΔWR, and isometric knee extension strength. However, %FMD was not significantly different between the groups. In the "Over AT" group, the rate of improvement in peak VO2 was positively correlated with the improvement in the isometric knee extension strength (r = 0.61, p < 0.001), but not with %FMD. These data suggest that exercise at an intensity above the AT improved exercise tolerance faster than that at the AT, and this improvement rate was associated with changes in isometric knee extension strength.
Keywords: Acute myocardial infarction; Cardiac rehabilitation; Exercise intensity above the anaerobic threshold; Exercise tolerance.