Evaluation of Exercise Capacity Using Submaximal Exercise at a Constant Work Rate in Patients With Cardiovascular Disease

BACKGROUND: Symptom-limited incremental exercise tests are used to estimate the severity of cardiovascular disease and the patient's daily activity. However, there is a need for objective parameters for submaximal exercise. To test the hypothesis that a decrease in maximal exercise capacity can be estimated by oxygen uptake (VO2) kinetics, we measured the time constant of VO2 both during the onset of constant work rate exercise at 50 W and during recovery from this exercise and compared it with data obtained during maximal exercise in patients with cardiovascular disease and in normal subjects. METHODS AND RESULTS: A total of 34 patients with cardiovascular disease and 14 normal subjects performed 6 minutes of 50-W constant work rate exercise and an incremental exercise test to the symptom-limited maximum on a cycle ergometer. VO2 was calculated from respiratory gas analysis on a breath-by-breath basis. The time constant of VO2 during the onset of 50-W exercise was 61.4 +/- 15.2 seconds in patients with cardiovascular disease, significantly longer (the kinetics of VO2 were slower) than that in normal subjects (48.8 +/- 10.4 seconds, P = .008). The time constant of VO2 during the onset of exercise was significantly negatively correlated with peak VO2 (r = -.67) and maximal work rate (r = -.66). The time constant during recovery, which did not differ significantly from that of exercise, was also prolonged in patients with cardiovascular disease; it showed a negative correlation with peak VO2 (r = -.63) and maximum work rate (r = -.54). CONCLUSIONS: The time constant of VO2 during and after recovery from 50 W of constant work rate exercise, which does not require the subject's maximal effort, is a useful and objective measure of exercise capacity in patients with mild to moderate cardiovascular disease.