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 (V̇ o 2 ) kinetics, we measured the time constant of V̇ o 2 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. V̇ o 2 was calculated from respiratory gas analysis on a breath-by-breath basis. The time constant of V̇ o 2 during the onset of 50-W exercise was 61.4±15.2 seconds in patients with cardiovascular disease, significantly longer (the kinetics of V̇ o 2 were slower) than that in normal subjects (48.8±10.4 seconds, P =.008). The time constant of V̇ o 2 during the onset of exercise was significantly negatively correlated with peak V̇ o 2 ( 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 V̇ o 2 ( r =−.63) and maximum work rate ( r =−.54). Conclusions The time constant of V̇ o 2 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.