Relation between oxygen uptake and carbon dioxide output during constant work rate exercise in patients with mild congestive heart failure

The gas exchange (anaerobic) threshold, a useful index of exercise capacity, is determined as the oxygen uptake (VO2) value at which the ratio of the increase in carbon dioxide output (VCO2) to the increase in VO2 becomes >1 during incremental exercise (V-slope method). However, this method has not been applied to the constant work rate exercise. We evaluated whether a similar threshold phenomenon (i.e., the gas exchange threshold) could be detected during exercise performed at a constant work rate. Thirty-seven patients with a variety of cardiovascular diseases and 27 normal subjects performed symptom-limited incremental exercise and 6 minutes of 50 W constant work rate exercise. The gas exchange threshold could be determined during both symptom-limited incremental exercise (GETi) and constant work rate exercise (GETc) in all subjects, except for 3 normal subjects, by using the V-slope method. There was a significant correlation between GETc and GETi (r = 0.80, p < 0.0001). GETc was significantly correlated with peak VO2 obtained during the incremental exercise test (r = 0.69, p < 0.0001). The results suggest that the gas exchange threshold during constant work rate exercise, which does not require the subjectʹs maximal effort, is a useful measure of exercise capacity in patients with cardiovascular disease.