Determination of the VE/VCO₂ Slope from a Constant Work-Rate Exercise Test in Cardiac Patients

Abstract

Incremental exercise testing to a symptom-limited maximum has been used to measure the ratio of the increase in ventilation (VE) to the increase in CO₂ output (VCO₂) during exercise (VE/VCO₂ slope), a valuable index reflecting the severity of the ventilation-perfusion mismatch in heart failure. Here we studied whether this same value for the slope of VE/VCO₂ could be determined from a short constant work-rate exercise test of moderate intensity. Twenty-three patients with a previous myocardial infarction underwent moderate-intensity (69 ± 15 W) constant work-rate exercise for 6 min and an incremental work-rate exercise test to the max. The VE/VCO₂ slope was calculated from the incremental exercise test from the start of increasing the work-rate to the ventilatory compensation point. The VE/VCO₂ slope was similarly calculated from the start of constant work-rate exercise until the 4th minute, when VE and VCO₂ changed minimally. The VE/VCO₂ slope determined from incremental exercise was 33.8 ± 5.9, ranging from 20.9 to 42.8. The slope obtained from constant work-rate exercise was 32.9 ± 5.7. The VE/VCO₂ slopes obtained from the two exercise tests did not differ significantly. The slope obtained from constant work-rate exercise was significantly positively correlated with the slope obtained from the incremental exercise (r = 0.84, p < 0.0001). The VE/VCO₂ slope can be determined from constant work-rate exercise at a moderate intensity. This indicates that the relationship between ventilation and CO₂ output is consistent and independent of the mode of exercise testing.