Lack of Ventilatory Threshold in Patients with Chronic Obstructive Pulmonary Disease

We investigated whether the ventilatory threshold (VET) could be detected in 25 patients with severe chronic obstructive pulmonary disease (COPD). Exercise on a treadmill was performed until symptom-limited maximum oxygen uptake (VO₂SL) was obtained. VET was absent in 14 patients (56%, VET(-) group) and present in the others (44%, VET(+) group). Basal pulmonary functions and dyspnea index (V_{E, SL}/MVV) were not different between the two groups. Endurance time and exercise tolerance (VO₂SL/bw) were significantly less in VET(-) than in VET(+). In the former group, Pao₂ and pH at maximal exercise decreased and Paco₂ increased significantly, but HCO₃⁻ did not change compared with the corresponding values before exercise. In the latter group, Paco₂ at maximal exercise increased significantly, and pH and HCO₃⁻ decreased significantly compared with the values before exercise, but Pao₂ did not. The changes in Pao₂ and Paco₂ were not different between the two groups, but changes in pH and HCO₃⁻ in VET(+) were greater than those in VET(-). These results suggest that the absence of VET in some COPD patients indicates a lower exercise capacity without producing metabolic acidosis. This may be caused by rapidly developing dyspnea.