Determinants of the exercise endurance capacity in patients with chronic obstructive pulmonary disease - The power-duration relationship

To characterize the determinants of the power-duration (W over dot -t) rela tionship in patients with chronic obstructive pulmonary disease (COPD), we evaluated 8 nonhypoxemic patients (FEV1 = 1.27 +/- 0.26 L) and 10 healthy c ontrols. After an initial maximum-incremental exercise test on cycle ergome ter (peak), the subjects underwent four high-intensity constant-load tests to the limit of tolerance (t), each on different days. The W over dot -t re lationship was found to be hyperbolic in both groups. Absolute values of bo th the critical power asymptote (theta(F)) and the curvature constant (W') were lower in patients than in control subjects. However, when expressed as percentage of peak work rate theta(F) was significantly higher in patients compared with control subjects (81.8 +/- 3.3% versus 67.5 +/- 3.7%, respec tively, p < 0.01). There were severe reductions in t in the patients that w ere consistently associated with higher breathlessness scores and V over do t (E)/MVV ratios. Interestingly, all patients were able to sustain exercise at BF for 20 min despite near-maximum physiological and subjective stresse s. We conclude that the reductions of both parameters of the hyperbolic W o ver dot -t relationship (theta(F) and W') in patients with COPD were due to the ventilatory constraints and their sensory consequences. Importantly, t heta(F) separated a "sustainable" from a "nonsustainable" exercise-intensit y domain: this parameter consistently occurred closer to peak work rate in patients than the healthy control subjects.