Am. J. Respir. Crit. Care Med., Volume 162, Number 2, August 2000, 497-504

Determinants of the Exercise Endurance Capacity in Patients with Chronic Obstructive Pulmonary Disease
The Power-Duration Relationship

J. ALBERTO NEDER, PAUL W. JONES, LUIZ E. NERY, and BRIAN J. WHIPP

Department of Physiology, and Department of Physiological Medicine, St. George's Hospital Medical School, University of London, London, United Kingdom; and Department of Medicine, Respiratory Division, Universidade Federal de São Paulo-Escola Paulista de Medicina (UNIFESP-EPM), São Paulo, Brazil

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

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