Ventilatory and metabolic adaptations to walking and cycling in patients with COPD

To test the hypothesis that in chronic obstructive pulmonary disease (COPD) patients the ventilatory and metabolic requirements during cycling and walking exercise are different, paralleling the level of breathlessness, we studied nine patients with moderate to severe, stable COPD. Each subject underwent two exercise protocols: a 1-min incremental cycle ergometer exercise (C) and a 'shuttle' walking test (W). Oxygen uptake (V̇O₂), CO₂ output (V̇CO₂), minute ventilation (V̇E), and heart rate (HR) were measured with a portable telemetric system. Venous blood lactates were monitored. Measurements of arterial blood gases and pH were obtained in seven patients. Physiological dead space-tidal volume ratio (VD/VT) was computed. At peak exercise W vs. C V̇O₂, V̇E, and HR values were similar, whereas V̇CO₂ (848 ± 69 vs. 1,225 ± 45 ml/min; P <0.001) and lactate (1.5 ± 0.2 vs. 4.1 ± 0.2 meq/l; P <0.001) were lower ΔV̇E/ΔV̇CO₂ (35.7 ± 1.7 vs. 25.9 ± 1.3; P <0.001) and ΔHR/ΔV̇O₂ values (51 ± 3 vs. 40 ± 4; P <0.05) were significantly higher. Analyses of arterial blood gases at peak exercise revealed higher VD/VT and lower arterial partial pressure of oxygen values for W compared with C. In COPD, reduced walking capacity is associated with an excessively high ventilatory demand. Decreased pulmonary gas exchange efficiency and arterial hypoxemia are likely to be responsible for the observed findings.