Pulmonary gas exchange abnormalities in mild chronic obstructive pulmonary disease: Implications for dyspnea and exercise intolerance

Rationale: Several studies in mild chronic obstructive pulmonary disease (COPD) have shown a higher than normal ventilatory equivalent for carbon dioxide (VE/VCO2) during exercise. Our objective was to examine pulmonary gas exchange abnormalities and the mechanisms of high VE/VCO2 in mild COPD and its impact on dyspnea and exercise intolerance. Methods: Twenty-two subjects (11 patients with GOLD [Global Initiative for Chronic Obstructive Lung Disease] grade 1B COPD, 11 age-matched healthy control subjects) undertook physiological testing and a symptom-limited incremental cycle exercise test with arterial blood gas collection. Measurements and Main Results: Patients (post-bronchodilator FEV1: 94 ± 10% predicted; mean ± SD) had evidence of peripheral airway dysfunction and reduced peak oxygen uptake compared with control subjects (80 ± 18 vs. 113 ± 24% predicted; P < 0.05). Arterial blood gases were within the normal range and effective alveolar ventilation was not significantly different from control subjects throughout exercise. The alveolar-arterial O2 tension gradient was elevated at rest and throughout exercise in COPD (P < 0.05). VE/VCO2, dead space to tidal volume ratio (VD/VT), and arterial to end-tidal CO2 difference were all higher (P < 0.05) in patients with COPD than in control subjects during exercise. In patients with COPD versus control subjects, there was significant dynamic hyperinflation and greater tidal volume constraints (P < 0.05). Standardized dyspnea intensity ratings were also higher (P < 0.05) in patients with COPD versus control subjects in association with higher ventilatory requirements. Within all subjects, VD/VT correlated with the VE/VCO2 ratio during submaximal exercise (r = 0.780, P < 0.001). Conclusions: High VD/VT was the most consistent gas exchange abnormality in smokers with only mild spirometric abnormalities. Compensatory increases in minute ventilation during exercise maintained alveolar ventilation and arterial blood gas homeostasis but at the expense of earlier dynamic mechanical constraints, greater dyspnea, and exercise intolerance in mild COPD.