Mechanisms of exercise intolerance in Global Initiative for Chronic Obstructive Lung Disease grade 1 COPD

The purpose of this study was to determine if a dissociation existed between respiratory drive, as estimated by diaphragmatic electromyography (EMGdi), and its pressure-generating capacity during exercise in mild chronic obstructive pulmonary disease (COPD) and whether this, if present, had negative sensory consequences. Subjects meeting spirometric criteria for mild COPD (n=16) and age and sex-matched controls (n=16) underwent detailed pulmonary function testing and a symptom limited cycle test while detailed ventilatory, sensory and respiratory mechanical responses were measured. Compared with controls, subjects with mild COPD had greater ventilatory requirements throughout submaximal exercise. At the highest equivalent work rate of 60 W, they had a significantly higher: total work of breathing (32 17 versus 16 +/- 7 J.min(-1); p<0.01); EMGdi (37.3 +/- 17.3 versus 17.9 +/- 11.7% of maximum; p<0.001); and EMGdi to transdiaphragmatic pressure ratio (0.87 +/- 0.38 versus 0.52 +/- 0.27; p<0.01). Dyspnoea-ventilation slopes were significantly higher in mild COPD than controls (0.17 +/- 0.12 versus 0.10 +/- 0.05; p<0.05). However, absolute dyspnoea ratings reached significant levels only at high levels of ventilation. Increased respiratory effort and work of breathing, and a wider dissociation between diaphragmatic activation and pressure-generating capacity were found at standardised work rates in subjects with mild COPD compared with controls. Despite these mechanical and neuromuscular abnormalities, significant dyspnoea was only experienced at higher work rates.