Coexistence of OSA may compensate for sleep related reduction in neural respiratory drive in patients with COPD.

Abstract

The mechanisms underlying sleep-related hypoventilation in patients with coexisting COPD and obstructive sleep apnoea (OSA), an overlap syndrome, are incompletely understood. We compared neural respiratory drive expressed as diaphragm electromyogram (EMG_di) and ventilation during stage 2 sleep in patients with COPD alone and patients with overlap syndrome. EMG_di and airflow were recorded during full polysomnography in 14 healthy subjects, 14 patients with OSA and 39 consecutive patients with COPD. The ratio of tidal volume to EMG_di was measured to indirectly assess upper airway resistance. Thirty-five patients with COPD, 12 healthy subjects and 14 patients with OSA completed the study. Of 35 patients with COPD, 19 had COPD alone (FEV₁ 38.5%±16.3%) whereas 16 had an overlap syndrome (FEV₁ 47.5±16.2%, AHI 20.5±14.1 events/hour). Ventilation (V_E) was lower during stage 2 sleep than wakefulness in both patients with COPD alone (8.6±2.0 to 6.5±1.5 L/min, p<0.001) and those with overlap syndrome (8.3±2.0 to 6.1±1.8 L/min). Neural respiratory drive from wakefulness to sleep decreased significantly for patients with COPD alone (29.5±13.3% to 23.0±8.9% of maximal, p<0.01) but it changed little in those with overlap syndrome. The ratio of tidal volume to EMG_di was unchanged from wakefulness to sleep in patients with COPD alone and healthy subjects but was significantly reduced in patients with OSA or overlap syndrome (p<0.05). Stage 2 sleep-related hypoventilation in COPD alone is due to reduction of neural respiratory drive, but in overlap syndrome it is due to increased upper airway resistance.