Ventilation and Gas Exchange During Sleep and Exercise in Severe COPD

¹ From the Department of Respiratory Medicine and the Respiratory Sleep Laboratory, University College Dublin, St. Vincent's Hospital, Dublin, Ireland

Ventilation and gas exchange were studied during sleep and incremental treadmill exercise in 19 patients with severe stable COPD with the primary aim of comparing the pathophysiology of oxygen desaturation in the two conditions. A secondary aim was to determine whether exercise studies could aid in the prediction of sleep desaturation. Full polysomnography was used, and ventilation, arterial oxygen saturation (SaO₂), and transcutaneous PCO₂ (PtcCO₂) were monitored continuously during sleep. No patient had significant sleep apnea. Mean (SD) FEV₁ was 32 (9.1)% predicted, PaO₂ was 71.2 (12.4) mm Hg, and PaCO₂ was 44.5 (4.6) mm Hg. SaO₂ fell twice as much during sleep as during maximum exercise: 13.1 (8.9) vs 6.0 (3.6)% (p<0.001). The mean sleep and exercise SaO₂, and minimum sleep and exercise SaO₂ were well correlated on linear regression (r=0.81 and 0.78, respectively, p<0.001), but on multiple regression analysis, awake PaO₂ was a better predictor of sleep desaturation than was exercise desaturation. The 12 major desaturators (minimum sleep SaO₂ <85%) had twice as great a fall in exercise SaO₂ as the 7 minor desaturators (3.6±2.8 vs 7.4±3.3%, p<0.05). The major desaturators also had a greater fall in estimated sleep PaO₂: 19.8 (5.1) vs 6.4 (7.1) mm Hg (p<0.01), which suggests that their greater sleep desaturation is not simply due to their position on the steep portion of the oxyhemoglobin dissociation curve. The rise in PtcCO₂ during sleep was similar among major and minor desaturators: 7.5 (2.9) vs 5.8 (3.7) mm Hg (p=NS), suggesting that all patients had a similar degree of hypoventilation during sleep, and that the greater fall in SaO₂ and estimated PaO₂ among some patients was secondary to other factors such as increased ventilation-perfusion mismatching.

Key Words: COPD • exercise • gas exchange • sleep • ventilation

Submitted on February 8, 1995
Accepted on September 19, 2007

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