Oxygen desaturation during sleep and exercise in patients with severe chronic obstructive pulmonary disease

Patients with chronic obstructive pulmonary disease (COPD) have varying degrees of arterial oxyhaemoglobin desaturation during sleep, which have been shown to correlate with awake oxygen levels. We wished to ascertain if exercise desaturation was a better predictor of nocturnal oxygen desaturation than daytime blood gases. We studied 25 COPD patients with PaO2 < 10 kPa (mean = 8.6 kPa), 12 of whom were normocapnic (PaCO2 < or = 6 kPa, Group A), and 13 of whom were hypercapnic (PaCO2 > 6 kPa, Group B), by means of overnight oximetry and maximum treadmill exercise testing. The overall group desaturated significantly more during sleep than exercise [12.9 +/- 10.5 fall in nocturnal oxygen saturation (SaO2) vs. 4.5 +/- 3.7, P < 0.01]. Group B had a lower minimum SaO2 during sleep than Group A (74.3 +/- 13.4 vs. 84.6 +/- 5.8, P < 0.05), despite very similar pre-sleep SaO2 (91.9 +/- 3.2 vs. 92.8 +/- 2.9, P = n.s.). Awake SaO2 correlated well with both mean values (r = 0.7, P < 0.001), and minimum sleep SaO2 (r = 0.44, P < 0.05), but not with the fall in sleep SaO2 (r = 0.21, P = n.s.). Minimum sleep and exercise SaO2 were also significantly correlated (r = 0.44, P < 0.05), but the fall in SaO2 during sleep and exercise was not (P = n.s.). We conclude that exercise studies add no extra information to awake blood gas analysis in predicting the likelihood of nocturnal oxygen desaturation in patients with COPD.