Predictors of nocturnal oxygen desaturation in patients with COPD.

The aim of this study was to identify factors which might predict nocturnal desaturation (defined as a fall of > 4% from awake baseline level for > or = 5 min) in normoxic or mildly hypoxic patients with stable COPD [arterial O2 saturation (SaO2) > or = 91%]. The study was prospective in nature, had full ethical approval and was performed in the Respiratory Department of a city teaching hospital. Thirty-three patients [mean (SD) age 67.2 (9) years] with stable COPD [mean (SD) FEV1 36.8 (11.0)% pred.] were recruited via the respiratory outpatient clinics and through the respiratory wards. The following parameters were measured: daytime arterial blood gases; spirometry; lung volumes (helium dilution); single breath CO transfer factor (TLCO and KCO); maximum inspiratory (IMP) and expiratory mouth pressures; pulse oximetry (SpO2) across a 6-min walk test, and SpO2 during sleep. Seventeen patients who experienced nocturnal desaturation had significantly lower mean PaO2 and SaO2, and higher PaCO2 values compared to non-desaturators. There was a positive correlation between mean nocturnal SpO2 and daytime PaO2, SaO2, and minimum exercise SpO2, and a negative correlation between mean nocturnal SpO2 and PaCO2, and FRC. Regression analysis revealed that daytime SaO2 was the only independent predictor of mean nocturnal saturation (accounting for 61% of the variability in the mean nocturnal SpO2). We observed nocturnal desaturation in all patients with a daytime SaO2 < or = 93% but in no patient with SaO2 > or = 95%. We conclude that daytime SaO2 can be used to predict nocturnal desaturation in normoxic or mildly hypoxic patients with stable COPD. Nocturnal desaturation is likely in patients with COPD where daytime SaO2 < or = 93%, and unlikely where daytime SaO2 > or = 95%.