Background: Daytime hypercapnia is common in morbidly obese patients with obstructive sleep apnea and is associated with serious complications. Our objective was to quantify the effect of adherence with positive airway pressure on hypercapnia and hypoxia in these patients.
Methods: We performed a retrospective cohort study of 75 patients using a multivariable general linear model analysis to identify variables that predicted changes in PaCO2 and PaO2 after therapy. Bootstrap resampling methods were used to calculate confidence intervals for the effects of significant predictors and to internally validate the predictive models.
Results: The variables that predicted the change in PaCO2 were average daily hours of positive pressure therapy, FEV1 percentage of predicted, and baseline PaCO2 (model R(2) = 0.70). The PaCO2 dropped 1.84 mm Hg per hour of adherence and plateaued at 7 hours of average daily use. The PaO2 improved by approximately 3 mm Hg per hour of adherence and plateaued after 4.5 hours of therapy (model R2 = 0.48). Patients who used therapy for more than 4.5 hours per day experienced significant improvements in PaCO2 and PaO2 compared with less-adherent patients (APaCO2 7.7+/-5 vs 2.4+/-4 mm Hg, p < .001; delta PaO2 9.2+/-11 vs 1.8+/-9 mm Hg, p < .001). For adherent patients, the need for daytime home oxygen therapy decreased from 30% to 6% (p = .02).
Conclusion: In hypercapnic patients with obstructive sleep apnea, adherence with positive airway pressure is an important modifiable predictor of improvements in PaCO2 and PaO2, and its benefit plateaus between 5 and 7 hours of daily therapy.