The effect of continuous positive airway pressure on pulmonary function may depend on the basal level of forced expiratory volume in 1 second

Background: The coexistence of chronic obstructive pulmonary disease (COPD) and obstructive sleep apnea (OSA), also referred to as overlap syndrome (OS), is associated with a high rate of morbidity, COPD exacerbations and mortality. Treatment with continuous positive airway pressure (CPAP) has proven to significantly decrease the rate of these complications. However, data concerning the effect of CPAP on pulmonary function are scarce and conflicting. The aim of our study was to evaluate the effect of 1 year of CPAP treatment on arterial blood gases (ABGs) and pulmonary function tests in patients with OS and its potential relationship with the baseline severity of airway obstruction. A secondary aim was to search for predictors of changes in the evaluated parameters. Methods: A retrospective study on a cohort of 92 patients (74 males) discharged from the Pulmonary Rehabilitation Unit of the Istituti Clinici Scientifici Maugeri in Pavia (Italy) from January 2013 to January 2016, with a diagnosis of OS and a prescription of CPAP, was conducted. Collected data at discharge were compared with 1-year follow-up data. Results: After 1 year of CPAP, we observed the following: (I) a significant improvement in ABGs in all patients [median pO2 65.0 (59.0-70.0) vs. 71 (64.8-77.1) mmHg, pCO2 39.8 (36.2-43.5) vs. 38.3 (32.3-44.2) at baseline and after 1 year respectively, P<0.001], which was more pronounced in patients who were hypercapnic at baseline; (II) no significant change in respiratory function in the whole population; (III) a significant change in forced expiratory volume in 1 second (FEV1) only under and above a threshold of 79.1% of basal FEV1 with an opposite trend. In particular, patients with a basal FEV1 below that threshold significantly improved [median FEV1 70 (-70 to 200) mL, P=0.001], whereas patients with a basal FEV1 above the same threshold significantly worsened [median FEV1 -270 (-370 to -130) mL, P=3.05μ10-5]. Conclusions: A population of overlap patients treated with CPAP may experience a different change in airflow obstruction after 1 year depending on the severity of baseline obstruction.