The purpose of this study was to evaluate whether CO2 rebreathing occurs in healthy subjects or patients with COPD ventilated during exercise assisted by the single-limb circuit with a plateau exhalation valve at a given inspiratory positive airway pressure and a minimal level of expiratory positive airway pressure (EPAP) and whether there is a potential threshold for predicting CO2 rebreathing.

Non-invasive ventilation circuit consists of single-limb tubing for inspiration and expiration, including a plateau exhalation valve. As exhaled gases was vented out the plateau exhalation valve at a constant rate, during exercise, with the increase of exhalation flow, single-limb circuit with a plateau exhalation valve might cause carbon dioxide(CO2) rebreathing. Therefore, our aim in the present study was to evaluate whether CO2 rebreathing occurs in COPD patients ventilated during exercise assisted by the single-limb circuit with a plateau exhalation valve and estimate a potential threshold for predicting CO2 rebreathing.

Inclusion Criteria:

• presented clinically stable (no exacerbation in the 4 weeks prior to study participation or with no change in medications);
• physician diagnosed COPD, forced expiratory volume in 1s (FEV1) < 50% predicted;
• dyspnea as a main symptom that limited daily activities.

Exclusion Criteria:

• subjects with obvious pulmonary bullae demonstrated by chest CT scan or X-ray;
• examination or facial trauma/malformation, recent facial, upper airway or upper gastrointestinal tract surgery, that would preclude receiving NIV therapy;
• a history of coronary artery disease or cardiac arrhythmias or potential electrocardiographic alterations;
• a history of uncontrolled hypertension, or other respiratory diseases;
• oxygen saturation(SpO2) < 88% at rest with a fraction of inspired oxygen (FiO2) ≥ 0.5;
• patients with musculoskeletal or neurological disorders;
• or failure to comply with the research protocol.