<qa>  What is the purpose of this study?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. 