We read with interest the study by Moretti et al concerning the incidence and causes of failure of non-invasive mechanical ventilation (NIV) after initial success.1 It is an interesting study, but some important factors associated with the initial treatment of chronic obstructive pulmonary disease (COPD) which could influence the results of the study are not addressed. The aim of NIV includes not only the correction of alveolar hypoventilation, but also the unloading of inspiratory muscles. NIV reduces the work of breathing, allowing resting of respiratory muscles and recovery of muscle function. How long had the patients in the two groups been in an acute state before admission to the intensive care units? Where were they before being enrolled in the study—in an emergency unit or in a pneumology ward? Was it the same for the two groups? Perhaps the patients in …