Controlled trials23–25 of PSV in patients with stable COPD reported conflicting results on short-term clinical and functional outcomes. In patients with stable COPD and chronic hypercapnia, Vitacca et al26evaluated the short-term physiologic effects of two settings of nasal PSV (empirical setting vs physiologic setting where the level of inspiratory pressure support and intrinsic positive end-expiratory pressure [PEEPi] was tailored to the patient according to an invasive evaluation). Their study showed that PSV was efficient in improving arterial blood gas measurements and in unloading inspiratory muscles independently, whether it was set taking into account the patient’s comfort and improvement in arterial blood gas measurements (empirical setting), or tailored to the patient’s respiratory muscle efforts and mechanics. However, there was a very important decrease of inefficient inspiratory efforts with the physiologic setting. In our study, we found that PSV increased V̇e, resulting in increased Vt, inspiratory flow, and Ti/Ttot; however, a significant decrease of f was observed in the hypercapnic group, at high PSV and biPAP pressures only. This change in the pattern of breathing should improve alveolar ventilation. Concerning inspiratory mechanics parameters during PSV, WOB and Wdi decreased largely proportionately to the level of pressure. Similar results had been shown by Brochard et al,27 who evaluated the physiologic effects of inspiratory positive airway pressure applied via a face mask in 11 patients with acute exacerbations of COPD. Treatment was associated with increased Vt, decreased f, improved gas exchange, and significant decrease in diaphragmatic activity (as measured by Pdi, the pressure-time product for the diaphragm, and integrated surface diaphragmatic electromyographic activity). Best results were obtained with higher inspiratory positive airway pressures (20 cm H2O vs 12 cm H2O), as it was performed in our study in both normocapnic and hypercapnic states. This also confirms the results of other clinical studies,7,20,27–30 performed in patients with acute exacerbations of COPD, which demonstrated that as the level of the inspiratory pressure was increased, the consequent Vt increased and the f decreased with improved gas exchange; these studies also demonstrated that PSV without continuous positive airway pressure could unload ventilatory muscles during SB, either totally or partially.