PSV is a pressure-targeted mode of ventilation, that provides breath-by-breath patient-triggered support, synchronized with the patient's inspiratory effort . Following the detection of patient's inspiratory effort, a demand valve allows the airway pressure to rise to a pre-set level, which is maintained until the detection of the patient's wish to expire. At this stage an expiratory trigger system stops the support and allows the airway pressure to drop down to the expiratory level. Expiration is therefore passive in principle, while a selected level of positive end-expiratory pressure (PEEP) can be applied. Thus, PSV requires a patient's intact drive to breathe. As increasing levels of pressure support are used, the inspiratory muscles progressively unload and the patient's inspiratory work of breathing (WOB) and the pressure time product (PTP) decrease [5-7]. These changes are commonly associated with a decrease in respiratory rate (RR), in respiratory neural drive and in P0.1 [6,8], while tidalvolume (Vt) shows a tendency to increase [2, 7]. Indeed, the clinician can set the ventilator to obtain a target patient effort by simply adjusting the pressure support level. However, despite these major features, and the claimed better synchronization and patient comfort compared to other popular weaning modes such as SIMV [2, 7], the advantages of PSV in weaning patients from the ventilator are non univocal [9, 10]. Moreover, new ventilatory modes promising better patient ventilator synchronization 
and comfort [12, 13] have partially overcome the advantages of PSV. Still, as recently reported, PSV alone (36% of patients) or in combination with SIMV (28% of patients), is the most frequent weaning ventilatory mode  in North America, South America, Portugal and Spain.
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