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The aim of our study was to assess the effect of periodic hyperinflations (sighs) during pressure support ventilation (PSV) on lung volume, gas exchange, and respiratory pattern in patients with early acute respiratory distress syndrome (ARDS).Thirteen patients undergoing PSV were enrolled. The study comprised 3 steps: baseline 1, sigh, and baseline 2, of 1 h each. During baseline 1 and baseline 2, patients underwent PSV. Sighs were administered once per minute by adding to baseline PSV a 3- to 5-s continuous positive airway pressure (CPAP) period, set at a level 20% higher than the peak airway pressure of the PSV breaths or at least 35 cm H2O. Mean airway pressure was kept constant by reducing the positive end-expiratory pressure (PEEP) during the sigh period as required. At the end of each study period, arterial blood gas tensions, air flow and pressures traces, end-expiratory lung volume (EELV), compliance of respiratory system (Crs), and ventilatory parameters were recorded.Pao2 improved (P < 0.001) from baseline 1 (91.4 ± 27.4 mmHg) to sigh (133 ± 42.5 mmHg), without changes of Paco2. EELV increased (P < 0.01) from baseline 1 (1,242 ± 507 ml) to sigh (1,377 ± 484 ml). Crs improved (P < 0.01) from baseline 1 (40.2 ± 12.5 ml/cm H2O) to sigh (45.1 ± 15.3 ml/cm H2O). Tidal volume of pressure-supported breaths and the airway occlusion pressure (P0.1) decreased (P < 0.01) during the sigh period. There were no significant differences between baselines 1 and 2 for all parameters.The addition of 1 sigh per minute during PSV in patients with early ARDS improved gas exchange and lung volume and decreased the respiratory drive.