Barotrauma is a frequent complication of mechanical ventilation and is associated with increased morbidity and mortality [45,46]. Investigators have reported incidences of barotrauma as low as 0.5% in postoperative patients  and as high as 87% in patients with ARDS . These variations in the reported incidence of ventilator-associated barotrauma may relate to differences in the patients' underlying disease. Barotrauma is increased in patients with severe underlying lung disease, especially in patients with ARDS [30-32, 45, 46, 48, 49, 51], aspiration pneumonia ,Pneumocystiscariniipneumonia ,andpre-existingCOPD [46, 52-55].
Barotrauma has been reported at variable time intervals after the start of mechanical ventilation. In a cohort of 5183 mechanically ventilated patients Anzueto et al.  observed that 80% of the barotrauma appeared within the first 3 days of mechanical ventilation. These results are similar to the report of Gammon et al.  in patients with ARDS and of Schnapp et al.  in patients with multiple underlying conditions.
The development of barotrauma has been associated with ventilator mode , high airway pressures , high PEEP , high Vt , and increased end-expiratory lung volume . However, in a large series of patients with ARDS  and in a cohort of patients mechanically ventilated for more than 24 hours , the development of barotrauma was not related to the use of conventional pressures and volumes. In the study by Anzueto et al.  there were no differences in the measured pressures and Vt among patients with and without barotrauma. Amato et al.  reported a statistically significant relationship with both Vt and PEEP to the frequency of barotrauma. These investigators found that 42% of patients with ARDS who were ventilated with a Vt of 12 ml/kg developed barotrauma compared to only 7% of patients ventilated with a Vt of 6 ml/kg (p< 0.001). In larger studies, however, the incidence of barotrauma did not differ between patients ventilated with and without strategies designed to limit airway pressures and Vt [12, 31, 32].
Ventilator associated pneumonia (VAP), defined as pneumonia occurring more than 48 hours after endotracheal intubation and initiation of mechanical ventilation, complicates the course of 8-28% of the patients receiving mechanical ventilation . Accurate data on the epidemiology of VAP are limited by the lack of standardized criteria for its diagnosis.
Prolonged mechanical ventilation, defined as that lasting for more than 48 hours, is the most important factor associated with nosocomial pneumonia. A prospective Italian study  found that the frequency of VAP rose from 5% for patients receiving mechanical ventilation for one day to 69% for patients who required mechanical ventilation for more than 30 days. Fagon et al. [6l] estimated the cumulative risk of pneumonia as 7% at 10 days and 19% at 20 days after the initiation of mechanical ventilation. In this study, the incremental risk of pneumonia was constant over the course of respiratory support, with a mean rate of 1% per day. Recently Cook et al.  observed, in a study involving 1014 mechanically ventilated patients, that although the cumulative risk for developing VAP increased over the time, the daily hazard rate decreased after day 5, so the risk per day was evaluated as 3% on day 5, 2% on day 10 and 1% on day 15.
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