Effect of Prone Positioning on Mortality in Patients With Severe and Persistent Acute Respiratory Distress Syndrome

The acute respiratory distress syndrome (ARDS) is an acute respiratory failure characterized by bilateral pulmonary infiltrates on frontal chest radiography, arterial oxygen tension (Pao2) to fraction of inspired oxygen (FiO2) ratio of 200 or less, and absence of evidence for left atrial hypertension. In-hospital mortality remains high: from 40 to 60%. Typical pathological findings include disruption of the alveolar epithelium, diffuse alveolar damage, hyaline membranes and pulmonary inflammation. Gas exchange is severely altered making of invasive mechanical ventilation required in most of the cases. Mechanical ventilation should be judiciously set in order to protect the lung from ventilator- associated lung injury (VALI). VALI is commonly seen as the expression of two main alveolar strains. The well documented one to date is over distension of alveoli when high tidal volumes are used. A North-American randomized controlled trial (RCT) has demonstrated that using low tidal volumes (6 ml per kilo of predicted weight) results in a 22% decrease in mortality when compared with high tidal volumes (12 ml per kilo of predicted weight). As suggested by an ancillary study of this trial, a reduced pulmonary inflammation is one of the contributing factors of this result. Therefore, minimizing VALI is a primary therapeutic goal. Low lung volume VALI is the second potential mechanism, though less well documented. It is expected to result from repetitive collapse and reopening of terminal respiratory units. Experimental data suggest that proning can reduce VALI by homogenizing lung strains. In ARDS patients, proning is mainly used due to its ability to improve gas exchange in 60 to 70% of patients. Nonetheless, 3 RCTs failed to demonstrate a significant effect on mortality of proning in three clinical situations: 1: acute lung injury, 2: hypoxemic acute respiratory failure, 3: ARDS. Post hoc analysis of one of them has suggested an effect of proning on mortality for the most hypoxemic sub-group. Studies limitations can explain lack of statistical significance as lack of power, heterogeneity of patients, insufficient duration of proning, high tidal volumes. Only one RCT was specifically designed for ARDS. Mortality was 25% lowest in the prone position arm, but this difference was not significant due to lack of power. So, the capability of proning to improve patient survival in ARDS is still an open question.