Effect of Prone Positioning on Mortality in Patients With Mild to Moderate Acute Respiratory Distress Syndrome.

Acute respiratory distress syndrome (ARDS) accounts for almost 10% of intensive care units (ICU) admissions. Three ARDS stages have been defined, based on the PaO2/FIO2 ratio measured with positive end-expiratory pressure (PEEP) ≥5 cmH2O: mild (201-300 mmHg), moderate (200-101 mmHg), and severe (≤100mmHg). They represent 30.0%, 46.6%, and 23.4% of ARDS, respectively. Mechanical invasive ventilation (MV), the cornerstone of ARDS patient care, has a primary goal to protect the lung from ventilator-induced lung injury (VILI). Delivering MV in a prone position (PP) has been shown to improve oxygenation, protect the lung through a better homogenization of lung stress/strain, and stabilize hemodynamics. A meta-analysis of four randomized controlled trials showed beneficial effect of PP vs. supine position (SP) in the most hypoxemic patients. A fifth randomized controlled trial further showed a significant reduction in mortality with PP in ARDS patients with PaO2/FIO2 \<150 mmHg, when neuromuscular blockade and long prone positioning sessions were used. Therefore, PP has since been strongly recommended for ARDS patients with PaO2/FIO2 \<150 mmHg. Yet, there is limited evidence in patients with mild to moderate ARDS. There are, however, strong arguments supporting the need for a new trial in ARDS patients with PaO2/FIO2 in the range 150-300 mmHg: 1. There is no trial that has specifically tested PP in this ARDS subset; 2. PP is safe and has become a standard of care in ICU; 3. Should VILI prevention be a mechanism through which PP improves survival, this should be involved in all ARDS patients; 4. The mortality at hospital discharge in this subset of ARDS remains significant, amounting to 34.9% (95% confidence intervals 31.4-38.5%) in mild and 40.3% (37.4-43.3) in moderate stages; 5. Among 580 patients with mild ARDS at admission to the ICU, in-hospital mortality was 10%, 30%, and 37% for those who improved, persisted, and worsened ARDS, respectively. 6. Finally, PP has