Early gas exchange effect of prone positioning is not associated with intensive care unit survival in moderate to severe acute respiratory distress syndrome

Background: Prone positioning enables the redistribution of lung weight, improving lung mechanics and preventing further ventilator-induced lung injury. Prone ventilation improves oxygenation and ventilation in most patients with the acute respiratory distress syndrome (ARDS). Whether the gas exchange achieved by prone positioning was associated with better survival outcome remained unanswered. Methods: We retrospectively analyzed patients who underwent prone positioning for moderate to severe ARDS. The median duration of prone positioning in this study was 16 hours. Arterial blood gases were analyzed prior to turning prone and 1-5 hours (median: 2 hours) after turning to first prone position. Patients with a PaO2/FiO2 ratio increasing by at least 20% or ≧ 20 mmHg after turning prone position were classified as “PaO2 responders”. Patients with a decrease in PaCO2 of ≧1 mm Hg after turning prone position were classified as “PaCO2 responders”. Results: Fifty-two patients were included, 34 (65.4%) were PaO2 responders and 18 (32.7%) were PaO2 non-responders while as 17 (32.7%) were PaCO2 responders and 35 (67.3%) were PaCO2 non-responders. No difference in duration of mechanical ventilation, ICU length of stay, proportion ventilator-free, ICU mortality and 60 days mortality within the PaO2 response category and the PaCO2 response category. Conclusions: Prone positioning induced improvement of gas exchange is distinct, the improvement in oxygenation is more prevalent than in ventilation. We found no association between the improvement in gas exchange and ICU survival or other ventilator related outcomes.