Clinical Meaning of Early Oxygenation Improvement in Severe Acute Respiratory Distress Syndrome underProlonged Prone Positioning

Ventilating patients with acute respiratory distress syndrome (ARDS) in the prone position has been shown to improve arterial oxygenation, but prolonged prone positioning frequently requires continuous deep sedation, which may be harmful to patients. We evaluated the meaning of early gas exchange in patients with severe ARDS under prolonged (${\geq}$ 12 hours) prone positioning. Methods: We retrospectively studied 96 patients (mean age, 60.1 ${\pm}$ 15.6 years;75% men) with severe ARDS ($PaO_2/FiO_2{\leq}$ 150 mmHg) admitted to a medical intensive care unit (MICU). The terms "$PaO_2$ response" and "$PaCO_2$ response" represented responses that resulted in increases in the $PaO_2/FiO_2$ ratio of ${\geq}$ 20 mmHg and decreases in $PaCO_2$ of ${\geq}$ 1 mmHg, respectively, 8 to 12 hours after first placement in the prone position. Results: The mean duration of prone positioning was 78.5 ${\pm}$ 61.2 hours, and the 28-day mortality rate after MICU admission was 56.3%. No significant difference in clinical characteristics was observed between $PaO_2$ and $PaCO_2$ responders and non-responders. The $PaO_2$ responders after prone positioning showed an improved 28-day outcome, compared with non-responders by Kaplan-Meier survival estimates (p < 0.05 by the log-rank test), but the $PaCO_2$ responders did not. Conclusions: Our results suggest that the early oxygenation improvement after prone positioning might be associated with an improved 28-day outcome and may be an indicator to maintain prolonged prone positioning in patients with severe ARDS.