Associations between changes in oxygenation, dead space and driving pressure induced by the first prone position session and mortality in patients with acute respiratory distress syndrome

Background: Outcome prediction in acute respiratory distress syndrome (ARDS) is challenging, especially in patients with severe hypoxemia. The aim of the current study was to determine the prognostic capacity of changes in PaO2/FiO2, dead space fraction (VD/VT) and respiratory system driving pressure (ΔPRS) induced by the first prone position (PP) session in patients with ARDS. Methods: This was a post hoc analysis of the conveniently-sized 'Molecular Diagnosis and Risk Stratification of Sepsis' study (MARS). The current analysis included ARDS patients who were placed in the PP. The primary endpoint was the prognostic capacity of the PP-induced changes in PaO2/FiO2, VD/VT, and ΔPRS for 28-day mortality. PaO2/FiO2, VD/VT, and ΔPRS was calculated using variables obtained in the supine position before and after completion of the first PP session. Receiving operator characteristic curves (ROC) were constructed, and sensitivity, specificity positive and negative predictive value were calculated based on the best cutoffs. Results: Ninety patients were included; 28-day mortality was 46%. PP-induced changes in PaO2/FiO2 and VD/VT were similar between survivors vs. non-survivors [+83 (+24 to +137) vs. +58 (+21 to +113) mmHg, and -0.06 (-0.17 to +0.05) vs. -0.08 (-0.16 to +0.08), respectively]. PP-induced changes in ΔPRS were different between survivors vs. non-survivors [-3 (-7 to 2) vs. 0 (-3 to +3) cmH2O; P=0.03]. The area under the ROC of PP-induced changes in ΔPRS for mortality, however, was low [0.63 (95% confidence interval (CI), 0.50 to 0.75]; PP-induced changes in ΔPRS had a sensitivity and specificity of 76% and 56%, and a positive and negative predictive value of 60% and 73%. Conclusions: Changes in PaO2/FiO2, VD/VT, and ΔPRS induced by the first PP session have poor prognostic capacities for 28-day mortality in ARDS patients.