PaCO

Background: Prone positioning has demonstrated decreased mortality in moderate to severe acute respiratory distress syndrome (ARDS) patients. This study aimed to investigate the gas exchange effect of prone positioning in moderate to severe ARDS patients. Methods: This retrospective study includes eight tertiary referral centers. Demographic data, laboratory data, treatment records, ventilator setting data, and outcomes were collected. Arterial blood gas data were collected before and 24 h after the initiation of prone positioning and under the supine position. PaO 2 responders were defined as the PaO 2 /FiO 2 ratio increasing by ≥ 20 % or ≥20 mm Hg, while PaCO 2 responders were defined as PaCO 2 decreasing by ≥ 1 mm Hg after prone positioning for one day. Results: One hundred thirty-eight patients receiving prone positioning were enrolled, with 86 (62.3 %) being PaO 2 responders and 67 (48.6 %) being PaCO 2 responders. Mortality rates were not significantly different between PaO 2 responders and non-responders at day 28 (29.1 % vs. 38.5 %, P = 0.254) or day 60 (39.5 % vs. 50.0 %, P = 0.229). PaCO 2 responders had a significantly lower mortality rate at day 28 (20.9 % vs. 43.7 %, P = 0.004) and day 60 (31.3 % vs. 54.9 %, P = 0.005). The PaCO 2 responders had significant improvement in minute ventilation (10.2 ± 2.5 vs. 10.9 ± 2.8 L/min, P = 0.024), dynamic driving pressure (18.7 ± 4.1 vs. 17.6 ± 4.0 cmH 2 O, P = 0.004), and lung compliance [21.4(17.8–27.6) vs. 23.5(19.9–29.0) L/cmH 2 O, P = 0.002] after prone positioning. Conclusion: In this multicenter retrospective cohort study of patients with ARDS receiving prone positioning, PaCO 2 responders had better clinical outcomes than PaCO 2 non-responders.