Prognostic classification based on P/F and PEEP in invasively ventilated ICU patients with hypoxemia—insights from the MARS study

Background Outcome prediction in patients with acute respiratory distress syndrome (ARDS) greatly improves when patients are reclassified based on predefined arterial oxygen partial pressure to fractional inspired oxygen ratios (PaO 2 /FiO 2 ) and positive end–expiratory pressure (PEEP) cutoffs 24 h after the initial ARDS diagnosis. The aim of this study was to test whether outcome prediction improves when patients are reclassified based on predefined PaO 2 /FiO 2 and PEEP cutoffs 24 h after development of mild hypoxemia while not having ARDS. Methods Post hoc analysis of a large prospective, multicenter, observational study that ran in the ICUs of two academic hospitals in the Netherlands between January 2011 and December 2013. Patients were classified into four groups using predefined cutoffs for PaO 2 /FiO 2 (250 mmHg) and PEEP (5 cm H 2 O), both at onset of hypoxemia and after 24 h: PaO 2 /FiO 2 ≥ 250 mmHg and PEEP < 6 cm H 2 O (group I), PaO 2 /FiO 2 ≥ 250 mmHg and PEEP ≥ 6 cm H 2 O (group II), PaO 2 /FiO 2 < 250 mmHg and PEEP < 6 cm H 2 O (group III), and PaO 2 /FiO 2 < 250 mmHg and PEEP ≥ 6 cm H 2 O (group IV), to look for trend association with all-cause in-hospital mortality, the primary outcome. Secondary outcome were ICU- and 90-day mortality, and the number of ventilator-free days or ICU-free days and alive at day 28. Results The analysis included 689 consecutive patients. All-cause in-hospital mortality was 35%. There was minimal variation in mortality between the four groups at onset of hypoxemia (33, 36, 38, and 34% in groups I to IV, respectively; P = 0.65). Reclassification after 24 h resulted in a strong trend with increasing mortality from group I to group IV (31, 31, 37, and 48% in groups I to IV, respectively; P < 0.01). Similar trends were found for the secondary endpoints. Conclusions Reclassification using PaO 2 /FiO 2 and PEEP cutoffs after 24 h improved classification for outcome in invasively ventilated ICU patients with hypoxemia not explained by ARDS, compared to classification at onset of hypoxemia. Trial registration ClinicalTrials.gov identifier: NCT01905033. Registered on July 11, 2013. Retrospectively registered.