A clinical classification of the acute respiratory distress syndrome for predicting outcome and guiding medical therapy*.

Objective: Current in-hospital mortality of the acute respiratory distress syndrome (ARDS) is above 40%. ARDS outcome depends on the lung injury severity within the first 24 hours of ARDS onset. We investigated whether two widely accepted cutoff values of Pao(2)/Ro(2) and positive end-expiratory pressure (PEEP) would identify subsets of patients with ARDS for predicting outcome and guiding therapy. Design: A 16-month (September 2008 to January 2010) prospective, multicenter, observational study. Setting: Seventeen multidisciplinary ICUs in Spain. Patients: We studied 300 consecutive, mechanically ventilated patients meeting American-European Consensus Conference criteria for ARDS (Pao(2)/Ro(2) 5 200 mm Hg) on PEEP greater than or equal to 5 cm H2O, and followed up until hospital discharge. Interventions: None. Measurements and Main Results: Based on threshold values for Pao2/Fio2 (150mm Hg) and PEEP (10 cm H20) at ARDS onset and at 24 hours, we assigned patients to four categories: group I (Pao(2)/Fio(2) 150 on PEEP < 10), group ll (Pao(2)/Ro(2) >= 150 on PEEP 10), group III (Pao(2)/Fio(2)< 150 on PEEP < 10), and group IV (Pao(2)/Fio(2) < 150 on PEEP 10). The primary outcome was all-cause in-hospital mortality. Overall hospital mortality was 46.3%. Although at study entry, patients with Pao(2)/Ro(2) less than 150 had a higher mortality than patients with a Pao(2)/Fio(2) greater than or equal to 150 (p = 0.044), there was minimal variability in mortality among the four groups (p = 0.186). However, classification of patients in each group changed markedly after. 24 hours of usual care. Group categorization at 24 hours provided a strong association with in-hospital mortality (p < 0.00001): group I had the lowest mortality (23.1%), whereas group IV had the highest mortality (60.3%). Conclusions: The degree of lung dysfunction established by a Pao2/Ro2 of 150 mm Hg and a PEEP of 10 cm H2O demonstrated that ARDS is not a homogeneous disorder. Rather, it is a series of four subsets that should be considered for enrollment in clinical trials and for guiding therapy. A major contribution of our study is the distinction between survival after 24 hours of care versus survival at the time of ARDS onset.