Influence of aetiology of acute respiratory distress syndrome and early differences in oxygenation on outcome

Introduction

Studies evaluating the relationship between the severity of hypoxemia in patients with acute respiratory distress syndrome (ARDS) and hospital outcome have not found a consistent outcome association. We hypothesized that severity of hypoxemia on admission after optimal ventilation may predict hospital outcome if the ARDS patients are categorised based on the primary etiology: pulmonary ARDS (ARDS_p) and extra pulmonary ARDS (ARDS_exp). Our aim was therefore to ascertain the relationship between hospital outcome and the severity of hypoxemia in patients with early ARDS (days 1 to 3 following admission) after categorising based on etiology.

Methods

We used a prospective cohort study design and enrolled 151 consecutive patients with a primary diagnosis of ARDS on the day of admission, admitted over a 2-year period to our adult general ICU. Patients enrolled in other clinical interventional trials were excluded from the study. Protocol-based management of mechanical ventilation was used to achieve optimal ventilation. Two authors independently designated patients as ARDS_p or ARDS_exp and a third reviewed any conflicts (only four cases). Patients were then subcategorised by severity of hypoxaemia based on PaO₂/FiO₂ ratio (P/F ratio) intervals. All other clinical interventions were at the discretion of treating clinician.

Results

The hospital mortality in all patients included in the study (n = 151) with ARDS was 44.1%. The patients classified as ARDS_p had a higher hospital mortality (50.6%) compared with ARDS_exp (36.4%), but the difference was not statistically significant (P = 0.12). Nonsurvivors with ARDS_p had a significantly lower P/F ratio on day 1 of ARDS diagnosis compared with survivors (12.05 ± 4.46 vs 15.39 ± 4.97 kPa; P = 0.002), a relationship not observed with ARDS_exp. This association between mortality and hypoxaemia in early ARDS_p persisted at even more serve levels of hypoxaemia (<15 kPa, <12.5 kPa, and <10 kPa; P = 0.01, P = 0.003 and P = 0.002, respectively), while in ARDS_exp the effect of hypoxaemia on mortality was not observed.

Conclusions

Our findings indicate that the hypoxaemia burden as assessed by P/F ratio intervals despite optimal ventilatory support on the day of ICU admission predicts increased risk of death in ARDS_p; but not in ARDS_exp. Interventional trials need to account for the influence of etiology and hypoxaemia burden on outcome prior to concluding this as a negative intervention.