Influence of aetiology of acute respiratory distress syndrome and early differences in oxygenation on outcome
© BioMed Central Ltd. 2010
Published: 1 March 2010
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 (ARDSp) and extra pulmonary ARDS (ARDSexp). 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.
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 ARDSp or ARDSexp 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.
The hospital mortality in all patients included in the study (n = 151) with ARDS was 44.1%. The patients classified as ARDSp had a higher hospital mortality (50.6%) compared with ARDSexp (36.4%), but the difference was not statistically significant (P = 0.12). Nonsurvivors with ARDSp 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 ARDSexp. This association between mortality and hypoxaemia in early ARDSp 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 ARDSexp the effect of hypoxaemia on mortality was not observed.
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 ARDSp; but not in ARDSexp. Interventional trials need to account for the influence of etiology and hypoxaemia burden on outcome prior to concluding this as a negative intervention.