Improved survival in ARDS: chance, technology or experience?

Randomised, clinical controlled trials in ARDS have shown that the use of historical control groups can produce misleading results.1 If , for example, the 90% mortality of the 1979 National Heart and Lung Institute ARDS Extracorporeal Membrane Oxygenation (ECMO) trial2 is used as a baseline, then all subsequent studies of ARDS would demonstrate an apparent increase in survival. Examination of the entry criteria and case mix of this study highlights many of the potential pitfalls in examining changes in mortality from ARDS over time. Abnormalities of gas exchange were the major entry criteria whilst no adjustment was made for the fact that many patients were admitted with severe, atypical pneumonia. Subsequent work has shown that the initial severity of gas exchange is not a strong predictor of survival in ARDS while case mix (in terms of patient selection, severity of disease, patient age and predisposing factors) is very important.3

Problems with patient selection have, to some extent, been overcome by the adoption of recent European/North American consensus conference definitions of ARDS.4 Gas exchange, plain chest radiology and, if available, haemodynamic data define ARDS in the context of an “at risk” patient. However, the ease of use of these criteria does not guarantee the selection of a homogeneous population. The …