Influence of Clinical Factors and Exclusion Criteria on Mortality in ARDS Observational Studies and Randomized Controlled Trials

ARDS has a high mortality in the acute setting, with long-term disability among disease survivors. In 1967, David Ashbaugh and colleagues first described the clinical features of ARDS, which were notably similar to the infantile respiratory distress syndrome. Half a century later, ARDS remains underrecognized and is associated with high mortality rates. Valuable insights from observational studies fail to demonstrate a mortality benefit in randomized controlled trials (RCTs). In the absence of a pharmacologic cure, supportive ventilator strategies limit rather than treat the ongoing lung injury. Interestingly, ARDS has higher mortality rates in observational studies compared to RCTs. Comparing mortality rates between ARDS studies and trials is problematic, partly due to varying time-points at which mortality is reported. Discerning the true mortality attributable to ARDS is also difficult. The diagnostic criteria for ARDS are mainly clinical and lack the objectivity of a laboratory test or biomarker. Nonetheless, these factors are common to both studies and trials, and fail to explain the higher mortality rate of ARDS observational studies. Disease heterogeneity and complex patient characteristics can also confound mortality estimation in ARDS. We therefore examined patient and trial factors that could influence mortality outcomes in ARDS observational studies and RCTs. Unlike RCTs, observational studies include ARDS subjects with severe comorbidities and those requesting limited care. Less stringent selection criteria could thereby contribute to high mortality rates in ARDS observational studies. In contrast, exclusion criteria in RCTs meticulously scrutinize patient characteristics, confining the type and number of eligible subjects. As a result, the task of identifying, consenting, and randomizing eligible patients within the enrollment window is challenging, further decreasing the number of subjects enrolled. Moreover, ARDS RCTs strictly adhere to lung-protective strategies, while ARDS observational studies continually demonstrate variable compliance. This review highlights the impact of patient- and trial-related factors on influencing mortality rates in ARDS observational studies and RCTs.