Imaging advances in ARDS evaluation

Acute respiratory distress syndrome (ARDS) is a non-specific yet catastrophic response of the lung to ‘injury’ — the nature of the injury varies but is a consequence of a direct insult (for instance, severe pneumonia) or an indirect effect (e.g. systemic sepsis, non-thoracic trauma). On histopathological examination, there is a stereotypical, overlapping sequence of changes in the lung culminating in the flooding of the lung with inflammatory oedema fluid and the formation of classical hyaline membranes; fibrosis of variable severity is part-and-parcel of the normal sequelae of ARDS. It is important to stress that the diagnosis of ARDS is made on physiological criteria (namely the PaO2/FiO2 ratio) with clinical features and imaging tests (CXR/CT) in a ‘supporting’ diagnostic role. That said, CT appearances in ARDS broadly fall into one of two groups: i) symmetrical airspace opacification with a gradient of increasing density from ventral to dorsal lung or ii) a more random distribution of increasing lung density with non-dependent foci of consolidation. Furthermore, detailed studies of CT appearances in concert with clinical and physiological abnormalities has provided valuable pathophysiological and prognostic insights.