Pulmonary and extrapulmonary acute respiratory distress syndrome are different

Acute respiratory distress syndrome (ARDS) can be derived from two pathogenetic pathways: a direct insult on lung cells (pulmonary ARDS (ARDSp)) or indirectly (extrapulmonary ARDS (ARDSexp)). This review reports and discusses differences in biochemical activation, histology, morphological aspects, respiratory mechanics and response to different ventilatory strategies between ARDSp and ARDSexp. In ARDSp the direct insult primarily affects the alveolar epithelium with a local alveolar inflammatory response while in ARDSexp the indirect insult affects the vascular endothelium by inflammatory mediators through the bloodstream. Radiological pattern in ARDSp is characterised by a prevalent alveolar consolidation while the ARDSexp by a prevalent ground-glass opacification. In ARDSp the lung elastance, while in ARDSexp the chest wall and intra-abdominal chest elastance are increased. The effects of positive end-expiratory pressure, recruitment manoeuvres and prone position are clearly greater in ARDSexp. Although these two types of acute respiratory distress syndrome have different pathogenic pathways, morphological aspects, respiratory mechanics, and different response to ventilatory strategies, at the present, is still not clear, if this distinction can really ameliorate the outcome.