Definitions Basic definition

In 1967 Ashbaugh and colleagues described a clinical syndrome of tachypnoea, hypoxaemia resistant to supplemental oxygen, diffuse alveolar infiltrates, and decreased pulmonary compliance in 12 patients who required positive pressure mechanical ventilation. The onset of the syndrome was acute, typically within hours of the inciting clinical disorder. The majority of patients did not have a history of pulmonary disease. Adequate oxygenation required the use of continuous positive pressure with end expiratory pressures (PEEP) of 5-10 cm H2O. The earliest radiographic findings were patchy infiltrates indistinguishable from cardiogenic pulmonary oedema that usually became confluent with progressive clinical deterioration. Lung compliance was substantially decreased. Gross lung specimens resembled hepatic tissue with large airways being free from obstruction. Histological examination revealed hyaline membranes in the alveoli with microscopic atelectasis and intra-alveolar haemorrhage similar to the infant respiratory distress syndrome.1

In a subsequent paper Petty and Ashbaugh refined and elaborated on what they coined the "adult respiratory distress syndrome".3 In a review of 40 cases the mechanism of lung injury was either direct (chest trauma, aspiration) or indirect (pancreatitis, sepsis) and, in some cases, was attributed to mechanical ventilation. Despite the heterogeneity of inciting events, the physiological and pathological response of the lung was uniform. The use of PEEP was critical in maintaining acceptable oxygen saturation by reducing the right to left intrapulmonary shunt and increasing the functional residual capacity. Recovery from lung injury could be rapid and complete or could progress to interstitial fibrosis and progressive respiratory failure. Fatalities were primarily due to septic complications.3

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