Definition and diagnosis
ARDS was first described in 1967 in patients with refractory cyanosis due to respiratory failure that necessitated mechanical ventilation. However, the criteria for defining the syndrome were not generally agreed until the American–European Consensus Conference (AECC) in 1994. This definition specified an acute onset, refractory hypoxaemia and radiographic evidence of bilateral pulmonary shadowing due to increased permeability of the alveolar–capillary membrane, with the exclusion of left ventricular failure as the cause (table 1). A cardiogenic cause of pulmonary oedema was to be excluded by pulmonary artery catheterisation showing a pulmonary artery occlusion pressure (PAOP) of less than 18 mmHg or by clinical evidence of left atrial hypertension as a sign of left heart failure. The severity of the condition was defined by the ratio of the arterial oxygen tension (PaO2 measured in mmHg) to the inspiratory oxygen fraction (FIO2; where room air is 0.21 and pure oxygen is 1.0).
This definition was superseded by the Berlin definition of 2012, which refined the AECC criteria. The onset of ARDS is now fixed as being within 7 days of an insult or of new or worsening respiratory symptoms. While bilateral radiographic ‘opacities’ are still necessary, other causes, such as effusions, nodules, and partial or complete collapse of a lobe or lung should be excluded. The exclusion of cardiac failure or fluid overload is stressed but the methods of exclusion now emphasise echocardiography, which can replace pulmonary artery catheterisation, mirroring actual clinical practice. For a clearer definition, a minimum positive end-expiratory pressure (PEEP) has been introduced and the name ALI has been omitted and replaced by grading of ARDS as mild (PaO2/FIO2 of more than 200 mmHg but not more than 300 mmHg), moderate (PaO2/FIO2 of more than 100 mmHg but not more than 200 mmHg) or severe (PaO2/FIO2 of not more than 100 mmHg).