The European—American Consensus Conference in 1993 provided working definitions of Acute Lung Injury and ARDS to improve diagnostic consistency and interpretation of the results of clinical and epidemiological studies.
Both Acute Lung Injury and ARDS are characterized by the presence of hypoxemia and pulmonary infiltrates without elevated left atrial pressure.
The two conditions are distinguished by severity of pulmonary gas exchange impairment: A Pao2—Fio, ratio of 300 or less defines acute lung injury, and a ratio of 200 or less defines ARDS regardless of the amount of positive end-expiratory pressure (PEEP) needed to support oxygenation.
The lung in Acute Lung Injury and ARDS is characterized by a major change in the elastic property of the alveoli. Surface tension plays an important role in determining the lung elasticity.
The forces created as a result of surface tension tend to collapse the alveoli. These collapsing forces are defined by the Laplace law: P = 2T / r (P = pressure in the alveolus; T = surface tension; r = radius of the alveolus).
The interstitial spaces, alveolar ducts, small vessels, and capillaries also contain macrophages, neutrophils, and erythrocytes. The acute phase may resolve or progress to a phase of fibrosis with persistent hypoxaemia, increased dead space, pulmonary hypertension, and further loss of lung compliance.