Acute lung injury (ALI) and ARDS are sudden onset inflammatory-mediated conditions of the lungs resulting in pulmonary edema and subsequent hypoxia. In 1994 the American-European Consensus Conference Committee established clinical criteria to define these conditions. ARDS is defined by the following:
1. acute onset;
2. bilateral pulmonary infiltrates on chest radiograph (Fig. 8.12);
3. PAWP < 18 mmHg or absence of clinical evidence of left atrial hypertension;
Evaluating these criteria closely, the bilateral pulmonary infiltrates on CXR are the clinical manifestation of pulmonary edema. The critical aspect of the definition is the requirement that the pulmonary capillary wedge pressure (PCWP) is less than 18 mmHg or that there is lack of clinical evidence of left atrial hypertension. These criteria eliminate heart failure as a cause of the observed edema, and thus point to an inflammatory etiology.
The resultant pulmonary edema leads to tissue hypoxia. Normally, an individual has a PaO2 of 13 kPa on 21% FiO2 (room air). The PaO2/FiO2 ratio then is 13/0.21 or 62. In ARDS, the pulmonary edema and disruption of the pulmonary
architecture is so profound that gas exchange is severely impaired. Oxygenation is poor despite increasing oxygen requirements. Thus the PaO2 decreases, while the FiO2 increases, driving down the ratio to less than 27. For example, a patient with ARDS may require a FiO2 of 70% to get a PaO2 of 10 kPa. The PaO2/FiO2 ratio is 10/0.70 = 14. ALI implies a less severe form of lung injury and shares the definition of ARDS with the exception that the PaO2/FiO2 ratio is ^40 kPa (300 mmHg).
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