ABSTRACT
DEFINITIONS AND EPIDEMIOLOGY Definitions Formally recognized over four decades ago (1), acute lung injury and the acute respiratory distress syndrome (ALI/ ARDS), are defined by descriptive physiologic and clinical criteria set forth by the American-European Consensus Conference (AECC) of 1994 (Table 1) (2). Three primary features define ALI/ARDS; a profound oxygenation defect (right-toleft transpulmonary shunt), impaired carbon dioxide excretion, and the presence of noncardiogenic pulmonary edema. The oxygenation defect is characterized by the partial pressure of oxygen in the arterial blood (PaO2)/fraction of inspired oxygen (FiO2) ratio (P/F ratio). When the P/F ratio is ,300, the criteria for ALI is met, whereas if ,200, the ARDS threshold is achieved. The AECC definitions for ALI and ARDS build upon the experience of the lung injury severity score introduced by Murray et al. (3). That scoring system combines the magnitude of physiologic impairment (severity of oxygenation defect, required level of positive end-expiratory pressure, and decreased respiratory compliance) along with the degree of alveolar consolidation on the chest radiograph (3). Several studies have compared the diagnostic accuracy between these criteria concluding statistical agreement (4). The intent of the simplified definition is to characterize a syndrome without reference to a specific etiology (5).
