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Description and Definitions

In 1967, Ashbaugh et al.1 described a syndrome characterized by the acute onset of dyspnea, severe hypoxemia, diffuse lung infiltrates, and decreased respiratory system compliance in the absence of evidence for congestive heart failure (CHF). The syndrome, initially called acute respiratory distress in adults (to contrast it with acute respiratory distress in neonates), is now known as the acute respiratory distress syndrome (ARDS). Following the initial report, other authors utilized various definitions that incorporated elements related to time of onset, presence of hypoxemia and radiographic infiltrates, and absence of overt CHF.

In 1988, Murray and others introduced the Lung Injury Score (LIS), an assessment tool for ARDS that reflects the extent of radiographic infiltrates, severity of hypoxemia and reduced respiratory system compliance, and level of positive end expiratory pressure (PEEP) used in mechanically ventilating affected patients.2 The LIS incorporates four parameters that are graded on a scale of 0 to 4: (1) ratio of PaO2 to FiO2(PaO2/FiO2); (2) total respiratory compliance; (3) level of PEEP; and (4) extent of radiographic infiltrates (assessed by noting the number of quadrants in the chest radiograph demonstrating infiltrates). The LIS equals the sum of the scores for the four variables divided by four. In clinical studies, a score of 2.5 or more is generally used as a threshold for severe disease.

Consensus Definitions of Acute Lung Injury and Acute Respiratory Distress Syndrome

Prior to 1994, published studies used nonuniform definitions of ARDS, prompting an American European Consensus Conference (AECC) to develop standardized definitions for ARDS and acute lung injury (ALI)—a broader category that encompasses ARDS.3 The AECC definitions included the acute onset of illness, bilateral chest radiographic infiltrates consistent with pulmonary edema, poor systemic oxygenation, and absence of evidence for left atrial hypertension (Table 141-1A). The syndrome was referred to as ALI when the ratio of PaO2 to FiO2 (PaO2/FiO2); was ≤300, and ARDS when the ratio was ≤200. The AECC definitions of ALI and ARDS were intentionally broad in order to encompass different types of acute hypoxemic respiratory failure occurring in a wide variety of settings.

Table 141-1AAmerican European Consensus Conference Criteria for Acute Lung Injury (ALI) and the Acute Respiratory Distress Syndrome (ARDS)

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