In 1967, Ashbaugh et al. described a syndrome of tachypnea, hypoxia, and decreased pulmonary compliance in a series of 11 adults and 1 child with respiratory failure. The pathologic features included interstitial and intra-alveolar edema and hemorrhage as well as hyaline membrane formation. On its face, this condition seemed to have many features in common with the previously described infant respiratory distress syndrome. Although the syndrome had been widely recognized and reported in the years that followed, it was not until 1994 that a consensus definition entered the scientific literature (1). The American-European Consensus Conference (AECC) proposed that the “adult” respiratory distress syndrome be renamed the “acute” respiratory distress syndrome (ARDS) to acknowledge the existence of this condition in children. The panel defined ARDS as a severe form of acute lung injury (ALI) characterized by acute, noncardiogenic pulmonary edema with bilateral pulmonary infiltrates on chest x-ray and a ratio of PaO₂ to FIO₂ of <200. The AECC designated ALI as a term comprehensively including all patients with PaO₂/FIO₂ <300 who otherwise meet criteria for ARDS (Table 49.1). This consensus document set the stage for a highly productive couple of decades in which the application of consistent diagnostic criteria facilitated the conduct of large-scale cohort studies and clinical trials that have shed considerable light on the epidemiology and pathophysiology of ALI and ARDS, creating many opportunities to rigorously evaluate the impact of novel therapeutic approaches.

Published incidence estimates for ALI and ARDS vary, and are best interpreted in the context of the study design, case ascertainment methods, population demographics, and mechanical ventilation practices that were used at the time patients were identified as meeting criteria for either condition. The data that are available suggest that there are striking differences in the incidence of ALI and ARDS in children as compared to adults. Population-based cohort studies in which investigators used AECC criteria to identify cases estimate that ALI occurs at a rate of 2.95-12.8 cases per 100,000 children per year, compared to as many as 78.9 cases per 100,000 adults per year (2,3,4,5,6,7,8,9). ARDS is estimated to occur at a rate of 2.2-9.5 cases per 100,000 children per year (2,5) compared to as many as 58.7 cases per 100,000 adults per year in adults (8). Among individuals at least 15 years old, the incidence of both ALI and ARDS increases dramatically with advancing age, reaching a peak incidence of 306 cases per 100,000 adults per year among individuals between 75 and 84 years of age (8). A recent downward trend in the incidence of adult ARDS is suggested by a carefully validated population-based cohort in suburban Minnesota. This study indicated that the incidence of ALI and ARDS among hospitalized adults appears to have declined between 2001 and 2008 from 81 to 38.3 cases per 100,000 adults per year (10).

Recent data indicate that ARDS occurs in 1%-4% of PICU admissions and as many as 8%-10% of children requiring mechanical ventilation (5,11,12). Historically, mortality for pediatric ARDS has varied between 20% and 75%, depending on the presence of coexisting risk factors such as immune compromise, the criteria that are used to identify cases, and the presence of nonpulmonary organ failures (11). The highest mortality rates tend to be reported in small, singlecentered, retrospective studies from the 1980s and early 1990s, before consensus diagnostic criteria were developed. Data from contemporary epidemiologic investigations and the control groups of multicentered clinical trials conducted during the past decade indicate that mortality in the pediatric ALI/ARDS population ranges between 8% and 36% (11,13

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