Adult Respiratory Distress Syndrome in Children

Adult respiratory distress syndrome (ARDS) is a clinical syndrome of acute respiratory failure that follows a diverse variety of primary severe physiologic insults. The primary insult may involve lung injury (hydrocarbon inhalation, oxygen toxicity) or the lungs may not be initially involved (trauma, sepsis). The hallmark of ARDS is increased permeability of the alveolar-capillary membrane with alteration of both capillary endothelial and alveolar epithelial barrier function. The result is development of pulmonary edema with severe hypoxemia, primarily due to a large intrapulmonary shunt, despite normal or low pulmonary intravascular pressure. While our focus is on lung injury, the mechanisms involved are not necessarily limited to the lungs in ARDS and, additionally, occur in pathologic processes other than ARDS. The first description of ARDS as a distinct clinical and pathophysiologic entity was in 1967, when Ashbaugh and coworkers described 12 adult patients without previous pulmonary disease who developed dyspnea, tachypnea, cyanosis refractory to oxygen therapy, decreased lung compliance, and diffuse alveolar infiltrates after several different primary insults. Because the pathologic appearance of the lungs was similar to neonatal respiratory distress syndrome, the term adult respiratory distress syndrome was adopted. Therefore, the designation "adult" is of historical origin and ARDS occurs in pediatric patients of any age in the same pathophysiologic form as seen in adults. Overall, more than 150,000 cases of ARDS occur per year, with a mortality rate of greater than 50%. In the pediatric age group, an incidence of 8.5 to 10.4 cases per 1000 pediatric intensive care unit admissions with a mortality rate of 59% can be estimated from five retrospective reports. These statistics may underestimate the incidence and overestimate the mortality with ARDS because of a tendency for frequently used diagnostic criteria in the past to select the more severely affected patient. ' Any difference in the pediatric population compared with adults would be due to a different spectrum of primary insults leading to ARDS and a difference in response to and recovery from severe injury rather than an age-related difference in the pathophysiology of the syndrome itself. The clinical course, altered cardiopulmonary physiology, and pathology associated with ARDS are well described and summarized in Figure 1. A thorough understanding of these aspects of ARDS is important in directing management, which is currently primarily supportive. Defining the underlying pathogenic mechanisms that lead to lung injury is presently an active area of research and multiple mediators have been proposed. Identification of the involvement of several systems, including inflammatory cells, complement, the coagulation system, and eicosanoid production, likely represents the endogenous response to the severe primary insult. It is unlikely that any single mediator or system will explain ARDS, but rather these diverse systems interact in different ways, depending on the nature of the insult and the stage of ARDS. There are limitations to what can be accomplished with supportive therapy in terms of improving outcome. The pathophysiologic alterations are well advanced, and potentially irreversible, by the time ARDS is clinically apparent. Perhaps a better understanding of the basic alterations in cellular function that lead to ARDS is needed to allow early identification of those patients destined to develop ARDS and early intervention in order to prevent progression to a potentially irreversible pathologic state. In this article I shall review ARDS as a clinical, pathophysiologic, and pathologic entity as well as describing the potential pathogenic mechanisms.