The first description of a syndrome of acute respiratory distress following trauma or severe hypovolemic shock appeared in 1967; this report emphasized the presence of tachypnea, severe hypoxemia related to an intrapulmonary shunt, decreased lung compliance, and diffuse bilateral infiltrates. The definition of acute respiratory distress syndrome (ARDS) was initially based on clinical criteria. Use of Murray's scoring system followed, and a standardized definition was applied from 1994 onwards. Under this system, ARDS is the most severe form of acute lung injury. Table 1. summarizes the principal elements of this definition (B.§ID§[d.„..§.La/: 1994).
Table 1 Major elements of the definition of acute lung injury and ARDS
The factors inciting ARDS can be categorized as either direct or indirect pulmonary insults ( Table, ..2). Infections, trauma, and aspiration of gastric contents are responsible for 70 to 80 per cent of cases. The incidence increases in the presence of several etiological factors and/or concomitant pulmonary pathologies.
The mortality associated with ARDS has been approximately 50 per cent from the 1970s until recently, with individual studies reporting variations from 10 to 90 per cent (Krafft ef a/ 1996). These variations are apparently due to differences in underlying pathological processes, presence of infections, and existence of multiple organ failure, and are not related to the PaO^/FiO2 ratio or the 'lung injury score'. However, recent studies appear to indicate decreases in mortality ( Milberg etaj
1995), particularly for trauma-associated ARDS.
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