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  • • Acute onset of respiratory failure.
  • • Bilateral infiltrates consistent with pulmonary edema on chest radiograph.
  • • Hypoxemia.
  • • No evidence of left atrial hypertension.

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The acute respiratory distress syndrome (ARDS) was first formally described by Dr. Ashbaugh and colleagues in 1967. They reported a total of 12 cases of patients with acute respiratory failure with an overall mortality rate of 57%. The syndrome was defined in that initial description as the acute onset of severe hypoxemia that responded to initiation of mechanical ventilation with positive end-expiratory pressure (PEEP), diffuse bilateral infiltrates on chest radiograph, and decreased pulmonary compliance. Since that original description the definition of ARDS has been debated and refined. Definitions have evolved to better incorporate the clinical spectrum of disease that was recognized to occur and to try to better correlate the diagnostic definition with the pathophysiology of the syndrome. In 1994 a group of experts convened and developed the American–European Consensus Conference Committee definition that is currently utilized by most practitioners and investigators to identify ARDS. This definition has four components:

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  • 1. Acute onset of respiratory failure.
  • 2. Pao2/Fio2:<300 mm Hg = acute lung injury (ALI) <200 mm Hg = acute respiratory distress syndrome (ARDS)
  • 3. Chest radiograph: bilateral alveolar infiltrates consistent with pulmonary edema.
  • 4. No evidence of left atrial hypertension. Pulmonary capillary wedge pressure <18 mm Hg if available.

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In addition, the consensus document suggests that patients should have an identified risk factor for the development of ALI and should not have significant chronic lung disease. This definition incorporates the concept that there is a spectrum of disease. All patients with an arterial oxygen tension–pressure/fraction of inspired oxygen (Pao2/Fio2) ratio < 300 mm Hg have acute lung injury; only that subset with more severe hypoxemia, as reflected in a Pao2/Fio2 < 200 mm Hg, are now categorized as having ARDS.

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The definition still has some elements that are being evaluated. The meaning of acute onset has been discussed. Studies of patients at risk for the development of ARDS have shown that the majority of patients who develop ARDS do so within 3–5 days so that is what most investigators use for the timeframe defined by acute. The chest radiograph description has also been identified as an area that needs to be more refined. Rubenfeld and colleagues had a panel of experts review a set of randomly selected radiographs from patients with Pao2/ Fio2 < 300. They found that there was significant interobserver variability in the interpretation of these films. For example, the percentage of radiographs identified as being consistent with ALI varied from 36 to 71%. Accordingly, there is now a taskforce working to establish more rigid criteria for the interpretation of radiographs for this syndrome.

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The diagnosis of left atrial hypertension may also be difficult. Recent studies suggest that as many as 25% of patients identified clinically as having ALI with no evidence of left atrial hypertension actually had pulmonary artery wedge pressures greater than 18 mm Hg when the measurements were made.

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A number of clinical syndromes have been identified as risk factors for ALI (Table 15–1). The most common predisposing condition for ALI is sepsis. The incidence of ALI from sepsis ranges from approximately 30 to 50%, depending on the definition used for sepsis. Other common predisposing conditions include aspiration of gastric contents and severe trauma. Pneumonia is identified as ...

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