Acute respiratory distress syndrome (ARDS) is a clinical syndrome of severe dyspnea of rapid onset, hypoxemia, and diffuse pulmonary infiltrates leading to respiratory failure. ARDS is caused by diffuse lung injury from many underlying medical and surgical disorders. The lung injury may be direct, as occurs in toxic inhalation, or indirect, as occurs in sepsis (Table 322-1). The clinical features of ARDS are listed in Table 322-2. By expert consensus, ARDS is defined by three categories based on the degrees of hypoxemia (Table 322-2). These stages of mild, moderate, and severe ARDS are associated with mortality risk and with the duration of mechanical ventilation in survivors.
TABLE 322-1Clinical Disorders Commonly Associated with ARDS |Favorite Table|Download (.pdf) TABLE 322-1 Clinical Disorders Commonly Associated with ARDS
|Direct Lung Injury ||Indirect Lung Injury |
|Pneumonia ||Sepsis |
|Aspiration of gastric contents ||Severe trauma |
|Pulmonary contusion || Multiple bone fractures |
|Near-drowning || Flail chest |
|Toxic inhalation injury || Head trauma |
| || Burns |
| ||Multiple transfusions |
| ||Drug overdose |
| ||Pancreatitis |
| ||Postcardiopulmonary bypass |
TABLE 322-2Diagnostic Criteria for ARDS |Favorite Table|Download (.pdf) TABLE 322-2 Diagnostic Criteria for ARDS
|Severity: Oxygenation ||Onset ||Chest Radiograph ||Absence of Left Atrial Hypertension |
Mild: 200 mmHg < Pao2/Fio2 ≤ 300 mmHg
Moderate: 100 mmHg < Pao2/Fio2 ≤ 200 mmHg
Severe: Pao2/Fio2 ≤ 100 mmHg
|Acute ||Bilateral alveolar or interstitial infiltrates ||PCWP ≤18 mmHg or no clinical evidence of increased left atrial pressure |
The annual incidence of ARDS is estimated to be as high as 60 cases/100,000 population. Approximately 10% of all intensive care unit (ICU) admissions involve patients with acute respiratory failure; ~20% of these patients meet the criteria for ARDS.
While many medical and surgical illnesses have been associated with the development of ARDS, most cases (>80%) are caused by a relatively small number of clinical disorders: severe sepsis syndrome and/or bacterial pneumonia (~40–50%), trauma, multiple transfusions, aspiration of gastric contents, and drug overdose. Among patients with trauma, the most frequently reported surgical conditions in ARDS are pulmonary contusion, multiple bone fractures, and chest wall trauma/flail chest, whereas head trauma, near-drowning, toxic inhalation, and burns are rare causes. The risks of developing ARDS are increased in patients with more than one predisposing medical or surgical condition.
Several other clinical variables have been associated with the development of ARDS. These include older age, chronic alcohol abuse, metabolic acidosis, and severity of critical illness. Trauma patients with an Acute Physiology and Chronic Health Evaluation (APACHE) II score ≥16 (Chap. 321) have a 2.5-fold increased risk of developing ARDS, and those with a score >20 ...