ESSENTIALS OF DIAGNOSIS
Onset of respiratory distress, often progressing to respiratory failure, within 7 days of a known clinical insult.
New, bilateral radiographic pulmonary opacities not explained by pleural effusion, atelectasis, or nodules.
Respiratory failure not fully explained by heart failure or volume overload.
Impaired oxygenation, with ratio of partial pressure of oxygen in arterial blood (PaO2) to fractional concentration of inspired oxygen (FIO2) less than 300 mm Hg, with PEEP 5 cm H2O or more.
Acute respiratory distress syndrome (ARDS) as a clinical syndrome is based on three inclusion criteria plus one exclusion criterion, as detailed above. The severity of ARDS is based on the level of oxygenation impairment: mild, PaO2/FIO2 ratio between 200 and 300 mm Hg; moderate, PaO2/FIO2 ratio between 100 and 200 mm Hg; and severe, PaO2/FIO2 ratio less than 100 mm Hg.
ARDS may follow a wide variety of clinical events (Table 9–28). Common risk factors for ARDS include sepsis, aspiration of gastric contents, shock, infection, lung contusion, nonthoracic trauma, toxic inhalation, near-drowning, and multiple blood transfusions. About one-third of ARDS patients initially have sepsis syndrome. Pro-inflammatory cytokines released from stimulated inflammatory cells appear to be pivotal in lung injury. Damage to capillary endothelial cells and alveolar epithelial cells is common to ARDS regardless of cause or mechanism of lung injury, resulting in increased vascular permeability and decreased production and activity of surfactant; these abnormalities lead to interstitial and alveolar pulmonary edema, alveolar collapse, and hypoxemia.
Table 9–28.Selected disorders associated with ARDS. ||Download (.pdf) Table 9–28. Selected disorders associated with ARDS.
|Systemic Insults ||Pulmonary Insults |
Disseminated intravascular coagulation
Drugs and drug overdose
Thrombotic thrombocytopenic purpura
Aspiration of gastric contents
Embolism of thrombus, fat, air, or amniotic fluid
Diffuse pneumonia (eg, SARS)
Acute eosinophilic pneumonia
Cryptogenic organizing pneumonitis
Upper airway obstruction
Free-base cocaine smoking
Toxic gas inhalation
Lung reexpansion or reperfusion
ARDS is marked by the rapid onset of profound dyspnea that usually occurs 12–48 hours after the initiating event. Labored breathing, tachypnea, intercostal retractions, and crackles are noted on physical examination(eFigure 9–34). Chest radiography shows diffuse or patchy bilateral infiltrates that rapidly become confluent; these characteristically spare the costophrenic angles. Air bronchograms occur in about 80% of cases. Upper lung zone venous engorgement is uncommon. Heart size is usually normal, and pleural effusions are small or nonexistent. Marked hypoxemia occurs ...