Acute Respiratory Distress Syndrome (ARDS)

KEY FEATURES

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 PaO₂/FIO₂ < 300 mm Hg, with positive end-expiratory pressure (PEEP) ≥ 5 cm H₂O

General Considerations

• Damage to capillary endothelial cells and alveolar epithelial cells is common and results in increased vascular permeability and decreased production and activity of surfactant, leading to

  • Interstitial and alveolar pulmonary edema

  • Alveolar collapse

  • Hypoxemia

• Pro-inflammatory cytokines released from stimulated inflammatory cells appear to be pivotal in lung injury

• Severity of ARDS is based on the level of oxygenation impairment

  • Mild: PaO₂/FIO₂ ratio of between 200 mm Hg and 300 mm Hg

  • Moderate: PaO₂/FIO₂ ratio between 100 mm Hg and 200 mm Hg

  • Severe: PaO₂/FIO₂ ratio < 100 mm Hg

Systemic insults associated with ARDS

• Sepsis (Table 9–26)

• Shock

• Pancreatitis

• Multiple blood-product transfusions

• Trauma

• Burns

• Disseminated intravascular coagulation (DIC) and thrombotic thrombocytopenia purpura (TTP)

• Drugs

  • Opioids

  • Aspirin

  • Phenothiazines

  • Tricyclic antidepressants

  • Amiodarone

  • Chemotherapeutics

  • Nitrofurantoin

  • Protamine

• Cardiopulmonary bypass

• Head injury

• Paraquat exposure

Pulmonary insults associated with ARDS

• Aspiration of gastric contents

• Near-drowning

• Pneumonia, bacterial, viral, or fungal

• Miliary tuberculosis

• Toxic gas inhalation

  • Nitrogen dioxide

  • Chlorine

  • Sulfur dioxide

  • Ammonia

  • Smoke inhalation

• Free-base cocaine smoking

• Embolism of thrombus, fat, air, or amniotic fluid

• Lung contusion

• Acute eosinophilic pneumonia

• Cryptogenic organizing pneumonia

• Acute upper airway obstruction

• Lung reexpansion or reperfusion

• Lung contusion

• Radiation exposure

• High-altitude exposure

• Oxygen toxicity

CLINICAL FINDINGS

Symptoms and Signs

• Rapid onset of profound dyspnea, usually 12–48 hours after the initiating event

• Labored breathing and tachypnea, with crackles on examination

• Marked hypoxemia refractory to supplemental oxygen

• Multiple-organ failure, particularly involving ...