Respiratory failure and ARDS
Also known as: acute respiratory distress syndrome
Acute respiratory distress syndrome (ARDS) is a form of acute hypoxemic respiratory failure in which diffuse injury to the alveolar-capillary membrane causes protein-rich, noncardiogenic pulmonary edema and severe impairment of oxygenation.
Acute respiratory distress syndrome (ARDS) is severe, rapidly developing respiratory failure caused by diffuse alveolar damage. Injury to the alveolar-capillary membrane makes it leaky, flooding the alveoli with protein-rich fluid — a noncardiogenic pulmonary edema — and forming the hyaline membranes seen on histology. The result is stiff, poorly compliant lungs and profound hypoxemia.
ARDS is always secondary to an underlying insult. Sepsis is the most common cause, followed by pneumonia, aspiration of gastric contents, severe trauma, pancreatitis, and transfusion-related injury. Diagnostically, think of the Berlin definition: onset within one week of a known insult, bilateral opacities on imaging, edema not fully explained by heart failure or fluid overload, and a PaO₂/FiO₂ ratio of 300 or less measured on at least 5 cm H₂O of PEEP or CPAP — with lower ratios marking more severe disease.
Physiologically, fluid-filled alveoli create intrapulmonary shunting: blood passes unventilated alveoli, so the hypoxemia responds poorly to supplemental oxygen alone. This distinguishes ARDS within the broader framework of respiratory failure — type 1 (hypoxemic) failure involves low PaO₂ with normal or low PaCO₂, while type 2 (hypercapnic) failure reflects inadequate ventilation. Management centers on treating the underlying cause and lung-protective mechanical ventilation with low tidal volumes and positive end-expiratory pressure (PEEP).
The USMLE Step 1 tests ARDS through vignettes pairing a septic or trauma patient with refractory hypoxemia and bilateral infiltrates — expect questions on hyaline membranes, the PaO₂/FiO₂ criterion, shunt physiology, and why low tidal volume ventilation is used.
Key takeaways
- ARDS is acute hypoxemic respiratory failure from diffuse alveolar damage and noncardiogenic pulmonary edema.
- Sepsis is the most common cause; pneumonia, aspiration, trauma, and pancreatitis are other major triggers.
- Diagnosis requires acute onset, bilateral opacities, edema not explained by cardiac failure, and PaO₂/FiO₂ ≤ 300 on at least 5 cm H₂O of PEEP or CPAP.
- Intrapulmonary shunting makes the hypoxemia poorly responsive to oxygen alone.
- Treatment is supportive: address the underlying cause and ventilate with low tidal volumes and PEEP.