A 41-year-old woman presents with severe community-acquired pneumococcal pneumonia. Chest radiography reveals diffuse bilateral infiltrates, and hypoxemic respiratory failure develops despite appropriate antibiotic therapy. She is intubated and mechanical ventilation is initiated with a volume- and pressure-limited approach for the acute respiratory distress syndrome (ARDS). Over the ensuing 24 hours, her partial pressure of arterial oxygen (Pao2) decreases to 40 mm Hg, despite ventilatory support with a fraction of inspired oxygen (Fio2) of 1.0 and a positive end-expiratory pressure (PEEP) of 20 cm of water. She is placed in the prone position and a neuromuscular blocking agent is administered, without improvement in her Pao2. An intensive care specialist recommends the initiation of extracorporeal membrane oxygenation (ECMO).