A 56-year-old man has been admitted to the ICU in respiratory distress. An endotracheal tube is placed for mechanical ventilation at a tidal volume of 900 mL, a rate of 12 breaths/min, and FiO2 50%. PEEP is 10 cm of water. Medications include subcutaneous heparin and aspirin. He now develops tachycardia and a blood pressure of 70/palpation mm Hg. Cardiac examination reveals multiple premature contractions. His arterial blood gas reveals a PO2 of 40 mm Hg. Most likely cause of this condition is?