A 16-month-old boy suffered a cardiac arrest as a result of acute myocarditis, and venoarterial extracorporeal membrane oxygenation was instituted. Twelve hours later, acute left heart distention developed with cessation of left ventricular ejection. Under transesophageal echocardiographic guidance, a long introducer was placed into the left atrium through a transseptal puncture and connected in-line to the venous circuit. Within hours, left ventricular function improved and ejection returned. Left heart decompression was continued for 5 days, and the patient was weaned from extracorporeal membrane oxygenation after 6 days with normal cardiac and neurologic function.