ObjectiveTo recognize cardiac stun syndrome and electromechanical dissociation in patients receiving extracorporeal membrane oxygenation (ECMO), and to define patients at risk.DesignRetrospective review.SettingTertiary neonatal ICU.PatientsFour newborn patients with car-diorespiratory failure who developed signs of cardiac stun syndrome and electromechanical dissociation early in the ECMO course.Measurements and Main ResultsInitially, these patients had metabolic acidosis, chest roentgenograms showing pulmonary granularity and moderate cardiomegaly, and symptoms of severe respiratory distress. Cardiac dysfunction was apparent after ECMO was begun, with poor perfusion, pale color, narrow pulse pressure, and tachycardia despite normovolemia. Within 1 to 2 hrs, electromechanical dissociation occurred manifested by the absence of pulse pressure, palpable pulse, cardiac sounds, and apical impulse while on 50% to 70% bypass. All patients survived.InterventionsPatients received ECMO, calcium gluconate, sodium bicarbonate, and dobutamine.ConclusionsPatients with cardiac stun syndrome have symptoms similar to severe respiratory distress syndrome, and may require ECMO support. In the ECMO patient, cardiac stun syndrome and electromechanical dissociation can be confused with low circuit volume, pneumothorax, or cardiac tamponade. Early recognition of electromechanical dissociation may improve care and outcome. Cardiac stun syndrome can be treated successfully with ECMO.