Stunned myocardium during extracorporeal membrane oxygenation. The "stunned myocardium" is a syndrome of reversible myocardial dysfunction that may be mediated by oxygen-derived free radicals. This phenomenon has been seen in some neonates undergoing extracorporeal membrane oxygenation. We performed echocardiograms and measured creatine phosphokinase isoenzymes and lipid peroxide levels in 16 neonates before, during, and after extracorporeal membrane oxygenation. Infants who developed stunned myocardia by echocardiography did so shortly after initiation of bypass and exhibited concurrent elevations of the MB fraction of creatine phosphokinase. Lipid peroxide levels did not simultaneously rise. These data suggest that oxygen-derived free radicals may not cause the stunned myocardium seen in neonates undergoing extracorporeal membrane oxygenation. 