Commotio cordis: Sudden cardiac arrest from a blunt, nonpenetrating blow to the chest. The basis of the cardiac arrest is ventricular fibrillation (a chaotically abnormal heart rhythm) triggered by chest wall impact immediately over the anatomic position of the heart. This can happen, for example, in sports such with baseballs, softballs, lacrosse balls, hockey pucks and a blow in boxing. The only effective treatment is cardiopulmonary resuscitation with defibrillation to reverse ventricular fibrillation. Commotio cordis ia also known as a "concussion of the heart."