Cardiac arrest presenting as pulseless electrical activity (PEA) is associated with poor outcome. Its major underlying causes (e.g. cardiac tamponade, pulmonary embolism, tension pneumothorax or hypovolaemia) are difficult to detect reliably in an out-of-hospital setting. We here present a case of a 14-year-old girl suffering from PEA because of cardiac tamponade 4 weeks after surgical closure of a ventricular septal defect. Immediate focused echocardiography on scene by an emergency physician showed a large pericardial effusion that had led to cardiac tamponade and finally to a PEA cardiac arrest. Immediate pericardiocentesis was carried out. The girl progressed to complete neurological recovery. This case report demonstrates that focused emergency echocardiography may be useful for the diagnosis of pericardial tamponade leading to cardiac arrest and this diagnosis can be made out-of-hospital by an appropriately trained physician.