More than 350,000 people each year will experience an out of hospital cardiac arrest. Cardiac arrest is an extremely dangerous circumstance that requires immediate treatment. In cardiac arrest, death results when the heart suddenly stops working properly. This may be caused by abnormal, or irregular, heart rhythms (called arrhythmias). Since prior heart attack, or myocardia infarction, is a major risk factor for arrhythmia, these patients are prime candidates for surgically implanted defibrillators, which monitor heart rhythm and deliver an electric shock if needed to keep the heart beating regularly. The current tools for assessing whether a patient is likely to actually suffer an arrhythmia and therefore benefit most from the defibrillator (which carries its own risks) are not highly predictive. Dr. Natalia Trayanova, the Murray B. Sachs Professor of Biomedical Engineering and Medicine at Johns Hopkins University, and a team of researchers are working to change this.