Heart failure is a common chronic disorder in which the heart loses its ability to pump blood efficiently, depriving other organs of the oxygen and nutrients needed to function properly. According to the American Heart Association, in the United States in 2006, heart failure caused 60,337 deaths and was an underlying or contributing cause of another 222,417 deaths. Obesity, diabetes, high blood pressure, and hardening of the arteries increase the risk of heart failure, which can develop as a result of a heart attack, progressive enlargement of the left ventricle, or heart valve disease. Although pacemakers, implantable defibrillators, and a variety of drugs have been developed to treat heart failure, some patients have end-stage disease that does not respond adequately to these interventions. Moreover, due to advanced age or poor medical condition, a number of patients who have unresponsive end-stage heart failure are not eligible for heart transplantation.