A CRT-D, also known as a biventricular device is a type of implanted pacemaker defibrillator (ICD) that can pace both the septal and lateral walls of the left ventricle. CRT devices have been shown to reduce mortality and improve quality of life in patients with heart failure symptoms. By being on both sides of the left ventricle, the device can resynchronize a heart whose opposing walls do not contract in synchrony, which occurs in approximately 25-50 % of heart failure patients. CRT devices have at least two leads, one in the right ventricle to stimulate the septum, and another inserted through the coronary sinus to pace the lateral wall of the left ventricle. Sometimes, for patients in normal sinus rhythm, there is also a lead in the right atrium to facilitate synchrony with the atrial contraction. Thus, timing between the atrial and ventricular contractions, as well as between the septal and lateral walls of the left ventricle can be adjusted to achieve optimal cardiac function.