Patients with heart failure have a pumping action of the ventricles in which one ventricle contracts before the other ventricle. This uncoordinated (unsynchronized) pumping is due to a delay in the stimulation of the left ventricle because of its increased size. Pacing both the right and the left side of the heart (or cardiac resynchronization therapy [CRT]) has been proven to be effective in the treatment of heart failure (HF). Current market-approved devices combine both pacing (CRT) and shocking (implantable cardioverter defibrillator [ICD]) therapy for patients who have severe heart failure and are at risk for developing life-threatening heart rhythms. These devices provide an electrical pacing stimulus to both ventricles and may help the heart contract in a more coordinated way and improve heart failure symptoms.