As opposed to traditional pacemakers, whose purpose is to make sure the heart rate does not fall below a certain level, biventricular pacemakers are meant to synchronize the contraction of the left and right sides of the heart in certain individuals with congestive heart failure and an electrical imbalance in the electrical conduction system. This is called cardiac resynchronization therapy (CRT) and is accomplished through an additional lead that is placed within the coronary sinus, a major vein in the left side of the heart.  Biventricular pacemakers are often coupled with implantable cardioverter defibrillators in the same device (CRT-D).   Preparation and the procedure itself are very similar to that involved in a defibrillator implant, except that due to the technical details involved in the placement of the third lead the procedure time can be longer (as long as 4-5 hours).