The unit is implanted under the skin and attached to the heart with leads. Electrical impulses are then used to correct the arrhythmia or arrhythmias.

The heart is essentially two pumps - the upper chambers called the atria and the lower chambers called the ventricles. Pumping of the heart is controlled by electrical impulses from the sinus node, a group of cells in the right atrium.

If something goes wrong with the functioning of the sinus node and normal pacing of the heart is disrupted, a number of arrhythmias may develop. Some are too fast, others too slow. Still others are irregular or out of proper sequence.

Defibrillators monitor the heart's rhythm and when an excessively rapid rhythm occurs, it delivers a shock to correct it. Pacemakers are typically used to prevent a slow rhythm or ensure that the chambers beat in the proper sequence (atrium then ventricle).

Many patients who require an implantable defibrillator to correct a dangerously fast heart rhythm also experience slow or out-of-sequence rhythms.

Unlike the earlier devices, the combination unit provides pacing of both the atrium and ventricle, as well as ventricular defibrillation.

Like other defibrillators, the device records and stores data about the heart's electrical activity in the form of electrocardiogram (EKG) readings that the physician can access. However, the Pacemaker Defibrillator combination unit takes that feature a step further with electrical information from the atrium as well as the ventricles.