A pacemaker is a small, battery-operated device that senses when your heart is beating irregularly or too slowly. It sends a signal to your heart that makes your heart beat at the correct pace.

Newer pacemakers weigh as little as 1 ounce. A pacemaker usually has 2 parts:

• The generator contains the battery and the information to control the heartbeat.
• The leads are wires that connect the heart to the generator and carry the electrical messages to the heart.

A pacemaker must be implanted under the skin. This procedure usually takes about 1 hour. You will be given a sedative to help you relax. You will be awake during the procedure.

A small incision (cut) is made, usually on the left side of the chest below your collarbone. The pacemaker generator is then placed under the skin at this location.

Using live x-rays to see the area, the doctor puts the leads through the incision, into a vein, and then into the heart. The leads are connected to the generator. The skin is closed with stitches. Most patients go home within 1 day of the procedure.

Two kinds of pacemakers -- transcutaneous and transvenous pacemakers -- are used only in medical emergencies. They are not permanent pacemakers.

Pacemakers may be used for people who have heart problems that cause their heart to beat too slowly. A slow heartbeat is called bradycardia. Two common problems that cause a slow heartbeat are sinus node disease and heart block.

When your heart beats too slowly, your body and brain may not get enough oxygen. Symptoms may be lightheadedness, tiredness, fainting spells, and shortness of breath.

Some, pacemakers can be used to stop a heart rate that is too fast (tachycardia) or that is irregular.

Other types of pacemakers can be used in severe heart failure. These are called biventricular pacemakers. They match up the beating of both sides of the heart.

All of today's biventricular pacemakers can also work as implantable cardio-defibrillators (ICD), which restore a normal heartbeat.