• What Are Electronic Pacemakers?
• What Types of Pacemakers are There?
• Which Type of Pacemaker is Right for Me?
• What Types of Problems Occur with Pacemakers?
• Where are the Web Sites for the Major Pacemaker Vendors?

Recall that the heart is like an automobile engine: it has an electrical system (like the battery, distributer, and spark plugs) and a mechanical system (like the valves, pistons, and driveshaft). In the heart, the sinus node normally generates the electrical impulses that drive the heart. These impulses are carried by the muscle in the atria (upper chambers) to the atrioventricular node, which connects the atria) and the lower chambers of the heart (the ventricles). The atrioventricular node carries the impulses to the His-purkinje system, which carries them to the ventricles. The atria and the ventricles contract in response to the stimulus, thus generating the blood flow in the body.

The atria and the ventricles are normally separated by electrical insulation, so that the impulses from the atria cannot get to the ventricles except via the atrioventricular node. (The exceptions are called "accessory pathways", which are responsible for Wolff-Parkinson-White Syndrome).

When the sinus node is diseased, the heart cannot beat as fast as it should. The normal heart rate is 60 to 100 beats per minute at rest, and up to (220 - your age) with exercise. Although other parts of the heart (specifically, the atrioventricular node and the His-Purkinje system) can generate impulses, they do so at a much lower rate (about 60 beats per minute and about 40 beats per minute, respectively).

When the atrioventricular node is diseased, the atria beat normally but the ventricles cannot. This is a problem because only the ventricles are powerful enough to maintain the blood pressure. In fact, one can live without effective atrial contraction (e.g., during atrial fibrillation). One cannot live without effective ventricular contraction (e.g., during ventricular asystole, ventricular tachycardia, or ventricular fibrillation).

When the sinus node is diseased, the pacemaker has no way to know the best heart rate to maintain without having an artificial sensor of body activity. When a pacemaker has one or more such sensors, the letter "R" is added as a fourth letter in the code. For example, "AAIR", "VVIR", "DDDR".

Other types of pacemakers exist, but these are the most common.

If the sinus node is diseased but the atrioventricular node is not, an AAIR pacemaker is reasonable. This is because an AAIR pacemaker substitutes for the sinus node. In many cases, however, the cardiologist will choose a DDDR pacemaker because the disease process that affects the sinus node can later affect the atrioventricular node.

If the atrioventricular node is diseased but the sinus node is not and atrial fibrillation is not present, a DDD pacemaker is reasonable. This is because the pacemaker will make the ventricles follow the atria, and the atria will follow the sinus node. In some cases, however, the cardiologist will choose a DDDR pacemaker because the disease process that affects the atrioventricular node can later affect the sinus node.

If the atrioventricular node is diseased and atrial fibrillation is not present, a DDDR pacemaker is reasonable. This is because both chambers are able to respond to the pacemaker stimuli, and the sinus node cannot tell the pacemaker what the best (most physiologic) rate should be at every moment. If atrial fibrillation is sometimes present, a DDDR pacemaker with "mode switching" capability is a good choice because this capability allows the pacemaker to function in DDDR mode during sinus rhythm and VVIR mode during atrial fibrillation.

If the atrioventricular node is diseased and chronic atrial fibrillation is present, a VVIR pacemaker is reasonable. In this case, the atria are driven at 500-600 beats per minute by the fibrillation, making them useless both electrically and mechanically. The only option is to block the atrioventricular node either reversibly with medications or permanently with radiofrequency ablation, and then to pace the ventricles by themselves.

It used to be thought that simple VVI or VVIR pacing would be satisfactory even during sinus rhythm. However, although the blood pressure is maintained, this type of pacing is more likely to lead to atrial fibrillation and to congestive heart failure. Since dual chamber pacemakers are not expensive compared with the cost of caring for patients with those complications, many physicians in the U.S. are now choosing dual chamber pacemakers when possible. Physicians in Europe, where health care is often paid by the government, often select single-chamber pacemakers.

Long-term problems are unusual, but include malfunction of the leads (either oversensing or undersensing), damage of the body from the lead (known to occur only with the Accufix atrial "J" lead from Telectronics, Inc.), swelling of the arm on the side on which the pacemaker is implanted, and infection.

Of these, infection is the most troublesome because it often requires that the pacemaker and the leads be removed and that intravenous antibiotics be given. If you have a pacemaker and you think it might be infected, call your doctor immediately. Signs of infection include redness, unusual soreness, swelling, and/or warmth at the pacemaker site, or signs of generalized infection such as fevers and/or sweats.

• CPI-Guidant
• Sulzer Intermedics
• Medtronic
• St. Jude Medical (includes former Telectronic Pacing Systems devices, Pacesetter pacemakers, and Ventritex cardioverter-defibrillators)

• American College of Cardiology
• Heart Rhythm Society