Pacemakers Health Article

Definition

A pacemaker is an implantable electronic device that delivers electrical stimulation to the heart to help regulate its beat.

Purpose

Pacemakers are used to correct abnormal rhythms of the heart, most notably, brachycardia, an abnormally slow heartbeat. Normal heartbeat is 60 to 100 beats per minute (bpm) and brachycardia occurs anywhere below 60. One cause of brachycardia is when the natural pacemaker of the heart, the sinoatrial (SA) node, does not function. Known as sick sinus syndrome, signals from the node are always slow or do not accelerate to accommodate exercise or stress. Considered a part of the normal aging process, this syndrome results in a heartbeat that is too slow to circulate enough blood to meet the needs of the body. Symptoms include fatigue, activity intolerance, or even unconsciousness (also known as syncope). Pacemakers cure this condition by providing the needed electrical stimulus when the SA node does not work.

Pacemakers can also be used to treat a condition known as heart block. This problem occurs when the electrical connection between the upper chambers of the heart (atria) and lower chambers of the heart (ventricles) either fails or is significantly slowed. The area of the heart where this signal travels is called the atrio-ventricular (AV) node. The ventricles, without other stimulus, will produce their own beat of about 20 to 40 bpm, which is insufficient to support the body. Accordingly, patients with this problem feel tired and can lose consciousness. A pacemaker can treat this condition by keeping the heart rate within the normal range.

Patients that have brachycardia or heart block are at high risk for developing a tendency to have very fast, very inefficient contractions of the atria known as atrial defibrillation. A pacemaker that senses this abnormal rhythm and can switch to a mode of firing that brings it under control has been developed. Once the defibrillation has stopped, the pacemaker automatically switches back to its usual mode of function.

Description

The two main parts of a pacemaker are the pulse generator and the leads. The pulse generator is made of a computer chip, other electronic circuitry, and a lithium battery, all enclosed in a titanium case about the size of three to four stacked fifty-cent pieces. There can be one or two leads that carry the electrical impulse produced by the generator to the heart. The generator works by sensing whether the heart is firing at the right rate and supplying the electrical signal needed to start the heartbeat if it is not. The leads are flexible, double insulated wires that are placed within the heart chambers so that the needed signal is supplied to the area of the heart as needed. The leads can be unipolar, where the implanted tip is the negative pole (the positive is the pacemaker case) or bipolar where both the negative and positive poles are in the tip. Because the electrical signal has to travel across the chest with unipolar leads, pacemakers with leads of this type are more susceptible to outside interference.

If the pacemaker has one lead, it is known as a single chamber pacemaker. The lead can be placed in either the right atrium or the right ventricle. This type of device can be used only if the signal from the SA node or the AV node is the problem, and all other electrical conduction in the patient's heart is working correctly. Patients with this type of pacemaker can sometimes feel an uncomfortable neck throbbing, chest fullness or faintness when the device fires, known as pacemaker syndrome. Because of this problem, and the general ability to pump a greater volume of blood, some patients are treated with a dual chamber system.

The dual chamber pacemaker has two leads, one that is implanted in the right atrium, and one in the right ventricle. These pacemakers are also called sequentially pacing because the electrical signal is produced in a sequence—first to the atrium, then to the ventricle. The signal generators in dual chamber systems evaluate the heart's own electrical production in both chambers and produce their own signal when either or both become inadequate.

A third type of pacemaker is a rate-responsive system. These devices have the ability to sense physical activity and alter the heart rate to accommodate it. The responsiveness of this system results from one or more types of sensors. Some conditions that are sensed include motion, depth and rate of breathing, and blood temperature. As any of these conditions increase, the pacemaker speeds the rate of firing. Rate-responsive pacemakers most closely mimic the way the heart works naturally.

To help treat patients who have atrial fibrillation, pacemakers have been developed that can switch how they work to treat the rapid abnormal heart beat, then return to the normal function.