Indications for Pacemakers

Contraction of the heart's chambers and its ability to pump blood to the lungs and throughout the body are dependent on the precise functioning of the heart's electrical system. Unfortunately, much like the electrical system in a car, which can occasionally malfunction, the heart's electrical system also can malfunction. Although there are many ways in which this can occur, there are generally two common problems that result in the need for pacemaker implantation.

An area of specialized tissue in the right atrium (called the SA node) functions as the heart's spark plug. Normally, the SA node generates an electrical impulse that travels throughout the heart about every second. In some persons, however, this spark plug-like tissue may only "fire," say, 30 or 40 times every minute, leading to heart rates of only 30 or 40 beats every minute. At such slow rates, the heart is unable to pump an adequate amount of blood throughout the body.

Inadequate blood flow to the muscles and tissues of the body may cause patients to feel tired or fatigued. Inadequate blood flow to the brain can lead to the sensation of lightheadedness or dizziness or can even cause patients to pass out. The SA node sometimes will not fire frequently enough if the patient is taking certain medications that can slow the firing rate. More frequently, it may simply start to give out over time; this often occurs in older persons. For patients whose SA node is no longer firing frequently enough and whose heart rate is too slow, a pacemaker will be implanted to ensure that the heart rate is rapid enough to pump adequate amounts of blood throughout the body.

Between the atria and ventricles of the heart is a small area of specialized tissue called the AV node. Although the function of the AV node is actually quite complex, it basically conducts electrical impulses from the atria down into the ventricles. This electrical impulse then stimulates the ventricles to contract and to pump blood. Just as the SA node can malfunction, so, too, can the AV node occasionally malfunction.

As the AV node begins to malfunction, it may no longer be able to conduct every impulse that reaches it from the atria down into the ventricles. As a result, the ventricles are no longer stimulated to contract frequently enough to pump adequate amounts of blood throughout the body. If the AV node stops working completely, the ventricles may no longer be stimulated to contract by any electrical impulses. When the AV node malfunctions to the point that too few electrical impulses are being conducted or when it appears there is a risk that the AV node may stop working completely, a pacemaker is implanted. This ensures that the ventricles continue to be stimulated frequently enough to maintain adequate blood flow in the body.

Publication Review By: Stanley J. Swierzewski, III, M.D.

Published: 01 Jul 2000

Last Modified: 31 Mar 2014