PACEMAKERS
The heart has an intrinsic system of electrical activity which accounts for its orderly and regular contraction. The rate and rhythm of the heart beat are modified by heart diseases and medications as well as psychological, neurological, chemical and hormonal factors.
When certain abnormal heart rhythms (arrhythmia’s) occur, the heart may be unable to pump enough blood to maintain either a normal blood pressure or an adequate supply of oxygen to body tissues. Blackouts and/or sudden death may result.
If arrhythmia’s cannot be reversed and prevented by medication, a pacemaker is often required. The vast majority of arrhythmia’s that necessitate pacemaker insertion are manifested by extremely slow heart rates (less than 50 beats per minute). Occasionally, pacers are used to shock abnormally fast heart rates back into a normal rhythm.
Types of Pacemakers
Pacemakers are comprised of a hockey puck-sized electrical impulse generator connected to one (unipolar) or two (bipolar) wire leads which are attached to the heart muscle. Although there are many power sources available--from nuclear to a variety of power cells--lithium batteries are most commonly employed.
Temporary pacers are utilized when the heart abnormality or disorder is reversible or short-lived, e.g. heart attack, chemical imbalance, or drug overdose. They are also used to treat certain fast heart rhythms, evaluate the effects of heart medications, and to tide the patient over until arrangements can be made for placement of a permanent pacemaker. Permanent pacers are designed to work indefinitely, limited only by the lifetime of their power source (about eight years for lithium batteries).
Fixed-rate pacers generate impulses without regard for the intrinsic heart beat. Once the pacer is turned on and the rate is set, it fires continually at regular intervals. Because it can be dangerous for the pacer to fire at certain points during the heart beat, fixed-rate pacers have been replaced, for the most part, by demand pacers. The demand feature refers to the ability of the pacer to sense the heart's intrinsic beat and transmit an impulse only when necessary. Demand pacers, therefore, function intermittently depending upon the heart's ability to beat on its own.
Programmable pacers are adjustable for a variety of parameters. While they are not truly programmable in the computer sense of the word, changes in the rate, electrical output, sensitivity, mode, etc. can be made without removing the device. The two techniques of
effecting alterations in generator function are radio-frequency signals and pulsating magnetic fields. The former is preferable and can be adapted to receive information from the pacer as well as transmit.
The two methods of pacemaker placement are described below. Endocardial leads are those attached to the inner lining of the heart; epicardial leads are attached to the outside lining.
Endocardial Lead Placement
Also known as transvenous pacing, about 90 percent of all pacemakers are placed by this method using either local or general anesthesia. First the wire lead(s) is inserted into a vein in the neck, beneath the collarbone or near the shoulder. Then, using fluoroscopic X-rays as a guide, the surgeon advances the lead(s) into the chambers on the right side of the heart where it becomes lodged in the muscular fibers. Some leads have hooks, tines or screws to facilitate attachment to the inner wall (endocardium) of the heart. Which chamber (atrium or ventricle) is used to anchor the lead(s) depends on the type of arrhythmia and the pacemaker used. For permanent pacing, the generator is connected and placed beneath a pocket of skin on the chest or near the shoulder. Temporary pacers utilize an external generator at the bedside.
Epicardial Lead Placement
An operation under general anesthesia is required. Either the chest cavity is opened through a small incision beneath the left breast or, more commonly, an incision is placed below the breastbone. The heart is visualized directly, and the lead(s) is screwed into its outer wall (epicardium). The generator may be placed inside the chest cavity or under the skin in the upper abdomen of flank. Although this method assures more accurate and secure lead placement, it involves a bigger operation at greater risk to the patient.
Postoperative Care
The placement of the pacer leads can be checked with X-rays. The average length of hospitalization is about one day for insertion of a permanent pacing device. Most patients are followed-up by their cardiologists for regular pacemaker checks.
Complications
Among the operative problems are the risks of anesthesia, infection, bleeding, damage to blood vessels, perforation of the heart, blood clots in the veins, life-threatening arrhythmia’s and death. Long-term complications include lead dislodgement and erosion, pacer malfunction, migration of the generator, broken wires, damage to heart valves and pacemaker stimulation of the diaphragm muscle.
Results
About 80 percent of pacemaker patients are alive at two years, 65 percent at five years, and 40 percent at ten years. These figures do not differ remarkably from those for the general population matched for age.
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