Pacemakers
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.
Operation
Installing a pacemaker is a relatively minor surgical procedure that generally takes about an hour. It is often performed by an electrophysiologist, a specialized cardiologist, or surgeon. Under local anesthesia, a small incision is made under the collarbone, then the lead or leads are threaded through the subclavian vein into the heart's right side. Fluroscopy, a type of x ray that involves projecting an image on a fluorescent screen, is used to guide the process and requires the wearing of a lead apron during operation. Often, right-handed patients have their pacemaker put in their left side and vice versa to speed return to normal activities.
Once the leads are in place, tests are performed to make sure the placement provides the needed connection for pacing. If the signals from the leads on the heart are too weak, the tip may have been placed in dead heart tissue and may need to be repositioned. The connection can be attached to the surface of the heart by a small corkscrew, known as active fixation, or a tined tip, known as passive fixation. With either passive or active fixation, a layer of fibrin (a blood protein) matures the lead connection within six weeks of the installation.
Next, the pulse generator is embedded into a pocket under the skin of the chest and the leads are connected. At this point the pulse generator has to be checked to make sure it is functioning correctly using a pacemaker system analyzer (PSA), a computer which checks the device is working correctly. If all checks out, the skin is sutured in place and a dressing placed over the wound.
Fine tuning of the pacemaker settings will occur in the recovery room using a programmer, a special computer equipped with a wand that is placed on the patient's chest over the pacemaker. The programmer and the pacemaker communicate in a method similar to a television remote control. Two important variables in this programming are the pacemaker's capture and sensing. Capture refers to the voltage and pulse width of the electrical signal the device will deliver. The programming is set to ensure that the capture is set high enough that two to three times the threshold (minimum) voltage necessary is delivered, called the margin of safety. However, the capture should not be so high as to unnecessarily drain the battery and require earlier replacement of the device.
Sensing involves the ability of the pacemaker to detect signals coming from the patient's heart and to shut itself off until a predetermined interval passes without a signal. Pacemakers see the heart signals much like an implanted electrocardiography unit. Poor sensing is what causes the pacemaker syndrome often seen with single chamber pacemakers. For proper sensing, the leads need to be adjusted so that the intra-cardial signals are seen at the highest voltage possible. This allows the sensitivity of the pacemaker to be set at a lower level. If the sensitivity has to be turned up too much, chest muscle activity could interfere with the heart signal.
Most patients stay in the hospital for one to two days after implantation, but some can leave the same day.
Safety
Once the pacemaker is installed environmental conditions can affect the functioning of the unit. These include:
- strong electromagnetic fields, such as those used in arc-welding
- contact sports
- shooting a rifle from that shoulder
- cell phones used on that side of the body
- some medical tests such as magnetic resonance imaging (MRI)
Environmental conditions often erroneously thought to affect pacemakers include:
- microwave ovens (The waves only affect old, unshielded pacemakers.)
- airport security (Although metal detector alarms could be set off—patients should carry a card stating they have a pacemaker implanted.)
Maintenance
In general, if the condition of the patient's heart, drug intake, and metabolic condition remain the same, the pacemaker requires only periodic checking every two months or so for battery strength and function. This is done by placing a special device over the pacemaker that allows signals to be sent over the telephone to the doctor, a process called trans-telephonic monitoring.
If changes in medications or physical condition occur, the doctor can adjust the pacemaker settings using a programmer, which involves placing the wand above the pacemaker and remotely changing the internal settings.
Drugs taken by the patient and metabolic conditions affect both capture and sensing thresholds. For example, drugs such as ephedrine or glucocosteroids cause lower thresholds, while some anti-arrhythmics cause higher thresholds. Hyperoxia (an excess of oxygen in the system) and hypocapnia (a deficiency of carbon dioxide) are two metabolic conditions that can lower thresholds and acidosis (an accumulation of acid in the body) or alkalosis (an accumulation of base in the body) can cause higher thresholds. Reprogramming of the pacemaker can accommodate the new capture and sensing values needed.
When the periodic testing indicates that the battery is getting low, an elective pacemaker replacement operation is scheduled. The entire signal generator is replaced because the batteries are sealed within the case. The leads can often be left in place and reattached to the new generator. Batteries usually last about six to eight years.
Health care team roles
Electrophysiologists are specially trained cardiologists who study and treat problems with the heart conduction system. They are often the type of physician that will implant the pacemaker system and oversee the programming or reprogramming of the device. They are assisted in the operating room by specially trained nurses, who can help with the testing of the pacemaker, and the anesthesiologist, who is responsible for numbing the area of the incision and keeping the patient comfortable. Pacemaker manufacturers often send representatives to be present for the implantation and initial programming.
The maintenance of the pacemaker can be overseen by the electrophysiologist or cardiologist and their staff, which can include specially trained cardiac medical assistants as well as nurses.
Training
The training for pacemakers and their use occurs during medical training (medical or nursing school) and on the job. Physicians, nurses, and other allied health professionals can also receive training in pacemakers as part of their continuing education courses. Such training often focuses on a particular aspect of pacemaker use, such as diagnosing problems in persons having pacemakers implanted, the installation of transient pacing, or the treatment of fibrillation or heart failure with pacemakers.
