Pacemaker

Although it weighs just about an ounce, a pacemaker contains a powerful battery, electronic circuits and computer memory that together generate electronic signals. The signals, or pacing pulses, are carried along thin, insulated wires, or leads, to the heart muscle. The signals cause the heart muscle to begin the contractions that cause a heartbeat.

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How Does a Pacemaker Work?

Your heartbeat is controlled by the heart's own bioelectrical triggering system. When that system ceases to work properly, the solution may be a pacemaker.

The pacemaker has two parts -- a battery-powered generator and the wires that connect it to the heart. The silver-dollar-size generator, which has an effective life of seven to 12 years, is implanted just beneath the skin below the collarbone. The leads are threaded into position through veins leading back to the heart. The entire implantation procedure requires only a local anesthetic, and takes about an hour.

Most patients with pacemakers suffer from a condition in which the heart beats too slowly -- bradyarrhythmia is the medical term. This is most commonly a result of deterioration in the heart's own pacing system in elderly patients, though high blood pressure, coronary artery disease or scarring from a heart attack can also cause bradyarrhythmias.

A pacemaker consists of a battery, a computerized generator, and wires with electrodes on one end. The battery powers the generator, and a thin metal box surrounds both it and the generator. The wires connect the generator to the heart.

The pacemaker's generator sends the electrical pulses that correct or set your heart rhythm. A computer chip figures out what types of electrical pulses to send to the heart and when those pulses are needed. To do this, the computer chip uses the information it receives from the wires connected to the heart. It also may use information from sensors in the wires that detect your movement, blood temperature, breathing, or other factors that indicate your level of physical activity. That way, it can make your heart beat faster when you exercise.

The computer chip also records your heart's electrical activity and heart rhythms. Your doctor will use these recordings to set your pacemaker so it works better at making sure you have a normal heart rhythm. Your doctor can program the computer in the pacemaker without having to use needles or directly contacting the pacemaker.

The wires in your pacemaker send electrical pulses to and from your heart and the generator. Pacemakers have one to three wires that are each placed in different chambers of the heart.

The wires in a single-chamber pacemaker usually carry pulses between the right ventricle (the lower right chamber of your heart) and the generator.

The wires in a dual-chamber pacemaker carry pulses between the right atrium and the right ventricle and the generator. The pulses help coordinate the timing of these two chambers' contractions.

The wires in a triple-chamber pacemaker are used for heart muscle weakness and carry pulses between an atrium and both ventricles and the generator. The pulses help coordinate the timing of the two ventricles with each other.

Cross-Section of the Chest With a Pacemaker

The illustration shows a cross-section of the chest with a pacemaker. Figure A shows the location and general size of a double chamber, or double lead, pacemaker in the upper chest. The wires with electrodes are inserted into the right atrium and ventricle of the heart through a vein in the upper chest. Figure B shows the electrode electrically stimulating the heart muscle. Figure C shows the location and general size of a single chamber, or single lead, pacemaker in the upper chest. The wire with the electrode is inserted into the right ventricle of the heart through a vein in the upper chest.

Types of Pacemaker Programming

There are two main types of programming for pacemakers—demand pacing and rate-responsive pacing.

A demand pacemaker monitors your heart rhythm. It only electrically stimulates your heart if it’s beating too slow or if it misses a beat.

A rate-responsive pacemaker will speed up or slow down your heart rate depending on how active you are. To do this, the rate-responsive pacemaker monitors your sinus node rate, breathing, blood temperature, or other factors to determine your activity level. Most people who need a pacemaker to continually set the pace of their heartbeat have rate-responsive pacemakers.