The heart is a unique organ that must function continuously to pump blood supplying oxygen to the body. It speeds up during special times of need, as when an individual is running or doing stressful work. It slows at night or during sleep when the demand for blood decreases.
This tiny pump, about the size of a fist, squeezes approximately 2.5 fl oz (75 ml) of blood out into the body with each beat. At a normal heart rhythm, this adds up to about 10 pt (5 l) of blood each minute. The heart pumps 2,500 gal (9500 l) of blood each day, and more than 100 million gal (400 million l) of blood in a lifetime. Every heartbeat must be regulated in time and intensity.
The heart muscle is driven by an internal pacemaker, a small nodule of tissue lodged in the right atrium (upper chamber), called the sinoatrial (SA) node. It generates a small electrical signal that travels through special fibers in the heart to stimulate a timed, sequential contraction of the heart muscle called the sinus rhythm.
The SA node may function irregularly over time or even stop functioning, which will interfere with the performance of the heart. There are other electrically active tissues that will issue regulatory signals if the SA node stops generating an electrical current. The heartbeat will slow considerably under guidance of the next layer of tissue. An abnormally slow heartbeat is called bradycardia. The heartbeat may also become irregular, developing an arrhythmia. On the other hand, the SA node may become overactive, causing the heart to race at an abnormally high speed, a condition called tachycardia.
To correct problems of rhythm disturbance or SA node malfunction, cardiologists often use a pacemaker, an electrical device implanted in the shoulder or abdomen of the patient with a wire leading to the heart. This mechanical pacemaker generates the electrical signal which regulates the heart's functions. The rate of heartbeat, which is set when the pacemaker is implanted, can be changed if necessary without surgery. Modern
pacemakers are available to correct virtually any form of arrhythmia.
The first pacemaker, the result of long, arduous research, was used in a patient in 1958. The pacemaker device was not implanted, but its wire was connected to the patient's heart. The pacemaker itself was so large that it had to be carted around in a grocery store cart. While it was a solution to the patient's arrhythmia, it was hardly practical. Fortunately, pacemakers were soon miniaturized.
The pacemaker was designed to regulate every single heartbeat. It took over the function of the SA node; from the time of implantation, the patient's heartbeat was directed by the pacemaker at a preset speed (usually about 70-72 beats per minute). Thus the patient's capacity for exercise was limited because no matter what conditions he was under, his heart maintained the same rate of beating. It would not speed up to provide additional oxygen needed by the tissues when the patient exercised. Since then, however, a great deal of progress has been made.
Current models of pacemakers monitor the heart to determine the heart rate and do not interfere with the heart function unless the heart rate drops below a predetermined speed (usually 66 to 68 beats per minute). Only then will the pacemaker deliver an electrical signal to drive the heart until the pacemaker determines that the SA node is again on track. The mechanical device then ceases its signals and returns to monitoring the heart rate. This is called demand pacing.
Current pacemakers weigh less than an ounce (25 g), are about the size of a quarter, and pace the upper and lower chamber as needed.
Some patients are at risk of a form of arrhythmia called fibrillation, which is a completely uncoordinated, quivering, nonfunctional heartbeat. If not corrected quickly, fibrillation can cause death. Since 1985, pacemakers have been available to monitor the speed of the heart and deliver an appropriate electrical shock to the heart muscle if it begins to fibrillate. The device can deliver a low-level pacing shock, an intermediate shock, or a jolting, defibrillating shock if necessary.
Surgeons prefer to implant pacemakers in the shoulder because the procedure can be carried out under local anesthetic. The wire from the pacemaker is inserted into one of the large veins in the shoulder and fed down into the heart, through the right atrium and into the ventricle where it is attached to the heart muscle. If the wire cannot be fed through veins that are too small or diseased, the pacemaker can be implanted in the abdomen.
Doctors must see patients with pacemakers frequently to check the battery power and make sure the circuitry is intact. Leads may become disconnected, the wire may break, or scarring may form around the electrode, all of which can render the pacemaker useless. Patients should avoid sources of electromagnetic radiation, including security scanning devices at airports and diagnostic tests using magnetic resonance imaging (MRI), both of which can turn off the pacemaker. Some states prohibit a person from driving an automobile for a period of time after he has received a pacemaker if he has previously experienced unconsciousness as a result of arrhythmia.
See also Circulatory system.
Doebele, J. "A Better Mousetrap." Forbes 154 (October 1994): 238+.
Farley, Dixie. "Implanted Defibrillators and Pacemakers: A Gentler Jolt and Tickle for Trembling Hearts." FDA Consumer 28 (April 1994): 10-14.