Implanted Defibrillators and Pacemakers

by Food and Drug Administration (FDA)

On a Boston subway train in December 1990, John Thomas' heart stopped its normal steady beating. Instead, it began to quiver ineffectively in a type of cardiac arrest called fibrillation.

Automatically, built-in protection came to the rescue. A medical device implanted in Thomas' abdomen delivered an electric shock to his malfunctioning heart, jolting it back to regular rhythm.

"I lost consciousness for just a few seconds," says Thomas, 36, a social worker in Boston. "Nobody on the subway knew. I wasn't even sure what had happened." That same day, his doctor confirmed the device had indeed responded.

The device is a cardioverter defibrillator, one of the newest heart-rhythm regulators in pacemaker-defibrillator evolution. ("Cardioverter" indicates the capability to deliver low-energy shocks.) Thomas needs the device to treat primary ventricular fibrillation, a condition in which the lower heart chambers (ventricles) periodically have disorganized electrical activity and are unable to effectively pump blood to the body. He received his defibrillator after collapsing with cardiac arrest at Cape Cod the previous Fourth of July.

Thomas' defibrillator gave his heart three other jump-starts that fall and winter — his last one occurring in January 1991. To reduce the frequency of these episodes, his doctor prescribed drug therapy with Tenormin (atenolol), and Thomas has been free of the episodes ever since.

The Stats

While most cardiac arrests result from rapid heartbeat (tachycardia), some are due to slowed heartbeat (bradycardia), which is often treated with an implanted pacemaker. The Food and Drug Administration estimates that doctors implant about 15,000 defibrillators and 110,000 pacemakers each year.

Clinical studies submitted to FDA show the newest heart- regulating device — a pacemaker-cardioverter-defibrillator — corrected nearly 98 percent of patients' abnormal heart rhythm or cardiac arrest episodes. Some 400,000 Americans die annually from abnormally fast or irregular heart rhythm, FDA said in announcing approval of the device in February 1993.

The Antiarrhythmics vs. Implantable Defibrillators (or AVID) pilot study at the National Heart, Lung, and Blood Institute is examining whether defibrillators or anti-arrhythmia drugs are more effective in reducing deaths.

Pacemaking, Naturally

Responsibility for pacing heartbeats, which circulate blood, belongs to the sinus node atop the heart's right atrium (one of two upper chambers). This natural pacemaker's specialized cells fire electrical impulses that cause the atria and their respective ventricles to contract to move the blood in perfect timing.

The impulses travel down the atria, which receive blood through the veins from the body and lungs, and cause their contraction to "top off" the amount of blood in the ventricles. The impulses continue through a conductive pathway into the ventricles and cause their contraction, resulting in the pumping of blood through the arteries to the body and lungs. The right chambers circulate oxygen-depleted blood from the body to the lungs, while the left chambers circulate oxygen-rich blood from the lungs to the body.

Aside from speeding up in situations such as physical activity and slowing during rest or sleep, the normal heart typically completes 72 of these cycles a minute.

Some hearts, however, beat less than 60 times a minute (bradycardia) or race at over 100 a minute (tachycardia). These "arrhythmias" may have any number of causes, such as a birth defect, injury, chemical imbalance, even anti-arrhythmia medication. But the main predisposing factor, according to the American Heart Association, is acquired heart disease. Some arrhythmias aren't serious enough to warrant treatment.

To treat serious arrhythmias, doctors can turn to a permanently implanted artificial pacemaker or defibrillator. (Temporary emergency pacing with an external pacemaker is possible by threading lead wires through a vein to the patient's heart. Emergency defibrillation is possible with external defibrillator paddles.) Alternative treatments are drug therapy and surgical correction.

Artificial Pacemakers

The pulse generator of the artificial pacemaker corrects for a defective sinus node or conduction pathway by emitting rhythmic electrical impulses similar to those of the sinus node. Usually, it's made with a titanium metal case and other materials compatible with the body and powered by a lithium battery system.

The doctor implants the generator under the skin in the upper left part of the chest, attaching it to lead wires threaded to the heart.

Traveling along the wires, the impulses "tickle" the heart, stimulating it to beat at a normal pace, says Donald Dahms, chief of the pacing and electrophysiology devices branch at FDA's Center for Devices and Radiological Health.

The first pacemaker was implanted in 1958. These early devices had only one wire and paced the ventricles at regular intervals. They paced at a single rate prescribed by the doctor — usually 70 beats a minute.

The first advance, around 1976, was a design change that allowed "demand" pacing: A pacemaker would only pace if the patient's heart didn't beat within a given time period.

About the Author

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FDA is A United States government body that oversees medical devices, including contact lenses, intraocular lenses, excimer lasers and eyedrops. In the US, these products must be approved by the FDA before they can be marketed.