Magazine article Science News

Calling Death's Bluff: Implantable Defibrillators Can Save Thousands of Lives-But Which Ones?

Magazine article Science News

Calling Death's Bluff: Implantable Defibrillators Can Save Thousands of Lives-But Which Ones?

Article excerpt

Some hearts seem to be beating normally one moment, but then quiver for an instant and fall still forever. In these cases, the heart's electrical circuitry goes haywire, and its contractions accelerate until the organ flails uselessly and shuts down. Sudden cardiac death awaits nearly half a million people nationwide in the next 12 months. Once a person's heart stops, there is usually little that any hospital can do.

However, doctors do have an earlier chance to intervene. By implanting a specialized device into a person's chest, they can equip the heart to recover instantly when death comes knocking. When the electrical leads of these so-called implantable cardioverter-defibrillators, or ICDs, detect an abnormal heart rhythm, the attached pager-size generator delivers a shock that restores the normal cadence.

Use of these devices began in the 1980s and has increased exponentially in recent years. This year, U.S. doctors will surgically implant some 200,000 defibrillators at a cost of about $30,000 to $50,000 per operation.

Yet most of the devices will never see action. About four-fifths of defibrillators sit quietly in people's chests until their batteries conk out. Replacing a battery, which is necessary every 5 years or so, requires another round of surgery.

Defibrillators also can be cantankerous. They sometimes mistake mild arrhythmia, or rhythm disruption, for a deadly spasm and shock the heart unnecessarily, causing pain but no permanent harm. And while some people die because a device fails to fire when it should--a problem that spawned massive recalls of certain models last year--many more perish because no one recognized that a defibrillator could have saved their lives.

Doctors would like to improve their ability to distinguish people who truly need defibrillators from those who don't. The current method uses a crude measure: the heart's mechanical efficiency at pumping blood. That value, called ejection fraction, has no direct relationship with the heart's electrical circuitry.

Recent studies suggest that measures of the heart's electrical function and its responsiveness to the nervous system may reveal additional information about the heart's vulnerability to arrhythmia. Medical researchers recently concluded the first systematic test in which they weighed one such promising factor--an electrical characteristic called T-wave alternans--for deciding who should get a defibrillator. They plan to announce their results at a medical meeting this November.

If such trials prove that T-wave alternans and other new measures can supplement ejection fraction in clinical evaluation of patients, doctors may save more lives even though they implant fewer devices than they do now.

HIGH PRICE AT THE PUMP With each contraction of the heart, the organ's left ventricle squeezes blood into the body's arteries. A healthy chamber pumps out half to three-quarters of its contents, while a defective heart might expel 30 percent or less. This ejection fraction, expressed on a 100-point scale, can be gauged painlessly by performing a sonogram of the chest.

An insult such as a myocardial infarction--commonly called a heart attack--causes muscle damage that can instantly trigger fatal arrhythmia, or it can permanently reduce ejection fraction. The same muscle damage can also interfere with the conduction of electrical impulses through the heart, thereby increasing the chance that a subsequent life-threatening arrhythmia will occur.

"Ejection fraction has been studied extensively and is dearly associated closely with sudden death" says cardiac electrophysiologist David S. Rosenbanm of Case Western Reserve University's MetroHealth Campus in Cleveland.

The form of arrhythmia that usually triggers cardiac arrest is called ventricular fibrillation. In that event, uncoordinated muscle contractions cause the ventricles to flutter ineffectually. …

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