"Hypertrophic cardiomyopathy is the most common cause of sudden death in young people, including athletes, and a defibrillator affords the opportunity, as it turns out, to change the natural course of the disease and prevent sudden death," said Dr. Barry Maron, director of the Hypertrophic Cardiomyopathy Center at the Minneapolis Heart Institute Foundation. "In a way, this highlights the use and effectiveness of the device in this genetic disease in these young patients and expands the number of patients that could be eligible at least for consideration of a defibrillator."

Maron was lead author of the study, which is published in the July 25 issue of the Journal of the American Medical Association.

Other experts, however, pointed out that much of the information in the study has already been known.

"Some of this data has been published in a smaller number of patients. This study confirms what has already been done," said Dr. Jose Joglar, director of clinical electrophysiology, and associate professor of medicine at the University of Texas Southwestern Medical Center at Dallas. "This is not going to expand the use of defibrillators."

Hypertrophic cardiomyopathy is a genetic disease that causes the heart muscles to thicken abnormally. This, in turn, can upset the heart's electric system and cause life-threatening rhythm disturbances called arrhythmias.

Implantable defibrillators (ICDs), which terminate these dangerous rhythm disturbances, have become routine over the past few years in high-risk patients with hypertrophic cardiomyopathy. But it's not always easy to identify the people with hypertrophic cardiomyopathy who would benefit from a defibrillator.

"It's traditionally difficult to determine precisely which patients among all with this disease are at high risk and would deserve consideration for an ICD, because HCM [hypertrophic cardiomyopathy] is a particularly heterogeneous disease," Maron explained." Most of the patients at high risk have no symptoms or only mild symptoms and are younger than those who have coronary disease by, on the average, 25 years. And there is no one single risk factor that has emerged such as with coronary disease following a heart attack."

"Because the patients are younger, the decision for these ICDs are bigger because of the time period they will have them is longer," he continued.

Although the new study isn't the first study to look at ICDs in patients with hypertrophic cardiomyopathy, it is the largest.

Maron and his colleagues analyzed data from an international, multi-center registry of defibrillators implanted between 1986 and 2003 in 506 patients with hypertrophic cardiomyopathy. All of the patients, whose average age was 42 years, were considered at high risk for sudden death, although 87 percent had no or only mild symptoms. Average follow-up was 3.7 years. The defibrillators were implanted both in patients who had never had a problem (primary prevention) and in patients who had already had a problem (secondary prevention).

Twenty percent of the patients experienced one or more incidents in which the ICD terminated ventricular fibrillation (severely abnormal heart rhythm that results in cardiac arrest) or ventricular tachycardia (abnormally rapid heartbeat).

The defibrillator terminated abnormal heart rhythms in 10.6 percent of patients annually who had a device for secondary prevention and in 3.6 percent of patients a year for primary prevention.

The probability of a defibrillator intervening five years after implantation was almost 25 percent, the study found.

More than 40 percent of patients in whom the device intervened appropriately were under 40 years of age.

Thirty-five percent of the patients who had a defibrillator for primary prevention had undergone implantation based on just one risk factor. The likelihood that the device would intervene was similar in patients with one, two or three or more risk factors.

The risk factors evaluated in the study included a history of hypertrophic cardiomyopathy-related sudden death in one or more relatives under the age of 50; fainting; and abnormally rapid heart rhythm.

"This is a select patient population. The risk factors are pretty significant; the patients were probably sicker," Joglar said. "The great majority of patients with HCM will have a better prognosis than that."

The study authors cautioned that defibrillators should not necessarily be implanted in everyone with hypertrophic cardiomyopathy. Patients will still have to rely on individual doctors' judgment.

More information

Visit the Heart Rhythm Society for more on implantable defibrillators.