Who Benefits from Implantable Heart Defibrillators?

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The summary below is from the full report titled "Implantable Cardioverter Defibrillators in Primary and Secondary Prevention: A Systematic Review of Randomized, Controlled Trials." It is in the 18 March 2003 issue of Annals of Internal Medicine (volume 138, pages 445-452). The authors are JA Ezekowitz, PW Armstrong, and FA McAlister.

What is the problem and what is known about it so far?

Cardiac arrest is a sudden failure of the heart to pump blood. The most common reason for cardiac arrest is an electrical problem called ventricular fibrillation. Ventricular fibrillation causes the lower chambers (ventricles) of the heart to rapidly contract in an uncoordinated manner. This prevents the heart from pumping blood. Death usually occurs within 5 minutes unless pumping of the heart is maintained by external cardiac massage or the heartbeat is returned to its normal rhythm by an electrical shock (electrical defibrillation). Risk for cardiac arrest is increased in various people, including patients who have been resuscitated from previous arrest, patients with symptoms from abnormally fast ventricular beats (ventricular tachycardia), patients with heart failure, and patients with a recent heart attack. Doctors can treat patients at risk for cardiac arrest with drugs that help prevent abnormal heart rhythms (antiarrhythmic agents) or with electrical devices that are inserted under the skin (implantable cardioverter defibrillators [ICDs]). The ICDs detect abnormally fast heartbeats and give an electronic shock to the heart to return it to normal rhythm. Although ICDs can prevent deaths from cardiac arrest, it is unclear which types of patients are most likely to benefit from them.

Why did the researchers do this particular study?

To see which types of patients benefit from ICDs.

Who was studied?

4909 patients with heart disease who had participated in eight different studies.

How was the study done?

Rather than doing a new study, the researchers analyzed information from eight randomized trials that had assessed long-term effects of ICDs. All of the patients in the trials had some problem with their hearts. They were randomly assigned to receive an ICD or "usual care." Usual care often included antiarrhythmic drug therapy. Patients in the trials were followed for about 2 to 5 years. The authors grouped and combined the results from all of the trials in a special way (meta-analysis) to see whether ICDs reduced deaths more often than usual care in particular types of patients.

What did the researchers find?

The researchers found that, compared with usual care, ICDs more often prevented death in certain types of patients. These included patients with previous cardiac arrest, patients with symptoms from abnormally fast beats of the ventricles, and some patients with decreased heart muscle strength (left ventricular dysfunction) from coronary artery disease or heart attack. Patients with left ventricular dysfunction who benefited from ICDs often had fast heartbeats of the ventricles (asymptomatic nonsustained ventricular tachycardia) and abnormal heartbeats (sustained ventricular tachycardia) when special tests of the heart's electrical conducting system (electrophysiologic studies) were done.

What were the limitations of the study?

The study did not address costs, and ICDs are very expensive. The study included only eight trials; however, this is all the information that is currently available. More trials with additional patients are needed to figure out more precisely which patients with heart attacks and which patients with left ventricular dysfunction benefit from ICDs.

What are the implications of the study?

Implantable cardioverter defibrillators prevent death from cardiac arrest in many types of patients with heart disease.