For most heart failure patients the whole point of an implantable cardioverter-defibrillator (ICD) is to prevent a heart attack later on. But a new study in the New England Journal of Medicine (NEJM) may have patients wondering if the benefits of an ICD are worth the risks.

The study, published Wednesday, concludes that heart failure patients with an ICD who receive therapeutic shocks from the device have a "substantially higher" risk of death than those who do not receive shocks. Organizations like the Heart Rhythm Society (HRS; Washington), however, were quick to criticize the study and emphasize the importance of ICDs.

In the study, patients who received "appropriate" shocks were three times more likely to die that number, not including the patients who died within 24 hours after the first appropriate shock. The study authors said the most common cause of death among patients who received any ICD shock was progressive heart failure.

The risk of death among patients who received more than one appropriate shock was double that among patients who received a single appropriate shock, the study noted. Inappropriate shocks also were significantly associated with an increased risk of death, although to a lesser extent.

According to the Heart Rhythm Society, ICDs are 99% effective in stopping life-threatening arrhythmias and are the most successful therapy to treat ventricular fibrillation, the major cause of sudden cardiac arrest. However, the organization said, only 35% of patients who could be helped by an ICD have one. The devices are designed to continuously monitor the heart rhythm, automatically function as pacemakers for heart rates that are too slow, and deliver life-saving electric shocks if a dangerously fast heart rhythm is detected.

The Sudden Cardiac Death in Heart Failure Trial (SCD-HeFT) and the Multicenter Automatic Defibrillator Implantation Trial II (MADIT II) both showed that ICD therapy improves survival among patients who are at risk for sudden cardiac death but who have not previously had a sustained ventricular arrhythmia. On the basis of these data, recent clinical guidelines consider the implantation of an ICD for "primary prevention" (prevention of a first life-threatening arrhythmic event) to be standard high-quality care for patients who meet the entry criteria for these trials.

While the study shows a link between ICD shocks and patient mortality, the HRS noted, patients who received shocks were in worse health prior to the shock than those who were not shocked. The increased death rate is related to their overall health condition rather than the device itself, the organization said.

"Like many other studies, the SCD-HeFT clearly demonstrates that ICD devices save lives," said Dr. N.A. (Mark) Estes, president of the HRS. "While there is an association between patients who are shocked and an increased mortality, receiving a life-saving shock does not independently predict a patient's survival rate."

SCD-HeFT was a multicenter clinical trial in which 2,521 patients with heart failure and a left ventricular ejection fraction of 35% or less, but no previous sustained ventricular arrhythmia, were randomly assigned in equal proportions to receive an ICD, amiodarone therapy, or placebo. The cause of the heart failure was ischemic in 52% of the patients and non-ischemic in 48%. The use of evidence-based medical therapies was at high levels both at baseline and at the final follow-up assessment.

After a median follow-up of 45.5 months, ICD therapy, as compared with medical therapy alone, was associated with a 23% decrease in the risk of death. Wyeth Pharmaceuticals (Madison, New Jersey) provided drugs used in the study and Medtronic (Minneapolis) provided the ICDs.

Each patient in the ICD-therapy group was assigned to receive a single-lead ICD (Medtronic Micro Jewel II model 7223Cx). This device was implanted in 804 of 811 patients (99.1%); four patients received other Medtronic single-lead models, and three patients received Medtronic dual-lead models, owing to deviations from the protocol. The protocol for the programming of the ICD was deliberately conservative, the authors said, specifying that the ICD should intervene only for rapid, life-threatening ventricular tachycardia or ventricular fibrillation.

An accompanying editorial by Jeff Healey, MD, and Stuart Connolly, MD, says it is "somewhat disturbing to realize that actually receiving a shock is such an important predictor of death." However they note that it should not be surprising that many patients in whom sudden death from arrhythmia is averted by an ICD ultimately die from heart failure.

"In severe chronic conditions, most worthwhile interventions only modestly delay death. If a specific therapy is effective against only one cause of death and does not address the underlying disease process, then death from competing causes is inevitable," Healey and Connolly wrote.

They also note that it is plausible, but unlikely, that ICD shocks somehow have an adverse effect on myocardial function. "What is much more likely is that the occurrence of a ventricular arrhythmia that causes a shock is signaling a meaningful change in the patient's clinical status. The important message is that the first occurrence of shocks is not a random event in an otherwise stable clinical course but a sign of clinical deterioration in the underlying disease process."

David Nexon, senior executive VP of the Advanced Medical Technology Association (AdvaMed; Washington), said it is important to put the study results within the context of the significant overall patient benefit of ICDs. "First and foremost, ICDs are incredibly successful in achieving their primary purpose: saving lives," he said in a statement.

Nexon said the SCD-HeFT data set used by the study's authors showed that ICD therapy significantly reduced all-cause mortality by 23% compared to placebo, and ICDs were superior to drug therapy. He said that since the SCD-HeFT trial commenced 10 years ago, ICD technology has evolved significantly, and that the latest generation of these devices can safely end most arrhythmias without shocking the patient, eliminating three out of four shocks for dangerously fast heart beats.