SAN FRANCISCO – Inappropriate implantable cardioverter defibrillator shocks for atrial fibrillation/flutter lead to increased risk of death during the next several years, while inappropriate shocks for sinus tachycardia, supraventricular tachycardia, artifact, noise, or device oversensing are associated with the same survival as in patients who did not receive a shock.

This was the key finding in the ALTITUDE study, the first study large enough to permit subgroup analysis in patients who receive inappropriate shocks for different heart rhythms.

The ALTITUDE results go a long way toward solving a mystery that has puzzled electrophysiologists in recent years: Why do the landmark clinical trials of implantable cardioverter defibrillator (ICD) therapy and cardiac resynchronization therapy plus defibrillation (CRT-D) consistently show that inappropriate shocks are associated with decreased survival, compared with no shocks? Could it be that inappropriate shocks, besides being painful and anxiety-provoking, are actually killing patients?

"In this study, the adverse prognosis following an ICD shock may likely be related to the underlying arrhythmia and its associated disease process rather than to an adverse effect from the shock itself," Dr. Brian Powell explained in presenting the ALTITUDE data at the annual meeting of the Heart Rhythm Society.

ALTITUDE involved 3,814 patients with an ICD or CRT-D enrolled in Boston Scientific’s proprietary Latitude remote monitoring system. The patients were followed for an average of 25 months after they experienced their first shock. A panel of seven electrophysiologists adjudicated the first shock episode rhythms. Within each rhythm category, the 3,814 patients with a shock were then matched one to one – based on device type, age at implant, and sex – to patients without a shock.

The most common inappropriately shocked rhythm was monomorphic ventricular tachycardia, accounting for 36% of all cases. This was followed by atrial fibrillation (AF)/flutter at 18%, sinus tachycardia or supraventricular tachycardia (SVT) at 17%, ventricular fibrillation (VF) or polymorphic ventricular tachycardia (VT) at 16%, polymorphic and monomorphic ventricular tachycardia at 7%, noise/artifact/oversensing in 5%, and nonsustained arrhythmia in 1%.

Three-year overall survival was best in patients shocked inappropriately for sinus tachycardia or SVT at 83.9%, followed by 81.3% for noise/artifact/oversensing, then a significantly worse rate of 76.4% in those shocked for AF/flutter, and 68% in patients shocked for VF/VT.

Within the group who received inappropriate shocks, patients shocked for sinus tachycardia or SVT had a 29% lower risk of death than did those shocked for AF/flutter. Patients shocked because of noise/artifact/oversensing had a 42% lower risk of death than did those shocked for atrial arrhythmias, reported Dr. Powell of the Mayo Clinic, Rochester, Minn.

Compared with patients who received no shocks in the separate matched-pair analysis, those who received a shock for AF/flutter were at 1.6-fold greater risk for mortality, similar to the 1.65-fold increase in patients shocked for monomorphic VT. Patients shocked for nonsustained VT had a 2.17-fold increased mortality risk, those shocked for monomorphic and polymorphic VT had a 2.77-fold elevated risk, and patients who received a shock for VF/polymorphic VT were at 2.1-fold increased risk of mortality. In contrast, patients shocked for sinus tachycardia/SVT or for noise/artifact/oversensing had a mortality rate similar to patients with the same arrhythmia who weren’t shocked, he continued.

Why should inappropriate shocks in the setting of AF/flutter be associated with an increased risk of death? One possibility is that some of the shocks that convert these arrhythmias to normal sinus rhythm may have occurred in patients who were not being anticoagulated at the time, thus exposing them to increased risk of stroke.

Another possibility is that delivery of a shock in a patient with AF/flutter may be a marker for inadequately dosed beta-blocker therapy. Beta-blockers are known to improve survival in this population. When the drug is underdosed, AF episodes can be accompanied by rapid ventricular rates that cause the device detection criteria to call for a shock, according to Dr. Powell.

He stressed that while the ALTITUDE findings are reassuring, it’s still very important to program ICDs and CRT-Ds using all available methods to avoid unnecessary shocks, since they cause patients considerable anxiety and discomfort.

Discussant Dr. Michael R. Gold said ALTITUDE nicely demonstrates the power of these very large telemonitoring databases to generate important research results when the studies are carefully conducted, like this one, with its rigorous adjudication of events. And he fully agreed with Dr. Powell’s interpretation of the findings.

"ALTITUDE provides further evidence that it’s likely the arrhythmia substrate and not the shocks themselves that are killing people," added Dr. Gold, professor of medicine, chief of cardiology, and medical director of the heart and vascular center at the Medical University of South Carolina, Charleston.

Dr. Powell disclosed that he serves as a consultant to Boston Scientific, which sponsored ALTITUDE.

ALTITUDE Study: Inappropriate ICD Shocks Don't Increase Mortality