'Inappropriate' ICD Shocks Exonerated in Mortality Increase

May 09, 2011

May 6, 2011 (San Francisco, California) — An analysis from the ALTITUDE study, in which patients with primary-prevention implantable cardioverter-defibrillators (ICDs), with or without biventricular pacing, were followed with remote monitoring for up to five years, confirms and expands on earlier studies suggesting that inappropriate ICD shocks up mortality.

The new wrinkle in the current analysis: all of the elevated risk of death from such shocks was in patients with atrial fibrillation (AF) or atrial flutter, whereas mortality didn't go up with inappropriate shocks triggered by other conditions, such as sinus tachycardia or device oversensing.

In his presentation here at the Heart Rhythm Society 2011 Scientific Sessions, Dr Brian Powell (Mayo Clinic, Rochester, MN) pointed out that the MADIT-2, SCD-HeFT, and COMPANION trials had suggested that up to 17% of primary-prevention ICD recipients get inappropriate shocks within four years of implantation. Moreover, other studies, including SCD-HeFT and prior analysis of ALTITUDE, suggest that inappropriate shocks are associated with an increase in mortality risk. There is some evidence that shocks themselves are damaging to the myocardium and may thereby increase mortality.

In the current analysis, inappropriate shocks were considered those that were in response to AF or atrial flutter or to sinus tachycardia or supraventricular tachycardia (SVT) or those that were triggered by "artifacts, noise, or oversensing."

Patients who received shocks for AF/atrial flutter showed a 61% increase in mortality, compared with patients who never received a shock, while those getting shocks that were inappropriate for the other two reasons showed no mortality increase, according to Powell. Mortality went up more sharply with appropriate shocks.

The implication: whether shocks are bad prognostically seems to depend on what triggered the shock. "In this study," Powell said, "it appears that the adverse prognosis following an ICD shock may be related to the underlying arrhythmia as opposed to an adverse effect from the shock itself."

Powell said ALTITUDE is the first study large enough to "determine the effect of underlying rhythm at the time of inappropriate shocks" on outcomes.

Expanding on the analysis for heartwire , Powell said that, in some cases, the shocks themselves may contribute to some of the associated mortality increase, but the patient's disease probably accounts for most of it. "Among patients who get shocked for ventricular arrhythmias, I think certainly the substrate, or underlying cardiomyopathy, is a strong component of long-term survival, although the shock may have some added effect." Supporting that, he said, are studies suggesting that cardiac enzymes go up slightly after ICD shocks.

But in ALTITUDE, he said, "patients who failed antitachycardia pacing and required a shock were sicker patients who had more severe heart failure and underlying cardiomyopathy, more rapid ventricular arrhythmias, and more polymorphic ventricular tachycardia, which may have been the driving factor for why those who received shocks had increased mortality over time."

As the invited commenter for Powell's presentation, Dr Michael R Gold (Medical University of South Carolina, Charleston) said the analysis "shows the power of these very large databases to be able to point out to us things we can't do in randomized studies." In this case, he said, ALTITUDE "shows further evidence that it's likely the substrate or arrhythmias, and not the shocks themselves, that are killing people."

In the first part of the analysis, which included 3809 of the approximately 28 000 patients in ALTITUDE who received shocks, followed for an average 3.1 years from implantation, shocks were adjudicated to have been in response to AF/atrial flutter in 18%, sinus tachycardia or SVT in 17%, and noise/artifact/oversensing in 5%. Aside from those inappropriate shocks, there were appropriate shocks for sustained monomorphic VT in 36%, polymorphic VT in 16%, both monomorphic and polymorphic VT in 7%, and nonsustained VT in 1%.

Compared with mortality after a first shock for AF/atrial flutter, mortality after a first shock for sinus tachycardia or SVT was significantly reduced; the same applied to first shocks for noise/artifacts/oversensing.

Hazard Ratio (95% CI) for Mortality, by Rhythm at the Time of Shock, Compared With Mortality After Shock for AF/Atrial Flutter

Rhythm at shock HR (95% CI)
Sustained monomorphic VT 1.19 (0.97–1.46)
Polymorphic VT 1.34 (1.06–1.69)
Sustained monomorphic VT and polymorphic VT 1.55 (1.16–2.06)
Nonsustained VT 1.80 (1.06–3.07)
Sinus tachycardia/SVT 0.71 (0.54–0.94)
Noise/artifact/oversensing 0.58 (0.39–0.88)


A second analysis compared 3630 patients receiving shocks with an equal number of patients who never received a shock during the ALTITUDE follow-up, who were propensity-matched on the basis of age, sex, whether their device provided cardiac resynchronization, and implantation year. It was in this analysis that the mortality increase associated with inappropriate shocks was concentrated in the AF/atrial-flutter group.

Hazard Ratio (95% CI) for Mortality for 3630 Patients Receiving Shocks vs 3630 Not Receiving Shocks in ALTITUDE, by Rhythm at Time of Shock

Rhythm at shock HR (95% CI)
Sustained monomorphic VT 1.65 (1.36–2.01)
Polymorphic VT 2.10 (1.54–2.86)
Sustained monomorphic VT and polymorphic VT 2.77 (1.70–4.51)
Nonsustained VT 2.17 (0.82–5.70)
Sinus tachycardia/SVT 0.97 (0.68-1.37)
Noise/artifact/oversensing 0.91 (0.50-1.67)
AF/atrial flutter 1.61 (1.17-2.21)

Even if inappropriate shocks don't themselves increase mortality, Powell said when interviewed, measures to minimize their frequency are still important because they can be painful and a source of anxiety for patients. "One take-home message here is that when patients have inappropriate shocks, it's important to look at the underlying rhythm that actually led to the shock and to treat the underlying rhythm--especially if it's atrial fibrillation with rapid ventricular rates. We need to get the heart rate under control and make sure they're on optimal medical therapy to potentially improve their survival."

ALTITUDE was supported by Boston Scientific, with which Powell discloses a financial relationship. Gold discloses such relationships with Boston Scientific, Medtronic, Cameron Health, and St Jude Medical.


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