Frequency of Inappropriate ICD Shocks
Abstract & Commentary
By John P. DiMarco, MD, PhD Dr. DiMarco is Professor of Medicine, Division of Cardiology, University of Virginia, Charlottesville Dr. DiMarco is a consultant for Novartis, and does research for Medtronic and Guidant.
Source: Daubert JP, et al. Inappropriate implantable cardioverter-defibrillator shocks in MADIT II: frequency, mechanisms, predictors, and survival impact. J Am Coll Cardiol. 2008;51:1357-1365.
Inappropriate shocks from implantable cardioverter defibrillators (ICDs) continue to be a major problem. In this study, the MADIT II investigators review the experience in that study with inappropriate shocks in a primary prevention, post-myocardial infarction cohort.
MADIT II was a study of the value of ICDs for the primary prevention of sudden death in patients with chronic ischemic heart disease and a left ventricular ejection fraction of less than 30%. In MADIT II, 719 patients received an ICD. After implant, ICDs were interrogated quarterly and after any ICD shock. Two of the authors then analyzed the stored electrograms from each episode and categorized the rhythm that led to ICD therapy. ICD therapy that was not delivered for an episode of ventricular tachycardia (VT) or ventricular fibrillation (VF) was deemed inappropriate. If multiple shocks were delivered during a single episode, or within five minutes of an initial episode, they were considered to be part of the original episode.
Among the 719 patients in MADIT II in whom an ICD was implanted, 83 (11.5%) received one or more inappropriate shocks. The point estimate at two years for having experienced one or more inappropriate shocks was 13%. Multiple inappropriate shocks were noted in 32 of the 83 patients, with an overall mean number of 2.2 ± 2.5 inappropriate shock episodes in those patients who experienced any inappropriate shock. There were 56 patients who experienced inappropriate shocks only, 27 patients who had both inappropriate and appropriate shocks, and 636 patients who had no inappropriate shock episodes. Of the latter, 101 had received appropriate shocks and 535 had no shock episodes. Among the inappropriate shock episodes, 81 of 184 were delivered for atrial fibrillation (AF) or atrial flutter, 67 for supraventricular tachycardia (not specified as either a sinus tachycardia or some other rhythm), and 36 because of abnormal sensing. The mean ventricular rate that resulted in an inappropriate shock for AF or SVT was 174 ± 22 beats per minute.
Several clinical characteristics were more common among patients who received inappropriate shocks. These were prior atrial fibrillation, a history of smoking, and a diastolic blood pressure > 80 mmHg. Most modern ICD models contain programmable discrimination algorithms to help distinguish between supraventricular and ventricular rhythms with similar rates. Discriminators for either RR interval stability or, among the patients with a dual chamber device, the V > A criteria, were less commonly programmed on in patients with inappropriate shocks. However, there was no significant difference in the proportion of patients with single chamber compared to dual chamber devices who received inappropriate shocks. The lowest VT detection zone was also not significantly different between patients with inappropriate shocks and no inappropriate shocks. When predictors of all-cause mortality was examined using a Cox proportional hazards regression analysis, blood urea nitrogen greater than 25 mg/dL and the absence of beta blocker use were baseline characteristics associated with mortality. Interim events associated with mortality included interim heart failure hospitalization and both appropriate and inappropriate shocks.
The authors conclude that in a primary sudden death ICD prevention trial, inappropriate shocks constitute about one-third of all shock episodes. Inappropriate shock occurrence was associated with an increased mortality during follow-up and a decrease quality of life.
Commentary
The high probability for patients to receive inappropriate shocks remains one of the largest problems in the management of patients with ICDs. VT and VF detection zones are based on the RR interval sensed on the right ventricular electrogram. The shortest cycle lengths (fastest rates) are in the VF zone, and 1-2 VT zones with longer RR intervals can also be programmed. The electrophysiologist tries to provide a 10-15 beat per minute safety margin between the fastest rates likely to occur during sinus or supraventricular rhythms and expected VT rates. In patients with slow clinical VTs, this may be quite difficult, and a number of detection algorithms are available to help. Interval stability may help identify AF. "Sudden onset" may prevent inappropriate therapy for the gradual heart rate increase seen with sinus tachycardia. Electrogram morphology analysis may determine when a wide complex tachycardia is present. If a dual chamber device is in place, correlating atrial and ventricular rates and relationships may be helpful. Despite these "detection enhancements," inappropriate shocks still occur. The data here show that in a primary prevention cohort, almost one-third of all shocks are inappropriate. The consequences for the patient may be considerable. When inappropriate shocks occur, they tend to be multiple. Sinus tachycardia and atrial fibrillation won't be terminated by a shock, and the pain from the shock will keep the rate high leading to more shocks. This can be devastating for the patient psychologically if not physically.
What can be done to prevent inappropriate shocks? First, VT zones and therapies should be carefully set-up to minimize the possibility for inappropriate shocks and to utilize antitachycardia pacing for VT termination whenever possible. In most patients who receive an ICD for primary prevention of sudden death, relatively high detection rates can be initially programmed. Beta blocking agents will lower peak rate during sinus rhythm and atrial arrhythmias and should be used whenever possible. Finally, selective antiarrhythmic drug use or even AV junctional ablation in patients with histories of atrial fibrillation or tachycardia may be necessary.
Inappropriate shocks from implantable cardioverter defibrillators (ICDs) continue to be a major problem. In this study, the MADIT II investigators review the experience in that study with inappropriate shocks in a primary prevention, post-myocardial infarction cohort.You have reached your article limit for the month. Subscribe now to access this article plus other member-only content.
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