eMedicine Specialties > Cardiology > Electrophysiology Procedures

Implantable Cardioverter-Defibrillators

Author: Daniel M Beyerbach, MD, PhD, Consulting Staff, Florida Electrophysiology Associates
Contributor Information and Disclosures

Updated: Sep 6, 2006

Introduction

The implantable cardioverter-defibrillator (ICD) has revolutionized the treatment of patients at risk for sudden cardiac death due to ventricular tachyarrhythmias. Initially introduced in humans in 1980 and approved by the Food and Drug Administration (FDA) in 1985, the ICD has evolved from a treatment of last resort to a first-line treatment and prophylactic therapy for patients at risk for ventricular tachycardia (VT) or ventricular fibrillation (VF).

For excellent patient education resources, visit eMedicine's Heart Center. Also, see eMedicine's patient education article Heart Failure.

History

Michel Mirowski conceived of and developed the implantable cardioverter-defibrillator (ICD) almost single-handedly. Prompted by the sudden death of a colleague, Mirowski conceived of an automatic, fully implantable defibrillator. After building a prototype device, Mirowski tested and refined it in animals. Despite considerable skepticism and criticism from many of his colleagues, Mirowski implanted the first device in a human in 1980. In 1985, the FDA initially approved the ICD, specifying that patients had to have survived 2 cardiac arrests to qualify for ICD implantation. Initially, lead systems were epicardial, requiring a thoracotomy for implantation. Pulse generators initially were large and bulky, requiring abdominal implantation.

Remarkable technological advances have made ICDs easier and safer to implant and better accepted by patients and physicians. The development of transvenous lead systems, more effective biphasic defibrillation waveforms, and "active can" technology allows implantation in nearly all patients without the need for thoracotomy. Significant miniaturization of the capacitors and other components has reduced the size of the pulse generator tremendously, permitting subcutaneous pectoral implantation in most patients.

Current devices are considerably smaller than early generations of ICDs, and therapeutic and diagnostic functions have progressed markedly. Early devices were simple shock boxes, offering only high-energy shocks when the patient's heart rate exceeded a cut-off point. Diagnostic information was limited to the number of shocks delivered. Current devices offer tiered therapy with programmable antitachycardia pacing schemes, as well as low-energy and high-energy shocks in multiple tachycardia zones. Dual-chamber, rate-responsive bradycardia pacing is now available in all ICDs, and sophisticated discrimination algorithms minimize shocks for atrial fibrillation, sinus tachycardia, and other non–life-threatening supraventricular tachyarrhythmias. Diagnostic functions, including stored electrograms, allow for verification of shock appropriateness. Device battery longevity has also increased; early devices lasted 2 years or less, while current devices are expected to last 6 years or longer.

Clinical Results

Early data regarding the implantable cardioverter-defibrillator (ICD) consisted primarily of uncontrolled series of patients for whom antiarrhythmic drug therapy for ventricular tachycardia (VT) or ventricular fibrillation (VF) had failed. Even in these refractory patients, initial series suggested a markedly reduced risk of sudden, presumed arrhythmic, death. In the early-to-mid 1990s, 3 clinical trials were conducted in patients who had survived life-threatening ventricular tachyarrhythmias.

The Antiarrhythmics Versus Implantable Defibrillators (AVID) trial, conducted in the United States, enrolled patients with prior cardiac arrest or hemodynamically significant sustained VT and randomized patients to either ICD implantation or antiarrhythmic drug therapy, including primarily amiodarone and, in a few cases, sotalol. The Canadian Implantable Defibrillator Study (CIDS) trial in Canada had a similar structure. The Cardiac Arrest Study Hamburg (CASH) trial in Hamburg enrolled cardiac arrest survivors and randomized them to amiodarone, metoprolol, propafenone, or ICD implantation.

The AVID trial, although sponsored by the National Institutes of Health (NIH), was extremely controversial, with many electrophysiologists protesting that a randomized trial was not necessary to prove the effectiveness of the ICD. This trial was terminated prematurely because of improved survival rates in the ICD-treated patients. The CIDS trial yielded similar results. In the CASH trial, the propafenone arm of the study was dropped relatively early because of an increased mortality rate. ICD therapy proved superior when compared to either amiodarone therapy or metoprolol therapy.

These trials firmly established the ICD as preferred therapy in patients who have survived cardiac arrest or hemodynamically significant sustained VT. Subgroup analyses of AVID, CIDS, and the Multicenter Automatic Defibrillator Implantation Trial (MADIT) suggest that the survival benefit of the ICD is realized primarily by the patients who are sicker, those with greater impairment of left ventricular (LV) systolic function, as measured by LV ejection fraction (EF); in these 3 trials, the ICD had less apparent benefit in patients with more preserved LV systolic function.

Several important trials have subsequently been performed examining the role of ICDs as primary therapy for patients who are at risk but who have not yet manifested sustained ventricular arrhythmias. These trials include the MADIT, MUSTT, MADIT II, SCD-HeFT, and COMPANION trials.

The MADIT study enrolled patients with ischemic cardiomyopathy (EF <35%) and asymptomatic nonsustained VT who had inducible sustained VT or VF not suppressible with procainamide during electrophysiologic studies (EPS). Enrolled patients were randomized to either ICD implantation or to therapy considered appropriate by the treating physician. Antiarrhythmic drug therapy was administered in both arms as considered appropriate by the treating physician. Similar to AVID, MADIT was terminated prematurely because of a significant survival benefit seen in patients treated with ICDs.

The Multicenter Unsustained Tachycardia Trial (MUSTT) had similar inclusion criteria (prior infarct, EF <40%, nonsustained VT inducible at EPS) and randomized patients to EPS-guided therapy versus no specific antiarrhythmic therapy. Early in the trial, EPS-guided therapy consisted of antiarrhythmic drug therapy guided by EPS testing, with ICD implantation reserved for subjects with ventricular arrhythmias refractory to antiarrhythmic drugs. Later in the trial, ICDs were used earlier in patients who were randomized to the EPS-guided therapy arm of the trial. MUSTT showed a survival benefit in the EPS-guided group. The survival benefit was attributable to the ICD. Subjects who were randomized to EPS-guided therapy and were treated with antiarrhythmic drugs fared no better or worse than patients assigned to the control arm of the trial.

In MUSTT, a registry was maintained of patients who met the clinical criteria for the study but were noninducible in the EP laboratory. During follow-up, the survival rate in this group was better than in the inducible patients assigned to the control group but not as good as in inducible patients who received ICDs. Although MUSTT was not designed to determine the optimal treatment in noninducible patients, many have concluded that, in the population studied, EPS testing may be used to stratify high-risk and moderate-risk patients rather than high-risk and low-risk patients.

The Multicenter Automatic Defibrillator Implantation Trial II (MADIT II) markedly expanded the potential pool of ICD recipients. MADIT II randomized patients with prior myocardial infarction (MI) and EF less than or equal to 30% to ICD therapy or a control group; nonsustained VT or inducible VT at EPS was not required. Patients who received an ICD had a 31% reduction in mortality rate. An important aspect of MADIT II was that subjects in both arms of the trial were well managed medically with a high rate of beta-blocker, ACE-inhibitor, and cholesterol-lowering medication usage.

In the Sudden Cardiac Death in Heart Failure Trial (SCD-HeFT), subjects with an EF of 35% or less and symptoms in New York Heart Association (NYHA) functional class II or III were randomly assigned to conventional heart failure therapy plus placebo, conventional therapy plus amiodarone, or conventional therapy plus ICD implant. In this primary prevention trial, ICD therapy, as compared with placebo, was associated with a 23% reduction in risk of death from any cause and an absolute 7% decrease in mortality over 5 years. No difference in mortality benefit was shown between subjects with ischemic cardiomyopathy (70% of enrollees) and those with nonischemic cardiomyopathy. ICD therapy benefited only functional class II subjects.

In the Comparison of Medical Therapy, Pacing, and Defibrillation in Chronic Heart Failure (COMPANION) trial, patients with advanced heart failure, NYHA functional class III or IV, an EF of 35% or less, and intraventricular conduction delay with QRS duration greater than 120 milliseconds, but with no indication for pacemaker or ICD implant, were randomized to optimal medical therapy alone or in combination with cardiac resynchronization therapy with either a biventricular pacemaker or biventricular pacemaker-defibrillator. Risk of hospitalization or death from heart failure was reduced by 34% in the pacemaker group and by 40% in the defibrillator group. Risk of death from any cause was reduced by 24% in the pacemaker group and by 36% in the defibrillator group.

The Defibrillators in Non-Ischemic Cardiomyopathy Treatment Evaluation Trial (DEFINITE) enrolled subjects with nonischemic cardiomyopathy and showed a trend toward mortality benefit in the ICD arm, with mortality at 29 months being 13.8% in the medical therapy arm and 8.1% in the ICD arm (P =0.06).

At least 3 published ICD trials have failed to demonstrate a survival benefit over optimal medical therapy, 2 examining ischemic cardiomyopathy and 1 examining nonischemic cardiomyopathy. In the Coronary Artery Bypass Graft (CABG)-Patch trial, subjects undergoing CABG with decreased LV function (EF <35%) and an abnormal signal-averaged ECG (SAECG) were randomized to epicardial ICD implantation at the time of CABG or to a control group. In this study, ICD implantation improved the sudden cardiac death mortality rate but not the total mortality rate, apparently because of the poor predictive value of a preoperative SAECG in identifying patients at risk for arrhythmic death, or the salutatory effects of coronary revascularization in reducing the risk of arrhythmic death. The Defibrillator in Acute Myocardial Infarction Trial (DINAMIT) enrolled subjects within 40 days of an acute myocardial infarction and randomized them to optimal medical therapy with or without a defibrillator. No difference was shown inmortality at a mean follow-up of 2.5 years.

Both the CABG-Patch and DINAMIT trials involved a possible confounder of revascularization. In the Cardiomyopathy Trial (CAT), subjects with recent (<9 mo) onset of nonischemic cardiomyopathy were randomized to ICD therapy or control group. At 1 year, after enrolling 104 subjects, CAT was terminated early due to lower than expected mortality in the control group, 3.7% versus the expected 30%. At a mean follow-up of 5.5 years, no significant difference was shown in cumulative survival.

One small trial directly compared ICD therapy with medical therapy of amiodarone. The Amiodarone Versus Implantable Cardioverter-Defibrillator Randomized Trial (AMIOVIRT) studied 103 subjects with nonischemic cardiomyopathy and was stopped early when a prespecified rule for futility was reached. No difference in mortality was shown at 3 years. The SCD-HeFT trial provides only an indirect comparison between amiodarone therapy and ICD therapy because the trial was designed only to compare each of these therapies individually with optimal medical therapy.

Indications

Multiple studies have shown the implantable cardioverter-defibrillator (ICD) to be superior to antiarrhythmic drug therapy in patients with a history of life-threatening ventricular tachyarrhythmias (VT) and ventricular fibrillation (VF). Thus, the ICD is usually recommended as initial therapy in patients who present with sustained VT in association with structural heart disease or those who present with resuscitated cardiac arrest. Exceptions include (1) patients with idiopathic VT in whom the prognosis is excellent, (2) VF due to rapid rates during preexcited atrial fibrillation (AF) in patients with Wolff-Parkinson-White syndrome in whom initial therapy should be catheter ablation, (3) patients in whom VT or VF occurs early in acute transmural infarction, and (4) patients in whom a transient or reversible cause for the arrhythmia is detected.

The proper treatment of patients with polymorphic VT or VF presumed to result from acute ischemia in the absence of acute transmural infarction is less certain. Although in the past these patients were frequently treated with revascularization only, the current trend appears to be ICD implantation following revascularization.

In addition to ICD use as secondary prophylaxis after ventricular arrhythmia, multiple studies support ICD use as primary prophylaxis in ischemic and nonischemic cardiomyopathy. Based on MADIT and MUSTT, patients with prior infarct, EF less than 40%, and spontaneous nonsustained VT are candidates for EPS with ICD implantation if they are inducible. Although in MADIT failure of procainamide to prevent inducibility was required for randomization, in practice procainamide rarely is tested. Patients who fit the MADIT II profile (at least 1 mo post-MI with EF £ 30%) are candidates for ICD implant. Heart failure patients with ischemic or nonischemic cardiomyopathy, EF 35% or less, and NYHA functional class II or III are candidates for ICD implant.

Patients are candidates for ICD implant if they present with syncope of undetermined origin and have clinically relevant, hemodynamically significant, inducible VT or VF upon EPS or when drug therapy is ineffective or not an option. Given the poor sensitivity and specificity of EPS in patients with nonischemic cardiomyopathy, patients with unexplained syncope in the setting of nonischemic cardiomyopathy are frequently treated empirically with ICD implantation.

High-risk patients with long QT syndrome (LQTS), hypertrophic cardiomyopathy, arrhythmogenic right ventricular cardiomyopathy, Brugada syndrome, and short QT syndrome are candidates for prophylactic ICD implantation, although what constitutes high risk in these patient populations is currently under investigation. Currently, high risk features include syncope and a family history of unexplained sudden cardiac death.

Ongoing clinical trials and registries of patients with LQTS, hypertrophic cardiomyopathy, Brugada syndrome, and arrhythmogenic right ventricular cardiomyopathy will better define prophylactic indications for ICD implantation.

Care of Patients With Implantable Cardioverter-Defibrillators

Patients with implantable cardioverter-defibrillators (ICDs) should be observed in a dedicated ICD clinic. Patients are seen more frequently early after implant—generally 1 week after implant for a wound check, 1 month after implant for device interrogation, and 3 months after implant for repeat device interrogation. The follow-up interval generally can be increased to every 6 months in patients who are clinically stable.

Although technologic advances have greatly reduced the potential effects of electromagnetic interference (EMI), patients should be advised to avoid strong electromagnetic fields because of potential interference with sensing circuitry; examples of potential hazards include arc welders, large generators, and magnetic resonance imaging (MRI) magnets. Household appliances, microwave ovens, and cell phones should not pose a serious threat.

An ICD shock is generally painful. Patients should be advised of this in advance. Advise patients and their families that someone touching them is not harmed if the ICD discharges.

Issues regarding driving can be problematic. In the absence of specific state laws, many electrophysiologists recommend that patients be shock-free for 6 months before resuming driving. Loss of driving privileges imposes an enormous burden and change of lifestyle on patients with this restriction. Rules and recommendations regarding commercial driving typically are more stringent.

Although ICDs are extremely effective in terminating life-threatening arrhythmias, many patients require adjunctive therapy to reduce the frequency of arrhythmic events that require therapy. This generally consists of pharmacologic therapy and/or radiofrequency catheter ablation. Inappropriate shocks also may be delivered for atrial fibrillation, sinus tachycardia, and other types of supraventricular tachycardia, prompting ICD reprogramming or adjunctive therapy.

Some patients with ICDs require emotional or psychological support for anxiety, depression, and difficulties in adjusting to life with an ICD. The dedicated ICD clinic staff can help with many of these issues. Referral to a psychologist or psychiatrist may also be helpful. Support groups are available for patients and their families, some of which are accessible online.

Keywords

implantable cardioverter-defibrillators, ICD, ventricular tachyarrhythmias, ventricular tachycardia, VT, ventricular fibrillation, VF, supraventricular tachyarrhythmias, electrograms, electrophysiology, electrophysiologic studies, EPS

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