Editorials

Implantable defibrillators for life threatening ventricular arrhythmias

Are more effective than antiarrhythmic drugs in selected high risk patients 

Increasing numbers of patients are surviving a first episode of life threatening ventricular arrhythmia. In the absence of an acute myocardial infarction, patients who survive either ventricular fibrillation or sustained ventricular tachycardia have a high risk of further episodes, which may be fatal.¹ Until recently class I and class III antiarrhythmic drugs have been the standard treatment for these patients, with amiodarone² and sotalol³ having been shown to be superior to class I drugs. Nevertheless, even with the best medical therapy, arrhythmia recurrence rates are still 40-50% at five years. There is now growing evidence to support the wider use of implantable cardioverter-defibrillator devices as primary treatment in some patients with serious ventricular arrhythmias.

These devices were developed in the 1970s, with the first human implant in 1980.⁴ Early devices had a single therapy option of defibrillation only; the generator was implanted abdominally; and thoracotomy was required for electrode placement. The units are now smaller (current devices are little bigger than a pacemaker) and can be implanted pectorally, and improvements in sensing mean that the latest devices offer graded therapeutic responses to a sensed ventricular arrhythmia. Anti-tachycardia pacing, low energy synchronised cardioversion, and high energy defibrillation shocks can be given via a single transvenous lead.

Implantable cardioverter-defibrillators are effective at treating ventricular arrhythmias but until recently there has been no clear evidence that they reduce mortality. One small randomised study of 60 patients from the Netherlands comparing early cardioverter-defibrillator implantation with a conventional strategy starting with antiarrhythmic drugs did show a significant reduction in total mortality in the defibrillator group,⁵ at a cost which compared favourably with drug treatment.⁶ More recently, however, the results of the antiarrhythmics versus implantable defibrillators (AVID) study have been published.⁷

This was a randomised prospective trial comparing total mortality in 1016 patients treated with either implantable cardioverter-defibrillators or class III antiarrhythmic drugs (mainly amiodarone). The trial was stopped early when it became clear that survival was significantly better in the defibrillator group (89% v 82% at 1 year, 82% v 75% at 2 years, and 75% v 64% at 3 years (P<0.02), representing relative reductions in mortality of 39%, 27%, and 31% at 1, 2, and 3 years respectively). The benefits of defibrillator therapy were shown for both ventricular tachycardia and fibrillation, for elderly as well as younger patients, and were particularly impressive in patients with a left ventricular ejection fraction below 35%.

In March 1998 two further studies were presented at the American College of Cardiology meeting confirming the results of the AVID study. The Canadian implantable defibrillator study (CIDS) randomised 659 patients to amiodarone or an implantable cardioverter-defibrillator and showed a 20% reduction in mortality in the defibrillator group at three years. The cardiac arrest study Hamburg (CASH) randomised 288 patients to amiodarone, metoprolol, or a cardioverter-defibrillator and showed a 37% reduction in mortality in the defibrillator group at two years.

Several studies are assessing the efficacy of defibrillator therapy for primary prevention in patients at high risk of sudden death who have not yet had a clinical event. One such study, the multicenter automatic defibrillator implantation trial (MADIT), was stopped in 1996 after showing a 54% reduction in mortality with defibrillator therapy in 196 high risk patients who were shown electrophysiologically to be at risk of sustained ventricular tachycardia that could not be suppressed by an antiarrhythmic drug.⁸ More recently, however, the CABG patch trial showed no benefit for prophylactic defibrillator therapy in 900 high risk patients undergoing surgical revascularisation.⁹ Other studies are in progress which should help clarify the issue of patient selection.

An implantable cardioverter-defibrillator is not a cure. Patients are still at risk of an arrhythmia which might cause syncope or cardiac arrest, if only for a few seconds before treatment is delivered. Inevitably many patients face lifestyle restrictions, and a few patients have severe psychological problems. Implanting the device is not without risk, though implant mortality is less than 1%.¹⁰ Moreover, it is important to remember that survivors of sudden cardiac death require full cardiac investigation; many will have poor left ventricular function, hyperlipidaemia, and coronary artery disease which may require revascularisation.

In 1996 in the United Kingdom 262 patients received an implantable cardioverter-defibrillatoran implant rate of five per million population per year, about half the average for Western Europe and less than 10% of the rate in the United States (D Cunningham, personal communication). As with other medical technologies, the United Kingdom seems to have lagged behind other industrialised countries. Whether this is a result of medical conservatism, concerns about cost or efficacy, or other factors such as accessibility of specialised centres, is uncertain. What is now certain is that, in selected high risk patients, implantable cardioverter-defibrillators are more effective than antiarrhythmic drugs in prolonging life. When faced with a patient who has had sustained ventricular tachycardia or successful resuscitation from ventricular fibrillation physicians should consider an implantable cardioverter-defibrillator as first line treatment.¹¹