The implantable cardioverter defibrillator

The implantable cardioverter defibrillator (ICD) was developed for the prevention of sudden cardiac death in patients with life-threatening ventricular arrhythmias, particularly sustained VT or VF. Observational studies and recent prospective studies have shown their effectiveness.

Technological advances have been rapid and modern cardioverter-defibrillators are much smaller than their predecessors. One or more electrodes are usually inserted transvenously, although a subcutaneous electrode is sometimes used. Some new designs use subcutaneous electrodes exclusively and are implanted over the heart; no transvenous or intracardiac electrodes are required.

Currently available models feature several tachycardia zones with rate detection criteria and tiered therapy (low-energy cardioversion and high-energy defibrillation shocks) independently programmable for each zone. All feature programmable ventricular demand pacing. Extensive diagnostic features are available, including stored ECGs of the rhythm before and after tachycardia detection and treatment. Programmable anti-tachycardia pacing is an option with many models.

Defibrillation is achieved by an electric charge applied between the anodal and cathodal electrodes. The site and number of anodes and cathodes, the shape of the shock waveform, and the timing and sequence of shocks can all be pre-programmed. Biphasic shocks (in which the polarity of the shock waveform reverses during the discharge) are widely used. The capacitors are charged from an integral battery, which takes 5-30 seconds after the recognition of the arrhythmia.

Implantable defibrillators incorporating an atrial lead are also available. These provide dual-chamber pacing and can also distinguish atrial from ventricular tachyarrhythmias. They are used in patients who require an ICD and concomitant dual-chamber pacing, and in patients with supraventricular tachycardias that may lead to inappropriate ICD discharge. Atrial defibrillators have also become available in recent years to treat paroxysmal atrial fibrillation. Detailed supervision and follow up are required with all devices.

Resuscitation in patients with an ICD

Should resuscitation be required in a patient with an ICD, basic life support should be carried out in the usual way. If defibrillation is attempted no substantial shock will be felt by the rescuer. If it is deemed necessary to turn the device off this may be accomplished by placing a magnet over the ICD. If external defibrillation is attempted the same precautions should be observed as for patients with pacemakers, placing the defibrillator electrodes as far from the unit as possible. If resuscitation is successful the ICD should be completely reassessed to ensure that it has not been adversely affected by the shock from the external defibrillator.

Indications for implantation of an ICD

It is important to recognise those patients who are successfully resuscitated from cardiac arrest yet remain at risk of developing a further lethal arrhythmia. ICDs have been shown to be

Defibrillation by an ICD

Changes in ICDs over 10 years (1992-2002). Apart from reduction in size, the implant technique and required hardware have also improved—from the sternotomy approach with four leads and abdominal implantation to the present two-lead transvenous endocardial approach that is no more invasive than a pacemaker requires

Changes in ICDs over 10 years (1992-2002). Apart from reduction in size, the implant technique and required hardware have also improved—from the sternotomy approach with four leads and abdominal implantation to the present two-lead transvenous endocardial approach that is no more invasive than a pacemaker requires

Cardioversion of ventricular tachycardia by an ICD

Abdominal insertion or thoracotomy (needed with earlier models) is rarely required because most devices are now placed in an infraclavicular position similar to that used for a pacemaker

ICDs for secondary prevention

• Cardiac arrest due to VT or VF

• Spontaneous VT causing syncope or significant haemodynamic compromise

• Sustained VT without syncope or cardiac arrest with an ejection rate of 35% but no worse than Classs 3 of the New York Heart Association classification of heart failure

• For patients who have not suffered life threatening arrhythmia but are at high risk of sudden cardiac death effective in the prevention of sudden cardiac death in these patients and are, therefore, indicated as a "secondary" preventative measure. In clinical trials ICDs have been shown to be more effective than anti-arrhythmic drugs in this role.

All patients who are resuscitated from cardiac arrest due to VF or VT should routinely be considered for implantation of an ICD unless a treatable cause for the arrest is found. Similarly, ICDs should be routinely considered in patients with sustained VT leading to syncope or other substantial haemodynamic compromise, again unless a treatable cause is discovered.

Patients with severe impairment of left ventricular function have a high incidence of sudden cardiac death. Implantation of an ICD may be indicated as a preventative measure if the left ventricular ejection fraction is less than 35% and they have experienced an episode of sustained VT, even without syncope or cardiac arrest.

Patients resuscitated from VF occurring in the early stages of MI do not usually remain at risk of further episodes in the absence of other complications.

It is also possible to identify patients who have not yet suffered a life-threatening arrhythmia yet remain at high risk of sudden cardiac death. The use of ICDs is justified as a "primary" preventative measure in these patients. One important group in this category comprises those patients with severe impairment of ventricular function after MI who have non-sustained VT on Holter monitoring and inducible VT on electrophysiological testing.

Another group of patients who may justify insertion of an ICD as a primary preventative strategy are those with inherited conditions associated with a high risk of sudden cardiac death. These include structural disorders of cardiac muscle as well as physiological disorders involving abnormal ion transport mechanisms in the cell membrane.

The photographs of the changes in ICDs over 10 years and the chest radiograph of the biventricular pacemaker are reproduced with permission from the chapter on 'Implantable devices for treating tachyarrythmias' by Timothy Houghton and Gerry C Kaye in the ABC of Interventional Cardiology. London: BMJ Publishing Group, 2004. The ICDs from 1992 and 2002 were supplied by C D Finlay, CRT coordinator, Guidant Canada Corporation, Toronto

ICDs for primary prevention

• Patients with severe impairment of ventricular function following MI

• Patients with inherited conditions linked with high risk of sudden cardiac death

Non sustained VT on 24 hour ECG monitoring Inductive VT on electrophysiological testing Left ventricular dysfunction with an ejection fraction of less than 35% but no worse than Class 3 of the New York Heart Association classification of heart failure

• Familial cardiac conditions with high risk of sudden death incuding:

- long QT syndrome

- hypertrophic cardiomyopathy

- bugada syndrome

- arrhythmogenic right ventricular dysplasia (AVRD)

- after repair of Tetralogy of Fallot

The National Institute for Clinical Excellence (NICE) published guidance for the implantation of ICDs applicable to the United Kingdom in September 2000. These include recommendations for their use in patients who have been successfully resuscitated from cardiac arrest or who have sustained life-threatening arrhythmias

The results of the MADIT II trial are likely to widen the indications for the prophylactic use of ICDs. This study investigated whether implantation of an ICD would reduce mortality in high-risk patients with coronary disease and impaired left ventricular function. The trial, which had no arrhythmia entry criteria, was stopped prematurely after a 30% reduction in mortality was observed in post-MI patients with impaired left ventricular function randomised to receive an ICD

Further reading

• American College of Cardiology/American Heart Association. Guidelines for the implantation of cardiac pacemakers and antiarrhythmia devices. JACC 1998;31:1175-209.

• Coats AJ. MADIT II, the Multicentre Automatic Defibrillator Implantation Trial II stopped early for mortality reduction. Has ICD therapy earned its evidence-based credentials? Int J Cardiol 2002;82:1-5.

• Griffith MJ, Garratt CJ. Implantable devices for ventricular fibrillation and ventricular tachycardia. In: Julian DG, Camm AJ, Fox KM, Hall RJC, Poole-Wilson PA, eds. Diseases of the heart. 2nd ed. London: WB Saunders, 1996.

• National Institute for Clinical Excellence. Guidance on the use of implantable cardioverter defibrillators for arrhythmias. Technology appraisal guidance no 11. London: NICE,

2000.

• Kishore AGR, Camm AJ, Bennett DH. Cardiac pacing. In: Julian DG, Camm AJ, Fox KM, Hall RJC, Poole-Wilson PA, eds. Diseases of the heart. 2nd ed. London: WB Saunders, 1996.

• Klein H, Auricchio A, Reek S, Geller C. New primary prevention trials of sudden cardiac death in patients with left ventricular dysfunction: SCD-HeFT and MADIT II. Am J Cardiol 1999;83:91D-97D.

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