Implantable Cardioverter Defibrillators

Implantable an MI and heart failure with significant left ventricular systolic dysfunction con- tinue to have a high rate of SCD. cardioverter The first implantable cardioverter defibrillator (ICD) to manage SCD was defibrillators implanted in a human by Michel Mirowski in 1980 (Fig 1). Since then there has been an explosion in technology and Stuart Harris BSc(Hons) MBBS MRCP(UK), randomised control trial data to support Consultant Cardiologist, Essex Cardiothoracic their use. Centre, Basildon and Thurrock University Hospitals NHS Foundation Trust Mehul Dhinoja BSc(Hons) MBBS MRCP(UK), What are the components of an Specialist Registrar in Cardiology, The Heart implantable cardioverter Hospital, University College London Hospitals defibrillator? NHS Foundation Trust An ICD comprises: Clin Med 2007;7:397–400 • a lithium silver vanadium oxide Fig 1. Michel Mirowski MD (1924–90). battery, which provides low voltage energy patients with symptomatic heart failure Who needs an implantable a transformer which multiplies this • and dyssynchrony of ventricular contrac- cardioverter defibrillator? voltage tion a further lead can be placed in the In the UK, sudden cardiac death (SCD) • an aluminium electrolytic capacitor lateral tributaries of the coronary sinus occurs in 70,000–100,000 patients annu- which can store the high energy for cardiac resynchronisation (Fig 2). ally, mainly caused by ventricular voltage for use, and The basic detection of ventricular arrhythmias. Most of these patients have • sensing circuitry which can sense arrhythmias involves measuring heart recognised heart disease with either a local electrograms and filter out rate above which therapy will be deliv- previous myocardial infarction (MI) or noise like skeletal myopotentials. ered. Rate detection is very reliable, but left ventricular dysfunction, but SCD is ICDs have at least one lead in the right unfortunately susceptible to delivering the first presentation of heart disease in ventricle for pacing, sensing and defibril- inappropriate therapy for sinus tachy- 20% of patients. lation. There may also be a second lead cardia or poorly controlled atrial fibrilla- The mainstay of treatment is neurohor- or coil in the superior vena cava to tion. Modern devices can be monal modulation with beta-blockers and improve the defibrillation threshold. programmed to have multiple detection angiotensin-converting enzyme inhibi- Additional leads can be placed in the zones with detection enhancements to tion. While this reduces morbidity and right atrium for dual chamber pace- help prevent inappropriate shocks. mortality across the spectrum of cardio- maker indications or to help in the detec- When ventricular arrhythmias are vascular disease, patients who have had tion or discrimination of arrhythmias. In detected the device will try to terminate them by either antitachycardia pacing or delivering a shock. Most ICDs can deliver Key Points a maximum energy of 30–41 J via their capacitors as a biphasic waveform from Sudden cardiac death occurs in 70,000–100,000 people annually in the UK, the tip of the right ventricular lead to the mostly in people with pre-existing heart disease but in 20% it is the first generator or ‘active can’. The shocking presentation vector travels superiorly from the right Previous myocardial infarction (MI) and significant left ventricular dysfunction are ventricle to include most of the interven- associated with a high rate of sudden cardiac death tricular septum and left ventricle (Fig 3).

Implantable defibrillators are recommended in the absence of a reversible cause for survivors of a cardiac arrest or in sustained ventricular arrhythmias if associated What are the indications for an with syncope, haemodynamic instability or a left ventricular ejection fraction implantable cardioverter (LVEF) below 35% defibrillator? Implantable defibrillators are recommended as primary prophylaxis in patients who Randomised trials comparing ICDs and have had an MI and have an LVEF below 30% based on trial evidence (MADIT II) and the National Institute for Health and Clinical Excellence guidelines medical therapy have been conducted largely in patients with underlying KEY WORDS: ischaemic heart disease, left ventricular dysfunction, sudden cardiac ischaemic heart disease (IHD) who make death, ventricular arrhythmias up the majority of recipients. Other

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CABG Patch,9 SCDHeFT10 and DEFINITE11 trials. The first four included only patients with IHD, but SCDHeFT and DEFINITE included non-ischaemic patients.

MADIT I and II, MUSTT, DINAMIT, CABG Patch. The MADIT II trial showed a 31% reduction in all-cause mortality in patients with a previous (more than 30 days) MI and LVEF less than 30%, given conventional drug therapy and an ICD. Unlike MADIT I and MUSTT, no docu- mented ventricular arrhythmias or electrophysiology studies were required. Conversely, the DINAMIT and CABG Patch10 trials both failed to demonstrate any benefit of ICDs. DINAMIT enrolled patients 6–40 days post-MI who also had indicators of autonomic dysfunction suggesting progressive heart failure. Consequently, this study either selected out a higher risk population or the bene- Fig 2. Dual chamber implantable cardioverter defibrillator. fits of ICD implantation may have been attenuated by conventional drug therapy. The lack of benefit from early ICD important patient groups include those sustained VT and syncope, haemo- implantation has also been confirmed by with cardiomyopathies and ion channel dynamic instability or LVEF below 35%.1 retrospective analysis of the MADIT II disorders. The National Institute for population. Clinical Excellence issued a revised tech- CABG Patch implanted ICDs prophy- nology appraisal for ICDs in January What is the trial evidence for lactically in patients with left ventricular 2006.1 These guidelines do not however these guidelines? dysfunction at the time of coronary apply to patients with non-ischaemic Secondary prevention trials artery bypass surgery (CABG). The lack dilated cardiomyopathy. of survival benefit from ICDs in this Primary prevention is prophylaxis The first trials to investigate the benefits study probably highlights the benefits of against a first life-threatening event. It is of ICDs versus antiarrhythmic drugs revascularisation in preventing SCD and recommended for patients at least four were conducted in survivors of SCD. the increased risks of device implanta- weeks after an MI who either have a left Important studies included the AVID,2 tion during cardiac surgery. ventricular ejection fraction (LVEF) of CIDS3 and CASH4 trials (the full names less than 35%, non-sustained ventricular of trials are given at the end of the text). SCDHeFT. SCDHeFT was the first study tachycardia (VT) on Holter monitoring These studies enrolled patients with and in heart failure patients with both and inducible VT during electrophysio- without IHD with LVEF below 40%, ran- ischaemic and non-ischaemic aetiology. logical testing, or LVEF of less than 30% domising them to ICD implantation or It found a 23% reduction in all-cause and QRS duration of at least 120 ms. drug therapy with either amiodarone or mortality in patients with New York Primary prevention is also recom- beta-blockers. The results showed a sig- Heart Association (NYHA) class 2 or 3 mended in familial cardiac diseases with nificant mortality reduction with ICD heart failure and LVEF below 35% given a high risk of SCD such as hypertrophic treatment. conventional drug therapy and an ICD. (HCM) and arrhythmogenic right DEFINITE. The DEFINITE trial11 ran- ventricular cardiomyopathies (ARVC), Primary prevention trials long QT and Brugada syndromes, and in domised patients with non-ischaemic surgically corrected congenital heart Several trials investigated the benefits of dilated cardiomyopathy with LVEF less disease. ICDs for prevention of SCD in patients than 36% and NYHA class 1–3 to best In the absence of a reversible cause, with no previous history of haemo- medical therapy with or without an secondary prevention is advised for dynamically significant ventricular ICD. All-cause mortality was not signifi- survivors of a cardiac arrest due to arrhythmias. Important studies included cantly reduced by ICDs but there was a ventricular arrhythmia, patients with the MADIT,5,6 MUSTT,7 DINAMIT,8 significant reduction in arrhythmic

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Fig 3. Detection and successful cardioversion for ventricular fibrillation.

deaths with the greatest benefit seen in include syncope, family history of SCD, by coved ST elevation in leads V1 and V2 NYHA class 3 patients. Risk stratification non-sustained VT on Holter monitoring, (type 1) that may occur spontaneously or of these patients is difficult since electro- interventricular septal thickness greater be unmasked by sodium channel blockers physiology studies are not helpful at than 30 mm and an abnormal blood such as flecainide and ajmaline. Patients predicting SCD. However, patients pressure response during exercise testing. presenting with aborted SCD, syncope, presenting with syncope who had ICDs seizures or nocturnal agonal respiration implanted go on to have appropriate ICD should undergo ICD implantation,15 12 Arrhythmogenic right ventricular therapies for ventricular arrhythmias. cardiomyopathy although no prospective, randomised Consequently, patients with dilated trials have evaluated ICD therapy in these cardiomyopathy who have a syncopal In ARVC, risk stratification for primary patients. episode should be offered ICD prevention is difficult due to a lack of Asymptomatic patients with a family implantation. data. However, patients with a family his- history of SCD should be risk stratified tory of SCD who present with syncope with a VT stimulation test and undergo Survivors of sudden cardiac should be deemed at high risk and ICD implantation if this is positive. death undergo electrophysiology studies. There However, the supporting data are equiv- is some evidence to suggest that inducible ocal. Asymptomatic patients without a Patients with HCM or ARVC who sur- ventricular arrhythmias are associated family history should be risk stratified vive SCD or have haemodynamically with appropriate ICD therapies.14 only if the type 1 changes are spontaneous. unstable VT should all have an ICD All other patients should be followed up 13 15 implanted for secondary prevention. Brugada syndrome closely.

Hypertrophic cardiomyopathy Both the Brugada and long QT syn- Long QT syndrome dromes (LQTS) arise because of inher- In HCM, the risk stratification of ited ion channel abnormalities that There are two well-known inherited patients to guide ICD implantation for predispose to ventricular arrhythmias forms of LQTS, but a modern gene based primary prevention is more difficult. and SCD. classification has now replaced these Currently, the major risk factors for SCD The Brugada syndrome is characterised eponymous syndromes with at least six

Clinical Medicine Vol 7 No 4 August 2007 399 Treatment Evaluation DINAMITCABGCASHCIDSDEFINITEMADITMUSTTSCDHeFTAVID Patch AntiarrhythmicsDefibrillatorCoronaryCardiacCanadianDEFibrillatorsMulticenterSudden ArrestCardiac ArteryImplantable AutomaticUnSustainedIN In StudyAcute VersusDeathBypassNon-Ischemic Hamburg Myocardial Defibrillator Defibrillator inImplantable Graft TachycardiaHeart Patchcardiomyopathy Failure Infarction Study ImplantationDefibrillators Trial Trial Trial Trial I and II

Explanation of trials with implantable cardioverter defibrillators

CME Cardiology

12 Phang R, Kang D, Tighiouart H, Estes NA Explanation of trials with implantable cardioverter defibrillators 3rd, Link MS. High risk of ventricular arrhythmias in patients with nonischemic AVID Antiarrhythmics Versus Implantable Defibrillators dilated cardiomyopathy presenting with CABG Patch Coronary Artery Bypass Graft Patch syncope. Am J Cardiol 2006;97:416–20. 13 Begley DA, Mohiddin SA, Tripodi D, CASH Cardiac Arrest Study Hamburg Winkler JB, Fananapazir L. Efficacy of CIDS Canadian Implantable Defibrillator Study implantable cardioverter defibrillator therapy for primary and secondary DEFINITE DEFibrillators In Non-Ischemic cardiomyopathy prevention of sudden cardiac death in Treatment Evaluation hypertrophic cardiomyopathy. Pacing Clin DINAMIT Defibrillator IN Acute Myocardial Infarction Trial Electrophysiol 2003;26:1887–96. MADIT Multicenter Automatic Defibrillator Implantation Trial I and II 14 Wichter T, Paul M, Wollmann C et al. Implantable cardioverter/defibrillator MUSTT Multicenter UnSustained Tachycardia Trial therapy in arrhythmogenic right SCDHeFT Sudden Cardiac Death in Heart Failure Trial ventricular cardiomyopathy: single center experience of long-term follow-up and complications in 60 patients. Circulation 2004;109:1503–8. chromosome loci (LQTS 1–6) coding for drug therapy with implantable 15 Shimizu W. The Brugada syndrome – an six genes identified. Each genetic syn- defibrillators in patients resuscitated from update. Review. Intern Med 2005;44: 1224–31. drome can be characterised by distinct cardiac arrest: the Cardiac Arrest Study Hamburg (CASH). Circulation 2000;102: 16 Priori SG, Schwartz PJ, Napolitano C et al. clinical features. Patients surviving SCD 748–54. Risk stratification in the long-QT or presenting with recurrent syncope are 5 Moss AJ, Hall WJ, Cannom DS et al. syndrome. N Engl J Med 2003;348:1866–74. considered at high risk and should Improved survival with an implanted undergo ICD implantation in addition to defibrillator in patients with coronary beta-blockers. disease at high risk for ventricular arrhythmia. Multicenter Automatic Most patients do not die from LQTS Defibrillator Implantation Trial but are at risk (13% risk of a fatal event Investigators. N Engl J Med 1996;335: over a lifetime if untreated), but pre- 1933–40. dicting risk is extremely difficult. All 6 Moss AJ, Zareba W, Hall WJ et al; patients will require careful counselling. Multicenter Automatic Defibrillator Implantation Trial II Investigators. A recent study has suggested high-risk Prophylactic implantation of a defibrillator patients include: in patients with myocardial infarction and • LQTS 1 if QTc is greater than reduced ejection fraction. N Engl J Med 500 ms in men and women 2002;346:877–83. 7 Buxton AE, Lee KL, Fisher JD et al. • all men with LQTS 2 A randomized study of the prevention of women with a QTc above 500 ms sudden death in patients with coronary • artery disease. Multicenter Unsustained 16 • all patients with LQTS 3. Tachycardia Trial Investigators. N Engl J Med 1999;341:1882–90. 8 Hohnloser SH, Kuck KH, Dorian P et al. References Prophylactic use of an implantable cardioverter-defibrillator after acute 1 National Institute for Health and Clinical myocardial infarction. N Engl J Med 2004; Excellence. Implantable cardioverter 351:2481–8. defibrillators for arrhythmia. London: 9 Bigger JT Jr, Whang W, Rottman JN et al. NICE, 2006. guidance.nice.org.uk/TA95 Mechanisms of death in the CABG Patch 2 A comparison of antiarrhythmic therapy trial: a randomized trial of implantable with implantable defibrillators in patients cardiac defibrillator prophylaxis in patients resuscitated from near-fatal ventricular at high risk of death after coronary artery arrhythmias. The Antiarrhythmics versus bypass graft surgery. Circulation 1999;99: Implantable Defibrillators (AVID) 1416–21. Investigators. N Engl J Med 1997;337: 10 Bardy GH, Lee KL, Mark DB et al. 1576–83. Amiodarone or an implantable 3 Connolly SJ, Gent M, Roberts RS et al. cardioverter-defibrillator for congestive Canadian implantable defibrillator study heart failure. N Engl J Med 2005;352: (CIDS): a randomized trial of the 225–37. implantable cardioverter defibrillator 11 Kadish A, Dyer A, Daubert JP et al. against amiodarone. Circulation Prophylactic defibrillator implantation in 2000;101:1297–302. patients with non-ischemic dilated 4 Kuck KH, Cappato R, Siebels J, Ruppel R. cardiomyopathy. N Engl J Med 2004;350: Randomized comparison of antiarrhythmic 2151–8.

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