Introduction
Sudden death is broadly defined as “unexpected death due to natural causes occurring within 1 hour of the onset of acute symptoms”. Since the vast majority of these deaths are cardiac in origin, SCD (sudden cardiac death) has become almost synonymous with sudden death. An implantable cardioverter-defibrillator (ICD) is a device that is implanted prepectorally in patients that are at risk of Sudden Cardiac Death (SCD) due to ventricular tachyarrhythmias. ‘Defibrillation’ refers to the electrical termination of ventricular fibrillation, while ‘Cardioversion’ refers to synchronised electrical termination of atrial fibrillation, atrial flutter, and supraventricular and ventricular tachycardias.

The total cardiac mortality rate in India has been on the rise in the last few decades. Although accurate data is not available, the proportion of SCD from malignant arrhythmias, in the overall cardiac mortality rate, is definitely high in India. SCD occurs often due to ventricular tachyarrhythmia and 80% of SCD events occur in patients with ischaemic heart disease.1,2 The fatal arrhythmia is usually either primary VF (ventricular fibrillation) or VT (ventricular tachycardia) degenerating into VF. The self-termination of sustained ventricular tachyarrhythmias is rare. Prompt electrical cardioversion/defibrillation, if done within 3 minutes, can be life-saving in many of these patients. The delay in providing defibrillation has resulted in high overall mortality rates associated with out of hospital cardiac arrest. Therefore the single most important factor determining survival is definitely the immediate intervention after onset of the arrhythmic event.

The techniques of defibrillation were developed after investigations in the John Hopkins Hospital in the 1930s, with a focus on immediate intervention after the onset of the arrhythmic event. The Implantable Cardioverter-Defibrillator (ICD) was conceived to circumvent the delay in providing definitive therapy to individuals with life-threatening ventricular arrhythmias and its development was pioneered at Sinai Hospital in Baltimore by Michel Mirowski and team. After commencing their research in 1969, Mirowski and coworkers implanted the first ICD at John Hopkins University Medical Center in 1980. The ICD received approval from the U.S.FDA for market release in 1985.

Since the market release of implantable cardioverter-defibrillators (ICDs) two decades ago, evidence has been constantly accumulating on the use of implantable cardioverter-defibrillators (ICDs) in the management of sudden cardiac death (SCD) in particular patient groups, including prophylactic use in patients at high-risk of SCD. In the past two decades, the indications for ICD implantation have greatly expanded and the number of devices implanted annually has steadily increased.

Design of an ICD (Fig 1):
An ICD is a device that consists of:

Fig. 1 : An ICD device.	Fig. 2 : VF induction by T wave shock.
Fig. 3 : Anti-Tachycardia pacing.	Fig. 4 : VF detection and termination by ICD.

• a pulse generator with lithium batteries and a defibrillator capacitor that can send a powerful shock to the heart
• an electronic logic circuit to tell the ICD when to discharge.
• lead electrodes, which incorporates defibrillation coils, are placed in the heart to sense the cardiac rhythm and deliver the shock to the heart muscle.

An ICD typically delivers an electrical shock to the heart within 10 to 15 seconds of an arrhythmia onset, converting the abnormal rhythm back to normal, effectively preventing sudden cardiac death (Fig. 2).

An ICD can also deliver pacing (antibradycardia, antitachycardia) in addition to internal defibrillation and synchronised cardioversion.3 The anti-tachycardia pacing (ATP) is very useful in patients with VT, since many of these episodes can be terminated without the need for a painful shock. ATP is a series of rapid pacing pulses faster than the VT. ATP is capable of terminating slower, stable VTs with 90-95% VTs being successfully terminated by ATP (Fig. 3).

Also, ATP prevents any significant battery drainage, allowing the ICD to last for upto 10 years.

ICD implantation
ICDs are implanted using techniques similar to those for permanent pacemaker implantation, usually performed in the cardiac catheterization laboratory. ICD and pacemaker implantation require similar pre-operational testing and operational skills. However, DFT (defibrillation threshold) testing is mandatory in ICD implantation. To do this, VF is induced during the procedure and it is confirmed that the ICD terminates the VF successfully. An external defibrillator with remote defibrillator pads is a mandatory back-up requirement during ICD implantation (Fig. 4).

Role of ICD therapy in management of ventricular arrhythmias
The three major therapeutic options in the long-term management of ventricular arrhythmias to prevent SCD are:

1. Antiarrhythmic drug therapy
2. Ablative techniques applied at cardiac surgery or percutaneously using catheter techniques, and
3. ICD device system.⁴

ICD therapy is indicated to prevent SCD in patients who have not yet experienced life-threatening ventricular tachyarrhythmias but have a high risk for SCD (Primary prevention) and in patients who have already experienced a life-threatening ventricular tachyarrhythmia (Secondary prevention).

Many randomized controlled trials have been conducted to evaluate the efficacy of ICD therapy in preventing SCD.^5-10

The first proof that ICD therapy decreased mortality in Secondary prevention of SCD compared with the conventional anti-arrhythmic drug therapy in patients with ventricular arrhythmias was the Antiarrhythmics Versus Implantable Defibrillator (AVID) trial.⁵

The Multicentre Automatic Defibrillator Trial (MADIT) and MUSTT studies, which evaluated the role of ICD therapy in Primary prevention of SCD, suggested that patients with nonsustained VT and EF of 0.40 or less should undergo EP studies and if they have inducible VT (i.e. VT not suppressed with procainamide) should have an ICD.^8,9

The MADIT II study suggested that patients with prior MI and an EF of 0.3 or less ought to have a prophylactic ICD regardless of EP study and in patients with QRS duration greater than 120 ms (i.e. with cardiac dysynchrony) the survival benefits changed significantly.10 However, in actual terms the benefit of ICD for primary prevention has been modest, with huge attendant cost, as is discussed later.

The difference in results obtained from these randomized controlled trials outlined the necessity to stratify patient categories at higher risk of sudden death, that might most benefit from an ICD.

The recently conducted DINAMIT trials showed that ICD therapy does not reduce over-all mortality in high-risk patients with a recent myocardial infarction (LVEF < 35% and impaired cardiac autonomic function).11 Although the ICD therapy was associated with the reduction in the rate of death due to arrhythmia, it was offset by an increase in the rate of death from non-arrhythmic causes. Besides, further analysis indicated that the increased risk of death from non-arrhythmic causes was confined to patients who had received a shock from ICD.12 Current medical therapy with beta blockers, ACE inhibitors, aldosterone blockers delays the adverse ventricular remodeling that occurs after myocardial infarction that contributes to the electrophysiological mechanisms of tachyarrhythmias. The survival benefits of ICD therapy were delayed for the first year after a myocardial infarction and were not significantly different from current medical therapy, as is shown in the following figure for Kaplan-Meier survival estimates for the cumulative mortality risk in the DINAMIT study (Fig. 5).

Cost-benefit and cost-effectiveness of ICD
For any therapy, especially if invasive and expensive, critical analysis of its usefulness is paramount. In this regard, several parameters have been published in literature. The most commonly quoted parameter for ICD effectiveness has been “Relative reduction of mortality”. This is a potentially misleading statistic which can make molehills appear like mountains and hence should be discarded in favour of “Absolute reduction in mortality”. For example, consider 2 populations (groups A and B) of 1000 each; group A received some treatment and group B received placebo. If 1 died in group A and 2 died in group B, the relative reduction in mortality would be 50%, but the absolute reduction in mortality would be only 0.1%!!

Possibly the most logical and transparent statistic to judge efficacy and cost-effectiveness is the NNT (number needed to treat) to save 1 life. In this regard, ICD therapy shows the following pattern (Fig. 6):

The MADIT study selected the high-risk patients for secondary prevention since the NNT was 4, this amounts to Rs.16 lakhs to save 1 life (taking the current cost of a single chamber ICD as Rs. 4 lakhs).

The MADIT II study showed that ICD therapy resulted in an absolute benefit of £ 5.6% per year. The cost effectiveness computed for ICD therapy in primary prevention of SCD, as per these studies, comes to Rs.44 lakhs per life saved.

Therefore, ICD therapy is more cost effective for the secondary prevention of SCD due to ventricular arrhythmias than for primary prevention.

Recently, Sudden Cardiac Death in Heart Failure Trial (SCD-HeFT) showed that if used in non-selected patients with NYHA class II or III CHF and LVEF of 35 per cent or less, there was no significant mortality reduction with ICD.¹³

Fig. 5 : Survival estimates for the cumulative mortality risk in the DINAMIT study.

Fig. 6 : Number needed to treat to save one life.

Role in ICD implantation in specific clinical conditions

1. History of cardiac arrest (excluding acute myocardial infarction)
   When someone has been resuscitated from a cardiac arrest, not due to a reversible cause, that patient is at high risk for SCD. It is obvious that an ICD is strongly recommended in this setting. Unfortunately, such instances are rare in our setting, since emergency resuscitation outside the ICU is hardly ever successful
2. Sustained monomorphic ventricular tachycardia. (SMVT)
   In patients with SMVT that has resulted in cardiac arrest, cardiac failure or syncope, the ICD is usually employed as first line therapy.3 Compared to other available treatments like drug, catheter ablation, surgery, ICD therapy is widely applicable, well tolerated and associated with good short and long term results in vast majority of patients with SMVT and structural heart disease.
3. Hypertrophic cardiomyopathy
   Well established risk factors for SCD in HCM patients include a family history of SCD in more than two primary relatives, a clear history of recurrent syncope, a flat blood pressure response (less than 20 mmHg rise) during exercise, evidence of NSVT on ECG monitoring, and left ventricular wall thickness greater than 30 mm. If two or more risk factors are identified ICD therapy is recommended.³
4. Heart disease with syncope of undetermined origin
   Syncope is usually a benign condition. However when associated with structural heart disease and inducible VT during electrophysiological study, carries a high risk of SCD. Such patients need an ICD.¹⁴
5. Inherited arrhythmogenic Ion channel syndromes
   a) Congenital Long QT syndrome
   Initial therapy of choice in majority of these patients is beta-blocker therapy. ICD implantation is recommended in select patients in whom recurrent syncope, sustained ventricular arrhythmias, or recurrent cardiac arrest occurs despite drug therapy. ³
   b) Brugada Syndrome
   The Brugada syndrome represents one distinctive sub-group of idiopathic VF characterized by RBBB, persistent ST-segment elevation in precordial leads V1-V3 and a history of syncope or aborted SCD. Unlike Long QT syndrome, there is no effective drug therapy and ICD is recommended as first line therapy.¹⁴
6. Arrhythmogenic RV dysplasia (ARVD).
   Drug therapy is often used as primary therapy but is often ineffective. In patients with drug refractory malignant arrhythmias, the ICD provides prophylaxis against syncope due to haemodynamically unstable VT and sudden death. ¹⁴

Contraindications and limitations of ICD therapy
ICD therapy is not recommended

• When a reversible triggering factor for VT/VF can be definitely identified, such as ventricular tachyarrhythmias in evolving AMI or electrolyte abnormalities.
• In coronary disease patients without inducible or spontaneous VT undergoing routine coronary bypass surgery.
• In patients with Wolff-Parkinson-White syndrome presenting with VF secondary to atrial fibrillation.
• In patients with terminal illnesses, NYHA class IV drug refractory congestive heart failure, who are not candidates for cardiac transplantation, or with a life expectancy not exceeding 6 months.
• In patients with a history of psychiatric disorders, including uncontrolled depression and substance abuse, which will interfere with the meticulous care and follow-up needed by these patients and acts as a relative contraindication.
• In patients who have frequent tachyarrhythmias that may trigger shock therapy, such as sustained VT not responsive to antitachycardia pacing or pharmacological therapy as these events would cause frequent device activation and multiple shocks.¹⁴

Complications
The most frequent device related complication is frequent ICD discharges which are a clinical emergency. These discharges may be appropriate or inappropriate.

Inappropriate discharges occur in up to 10% of patients and are usually due to supraventricular arrhythmias.

A severe complication after ICD implantation is the infection of the ICD system. It necessitates explantation of the entire ICD system.³

Follow-up
Follow up of the patient with an ICD must include periodic visit preferably 6 monthly to assess pace-sense and impedance characteristics, to assess charge times, to diagnose the cause of any delivered therapy. Aspects of follow-up include history with specific emphasis on awareness of delivered therapy and any tachyarrhythmic events, device interrogation, assessment of battery status and charge time, retrieval and assessment of stored diagnostic data.

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