Implantable Cardiac Defibrillators

A number of international trials have concluded that im-plantable cardiac defibrillators (ICDs) provide superior survival outcomes when compared to traditional pharmacological interventions for specific patients who are at risk of sudden cardiac death from ventricular dysrhythmias (AVID Investigators 1997, Moss et al. 1996, 2002, Trappe et al. 1997).

ICDs are implanted subcutaneously via the transvenous route in the left or right deltopectoral area, similar to a permanent pacemaker (Houghton & Kaye 2003, Porterfield et al. 1999). The procedure can be undertaken using local anaesthesia and intravenous sedation. An ICD comprises a generator (containing microprocessors, integrated circuits, memory boards and battery) plus one or more leads for pacing and defibrillation electrodes (Di Marco 2003). The software co-ordinates, senses, interprets, stores data and links with the different subsections of the system to provide an appropriate response (Pinski 2000). Current indications and guidelines for use are divided into primary and secondary prevention. For primary prevention:

• inducible VT on electrophysiological testing;

• myocardial infarction, depressed left ventricular function with an ejection fraction (EF) of 35% or less (normal ejection fraction 65-70%);

• high risk of sudden death due to familial cardiac conditions: long QT syndrome or arrhythmogenic right ventricular dysplasia.

For secondary prevention:

• survivors of cardiac arrest due to either VT or VF;

• spontaneous sustained VT associated with symptomatic haemodynamic compromise;

• sustained VT without causing syncope or a cardiac arrest in highly symptomatic patients who have an associated reduced EF (<35%) but whose heart failure is not totally debilitating (Houghton & Kaye 2003).

Modern ICDs are highly sophisticated, with a battery that may last anything from five to nine years, and can be programmed to perform multiple tasks including:

• record intracardiac electrograms, allowing review of episodes of rhythm disturbances and defibrillation;

• arrhythmia detection;

• delivery of high-energy unsynchronised defibrillation shocks;

• discharge low-energy synchronised shocks (cardioversion) for VT;

• delivery of low- and high-energy shocks (tiered therapy);

• antitachycardia pacing;

• back-up pacing for bradyarrhythmias (Di Marco 2003, Pinski 2000)

Worldwide, the number of patients having these devices fitted is increasing annually (Garratt 1998, Tagney 2003). It is important to know how to manage such individuals should the ICD fail or if defibrillation is required. Current guidelines suggest that paddles should be positioned at least 10-12 cm from the device. Shocks will not damage the equipment but it is advisable that the ICD should be deactivated to avoid the device potentially inducing VT or VF (Pinski 2000).

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