How do I manage the patient with an ICD?

An implantable cardioverter defibrillator (ICD) serves as prophylaxis against sudden collapse and death from rapid ventricular arrhythmias. In general, all ICDs sense the heart rate and provide anti-tachycardia pacing or deliver synchronised (cardioversion) or unsynchronised (defibrillation) shocks. Some of the modern ICDs also incorporate dedicated pacing function; patients with heart block or sinus node disease may be dependent on these devices just like any patient with an implanted cardiac pacemaker. Like pacemakers, ICDs have to be checked by telemetric interrogation at periodic intervals to confirm integrity of the lead systems and proper function of ICD components including adequacy of battery voltage. Reprogramming of the various parameters that govern pacing, arrhythmia detection and therapy may be necessary from time to time. Such routine follow up, usually undertaken at established arrhythmia centres, should occur at 3 to 6 monthly intervals in the absence of major intercurrent events. Some issues specific to this group of patients can be summarised as follows:

1. Avoid rapid heart rates
In its basic form, arrhythmia detection algorithms of ICDs rely on a programmed heart rate threshold. Once this is exceeded for a defined period of time, the device may deliver therapy irrespective of whether the arrhythmia is of ventricular or supraventricular origin. In a ventricular-based ICD, the shock energy vector is designed primarily to encompass the ventricles. Consequently, atrial arrhythmias may fail to convert such that multiple inappropriate ICD shocks may result. Further, if antitachycardia
pacing is delivered in the ventricle for an atrial arrhythmia, ventricular arrhythmias may be provoked creating a pro-arrhythmic situation. The newer ICDs incorporate atrial sensing to improve arrhythmia discrimination but it must be remembered that any algorithm that improves specificity for ventricular arrhythmia will entail some loss of sensitivity. Cognisant of the above, it is imperative that atrial arrhythmias are adequately treated in these patients, particularly the paroxysmal form of atrial fibrillation that is commonly associated with rapid rates at its onset. Occasionally, RF ablation of the AV node is necessary. Beta adrenergic blockers should be an integral part of therapy in most ICD patients.

2. Recognise ICD—drug interactions
Antiarrhythmic drugs have the potential for interacting with an ICD in several ways. Drugs such as flecainide and amiodarone can increase pacing and defibrillation thresholds. In patients with a low margin of safety for these parameters, use of these drugs may result in failure of pacing or defibrillation. Secondly, these drugs can slow the rate of ventricular tachycardia below the programmed rate threshold for detection by the ICD; failure of arrhythmia detection can result. Some rarer interactions include
alteration of the T wave voltage by drugs or hyperkalaemia resulting in double counting and inappropriate shocks.

3. ICD wound management
As an implanted device, the system is susceptible to infections. Pain and inflammation of the skin over the ICD may herald an infective process. Similarly, unexplained fever, particularly staphylococcal septicaemia may indicate endocarditis involving the leads and/or tricuspid valve.