What is an implantable cardioverter defibrillator (ICD)? Cardiology Basics

What is an implantable cardioverter defibrillator (ICD)? Cardiology Basics

Implantable cardioverter defibrillator or ICD is a cardiac implantable electronic device or CIED. The device is implanted subcutaneously under local anaesthesia, usually below the left clavicle. The electrodes from the device are introduced into the right sided cardiac chambers through a subclavian vein puncture. The ICD senses and analyzes heart rhythm continuously. It can detect life threatening ventricular arrhythmias and treat them electrically.

If a ventricular tachycardia is detected, the ICD tries to overdrive it by fast pacing. If it does not suppress the ventricular tachycardia, the ICD delivers a shock through its high voltage coils and terminates the arrhythmia.

The X-ray picture shown here depicts a high voltage coil in the superior vena cava. There is another high voltage coil in the right ventricle. ICDs can also give back up pacing if there is post shock bradycardia. Pacing electrodes are seen at the tip of the lead in the X-ray.

If a ventricular fibrillation is detected by the ICD, it immediately gives a direct current shock to abolish it. Thus, ICD is an important life saving device which safeguards the recipient from sudden death by dangerous cardiac arrhythmias.

Occasionally, an ICD may give an inappropriate shock when actually one is not needed. Most of these are due to supraventricular tachyarrhythmias being detected by the device as ventricular arrhythmias. Dual chamber ICDs with an additional atrial sensing lead may be helpful to avoid this. Inappropriate shocks can be averted to some extent by meticulous device programming. Some patients do have psychological problems due to the fear of shocks. Good counselling is often useful to allay this anxiety.

The most commonly implanted ICD is also called transvenous ICD because the leads are introduced through the veins into the heart. A purely subcutaneous ICD with lead implanted subcutaneously is also available. Recently an extravascular ICD has been tested, with a substernal lead, with performance better than purely subcutaneous ICD and almost nearing the transvenous ICD.

These new types of ICDs have been innovated to avoid the potential problems with  intravascular leads in conventional ICD system. Intravascular leads can get infected, fractured or dislodged and occasionally perforate the cardiac chambers. Leads can also lead to formation of intravascular thrombi and cause thromboembolism sometimes or venous obstruction. For these reasons, lead has been called as the ‘Achilles Heel’ of most cardiac implantable electronic devices, which include pacemakers and ICDs.

The ICD system consists of a pulse generator with battery, usually implanted below the left clavicle for transvenous ICDs. Pulse generator is implanted below the left axilla in case of purely subcutaneous ICD and extravascular ICD. Left sided implants are preferred because ICD shocks will be more effective with an ICD pulse generator can on the left side. ICD pulse generator can is one active electrode while high voltage coils located in superior vena cava and right ventricle are the other active shock electrodes for a transvenous ICD.

Overdrive pacing for ventricular tachycardia is an option available in transvenous and extravascular ICDs, but not in purely subcutaneous ICDs. Overdrive pacing is a painless form of treatment for ventricular tachycardia which is not too fast. Most ICDs can be programmed to try overdrive pacing when the high voltage capacitor is being charged for shock delivery. If overdrive pacing is able to terminate the ventricular tachycardia, a shock is averted. All fast ventricular tachycardias and ventricular fibrillation usually receive shock as the first option.

The battery life of the ICD depends on the number shocks it is called upon to deliver and the energy levels needed for each shock. Antiarrhythmic medications can be used to suppress ventricular tachycardia to reduce the potential need for shocks. But there is a caveat. Breakthrough ventricular tachycardia can occur at a lower rate, below the programmed detection rate of the ICD. Recurrent slow ventricular tachycardia can cause tachycardiomyopathy with heart failure in the long run. Antiarrhythmic medications can also sometimes increase the defibrillation threshold, which can be deleterious.

Another option often used to reduce the number of ICD shocks is mapping the site of origin of the ventricular tachycardia using intracardiac electrodes, known as electrophysiology study. The abnormal focus can then be ablated by delivering short pulses of radiofrequency current, known as radiofrequency catheter ablation. This does not have the disadvantages mentioned in case of medications. But mapping and ablation may not be that successful in all cases.

Good follow up programming is a must in case of ICD. ICD programming is done by a device which send radio signals to the device and receives information stored in it. ICD interrogation is needed after each shock to ascertain whether it was appropriate or inappropriate. Interrogation is also needed to check battery status. Recorded events can give an idea of the types of arrhythmias which have occurred.

With modern ICDs remote follow up is also possible with basic data from the ICD being transmitted over a wireless network to a physician located even in another continent. But changing programmed parameters will need a visit to the nearest specialist centre. Wireless access to medical devices has always raised concerns about unauthorized malicious access.

Another concern about ICDs is during end of life care. There may be situations in which ICDs may have to be inactivated in a near terminal situation, raising ethical concerns. Like a pacemaker, ICD has to be explanted when cremation is being planned. Care has to be taken to suspend ICD shocks by programming to avoid shocks to the persons doing the explantation. In other situations, like while surgery using electrocautery are being planned, it is possible to suspend ICD shocks by applying a magnet over the ICD, when a programmer is not available.

Precautions with electromagnetic radiation and magnetic resonance imaging are applicable to ICDs like pacemakers. This is because electromagnetic waves can interfere with functioning of the ICD. It can also interfere with the communication between the ICD and the programming device. Mobile phones can be used on the opposite ear, at a distance of at least 15 cm from the location of the ICD. Person with ICD should not drive a vehicle if an ICD shock has been received in the preceding few months. Beyond that period, opinion should be sought from the medical personnel who is following up the device function. This is only a brief overview about ICDs as the topic is very vast!