Implantable cardioverter defibrillator (ICD)

Implantable cardioverter defibrillator (ICD)

Implantable cardioverter defibrillator, known in short as ICD, is a device which monitors the heart rhythm continuously and treats it electrically when needed. As it functions automatically without the need for a bystander intervention as in automated external defibrillator or AED, it is also known as automatic implantable cardioverter defibrillator or AICD. When Michel Mirowski reported on a standby automatic defibrillator in 1970, it was met with a lot of skepticism [1]. But now ICD is a time tested life-saving equipment.

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 in the left parasternal region is also available. Recently an extravascular ICD has been tested, with a lead in the retrosternal space, 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 transvenous leads in the conventional ICD system. Leads in the vascular system can get infected, can get fractured or dislodged and occasionally perforate the cardiac chambers. Leads can also lead to intravascular thrombus formation. 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 in the left infraclavicular region for transvenous ICDs. Pulse generator is implanted in the left axillary region 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.

An important disadvantage common to all types of ICDs is an inappropriate shock. This could make up to one fourth of all shocks and there are ongoing innovations to have better diagnostic algorithms to avoid inappropriate shocks for a supraventricular rhythm. Oversensing of T waves and sometimes P waves as in case of extravascular ICDs can also cause an inappropriate shock. The pain caused by inappropriate shocks can cause psychological problems as well. Though the appropriate shocks are also painful, at least there is a relief that a potential sudden death has been averted.

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, though overdrive pacing has been used in fast ventricular tachycardias as well [2].

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 drugs 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 VT has a potential to cause tachycardiomyopathy. Antiarrhythmic drugs can also increase the defibrillation threshold and require higher energies for a shock to terminate VF.

Another option often used to reduce the number of ICD shocks is mapping and ablation of potentially ablatable tachycardias. This does not have the disadvantages mentioned in case of antiarrhythmic drugs. But mapping and ablation may not be that successful in case of ischemic VTs with multiple circuits related to the border zones of myocardial scars. Similarly, mapping and ablation have only limited role in those with certain cardiac channelopathies.

ICDs can be given both as a primary prevention and secondary prevention. They are given for primary prevention of sudden cardiac death in those with cardiac channelopathies with high risk of life threatening ventricular arrhythmias like Brugada syndrome and long QT syndromes. Post myocardial infarction left ventricular dysfunction is another potential indication for primary prevention. But in this case the utility of the device may not be that high even though studies have documented benefit [3]. Secondary prevention after resuscitated cardiac arrest and in those with recurrent life-threatening ventricular tachyarrhythmias are situations in which ICD implantation cannot be avoided.

Implantation of ICDs can be under local or general anaesthesia. If defibrillation threshold testing is done, general anaesthesia may be better because of the pain involved. But for modern ICDs with good performance, defibrillation threshold testing is seldom a necessity. For defibrillation threshold testing, VF can be induced by a low energy shock delivered on the T wave. Defibrillation threshold testing is not without risks, as occasionally the induced VF may be refractory, sometimes requiring even simultaneous defibrillation with two external defibrillators at full energy.

Good follow up programming is a must in case of ICD. ICD interrogation is needed after each shock to ascertain whether it was appropriate or inappropriate. Programmed parameters like zones of therapy and types of antitachycardia pacing cycles may have to be revised depending on the interrogation results. Interrogation is also needed to check battery status. Recorded events can give an idea of the types of arrhythmias, including atrial high rate events (AHRE) which have been considered as surrogates of atrial fibrillation.

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 have always raised concerns about unauthorized malicious access.

Potential complications other than those related to the leads as mentioned earlier and inappropriate shocks are those related to device pocket. In the early period after an implantation, pocket hematomas can form especially in those on antithrombotic medications. Hematomas have a potential risk of infection and may occasionally need surgical drainage. Erosion of device from the pocket is another potential problem. Device pocket infection is another dreaded complication which may require device explantation and implantation at another site after control of infection.

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 condition, raising ethical concerns. Like a pacemaker, ICD has to be explanted when cremation is being planned. Care has to be taken to suspend ICD therapies 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 therapy 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 only a brief overview about ICDs as the topic is very vast with so many ramifications!

References

1. Mirowski M, Mower MM, Staewen WS, Tabatznik B, Mendeloff AI. Standby automatic defibrillator. An approach to prevention of sudden coronary death. Arch Intern Med. 1970 Jul;126(1):158-61. doi: 10.1001/archinte.126.1.158. PMID: 5425512.
2. Wathen MS, DeGroot PJ, Sweeney MO, Stark AJ, Otterness MF, Adkisson WO, Canby RC, Khalighi K, Machado C, Rubenstein DS, Volosin KJ; PainFREE Rx II Investigators. Prospective randomized multicenter trial of empirical antitachycardia pacing versus shocks for spontaneous rapid ventricular tachycardia in patients with implantable cardioverter-defibrillators: Pacing Fast Ventricular Tachycardia Reduces Shock Therapies (PainFREE Rx II) trial results. Circulation. 2004 Oct 26;110(17):2591-6. doi: 10.1161/01.CIR.0000145610.64014.E4. Epub 2004 Oct 18. PMID: 15492306.
3. Moss AJ, Zareba W, Hall WJ, Klein H, Wilber DJ, Cannom DS, Daubert JP, Higgins SL, Brown MW, Andrews ML; Multicenter Automatic Defibrillator Implantation Trial II Investigators. Prophylactic implantation of a defibrillator in patients with myocardial infarction and reduced ejection fraction. N Engl J Med. 2002 Mar 21;346(12):877-83. doi: 10.1056/NEJMoa013474. Epub 2002 Mar 19. PMID: 11907286.