Implantable Cardioverter Defibrillator (ICD) Shock

ICD Shock Therapy


An implantable cardioverter defibrillator (ICD) continuously monitors the heartbeat and delivers an electric ICD shock, when needed, to restore a regular heart rhythm. They are used for (1) the primary prevention of sudden cardiac death in individuals with increased risk of life-threatening ventricular tachycardia (VT) or ventricular fibrillation (VF), and (2) the secondary prevention of sudden cardiac death in individuals with previous sustained VT, VF, or who were resuscitated due to likely VT or VF.1

An implantable cardioverter defibrillator also is used in cardiac resynchronization therapy, bradycardia pacing, and hemodynamic monitoring.2,3,4

ICD Indications

An implantable cardioverter defibrillator is used to prevent ventricular arrhythmia-related sudden cardiac death. As primary prevention, ICDs are used in patients at high risk for ventricular arrhythmia and sudden cardiac arrest who do not have experience with these conditions. As secondary prevention, ICDs are used in patients with prior episodes of sudden cardiac arrest, sustained ventricular tachycardia (VT), and syncope caused by ventricular arrhythmia.1

ICD shock therapy is not indicated for individuals who do not have a reasonable expectation of survival with an acceptable functional status for at least one year, even if they otherwise meet ICD implantation criteria. It is also not indicated in patients with ventricular tachyarrhythmias due to a completely reversible disorder in the absence of structural heart disease, as well as individuals with VF or VT that is amenable to surgical or catheter ablation where their risk of sudden cardiac death is normalized after successful ablation. Placement of an implantable cardioverter defibrillator should be delayed for active infections or acute medical problems; they can be bridged with a wearable cardioverter-defibrillator.

ICD indications include:

  • Prior myocardial infarction and reduced left ventricular function (though not for VT or VF limited to the first 48 hours after an acute myocardial infarction)5,6
  • Non-sustained ventricular tachycardia due to prior MI, LVEF of 40% or less, and inducible, sustained VT or ventricular fibrillation (VF) at electrophysiological study7
  • Prior sudden cardiac arrest due to VT/VF8
  • Unstable VF1
  • Stable sustained VT not due to reversible causes1
  • Unexplained syncope with inducible sustained monomorphic VT1
  • Non-ischemic heart disease with LVEF of 35% or less, despite guideline-directed management and therapy1
  • Non-ischemic cardiomyopathy with previous sudden cardiac arrest due to VT/VF, hemodynamically unstable VT, or stable sustained VT, not due to reversible causes1
  • Left ventricular systolic dysfunction9
  • Severe, dilated cardiomyopathy (LVEF less than 36)10
  • Congestive heart failure (LVEF less than 35)11
  • Cardiac sarcoidosis with sustained VT or previous sudden cardiac arrest, or LVEF of 35% or less12
  • Neuromuscular disorders for primary and secondary prevention13
  • Cardiac channelopathies (high-risk long QT syndrome, catecholaminergic polymorphic ventricular tachycardia, Brugada syndrome, short QT syndrome)14

Device Overview & Selection Options

Two types of ICDs are currently available: transvenous (TV-) ICD and subcutaneous (S-) ICD. The first available type of implantable cardioverter defibrillator was TV-ICD. This device is implanted through the venous system ending in the chambers of the heart. TV-ICDs consist of a pulse generator, sensing/pacing electrodes, and defibrillation coils. The pulse generator contains a microprocessor that analyzes the cardiac rhythm and controls the delivery of the therapy, a memory component that stores electrocardiographic data, a high-voltage capacitor, and the battery.15

An electrode is transvenously placed at the endocardium of the right ventricular apex. Dual chamber ICDs have additional electrodes placed in the right atrium. Biventricular ICDs have a third electrode placed transcutaneously in a branch of the coronary sinus, or surgically on the epicardium of the left ventricle. Defibrillation coils are positioned on the right ventricular electrode. In most ICDs, current flows from the distal defibrillation coil simultaneously to the pulse generator and the proximal defibrillation coil. TV-ICDs can deliver multiple therapies when ventricular arrhythmia is detected, including anti-tachycardia pacing, low-energy cardioversion, and high-energy defibrillation.15

  • TV-ICDs have a considerable rate of major complications at the time of implantation (e.g., hemorrhage, infection, pneumothorax, cardiac perforation, and death) and after implantation (e.g., inappropriate device therapy, endocarditis, vessel occlusion, valvular damage, lead dislodgement, and malfunction).16
  • S-ICDs are a newer, less invasive version of ICDs that can avoid some of the complications arising from TV-ICDs because they don’t require venous access. S-ICDs show comparable sensitivity to TV-ICDs in arrhythmia detection, improve specificity, and lower rates of inappropriate shocks compared to TV-ICDs.17

S-ICDs comprise a pulse generator placed subcutaneously over the left thorax and a single subcutaneous lead placed along the left side of the sternum. The lead includes a single high-voltage, low-impedance shock coil, and two low-voltage, high-impedance sensing electrodes. Three distinct sensing vectors are available: proximal ring of electrode to pulse generator, distal tip of electrode to generator, and, distal tip of electrode to proximal ring of electrode.

The device is controlled by a programmer console that programs all S-ICD diagnostics, including therapy and post-shock pacing activation, conditional shock VT and shock VF activation zone, stored arrhythmic events, shock therapies, battery status, and shock coil integrity.16 S-ICDs are approved for treatment of ventricular arrhythmias in patients who do not have any of the following:16

  • Symptomatic bradycardia
  • Incessant ventricular tachycardia
  • Spontaneous, frequently recurring VT that is reliably terminated with antitachycardia pacing

S-ICDs are contraindicated in patients that require permanent pacing, as it is unable to deliver bradycardia pacing, biventricular pacing, or anti-tachycardia pacing. S-ICDs are only able to provide limited post-shock pacing.18

ICD Shock: Patient and Special Populations Considerations

The use of ICDs as primary and secondary therapy of ventricular arrhythmias is well-established. Certain populations that tend to be under-represented in clinical trials require special consideration.

Older patients have an increased risk of complications, particularly pneumothorax and lead dislodgement.19 They are also more likely to have multiple comorbidities that could affect post-implantation survival.

ICDs are common in patients with continuous flow left ventricular assist devices (LVAD). However, there is no apparent mortality benefit associated with ICDs in this patient population.20 Given considerable morbidity and complications involving ICDs and lack of randomized trials, use of ICDs in patients with LVADs require special consideration.

Pediatric patients at increased risk for sudden cardiac arrest due to dilated cardiomyopathy, hypertrophic cardiomyopathy, arrhythmogenic right ventricular cardiomyopathy, long or short QT syndrome, Brugada syndrome, catecholaminergic polymorphic VT, and congenital heart disease, require special consideration because of structural heart defects, small size and future growth. For pediatric patients, S-ICDs are particularly suitable to avoid complication due to the small venous access.21

ICD Shock: Monitoring & Post-implantation Management

Traditional follow-up of implantable cardioverter defibrillator patients includes in-person visits every three months. Scheduled in-person approaches generate a large number of non-actionable visits and enormous clinic workload. Remote patient monitoring (RPM) collects near continuous data from ICDs for automatic cloud-based analysis and then to the clinic, without the need for connection with clinic or patient action. RPM reduces the number of non-actionable in-person evaluations without compromising detection of at-risk patients.22

ICDs are programmed to detect arrhythmias in the VF zone, with rates faster than 180/min to 200/min. Unfortunately, not all electrograms that meet the diagnostic criteria of VF zone are VF or ventricular arrhythmias. These are the events that trigger inappropriate ICD shock. Supraventricular tachycardias, including sinus tachycardias, atrial fibrillation and atrial flutter are accountable for more than 90% of inappropriate ICD shocks.23

Technical causes of inappropriate ICD shocks include faulty components, lead fractures, magnetic interference, oversensing of electrical noise, myopotentials and T-waves, and double counting QRS complexes.15 Certain patients are at higher risk for inappropriate ICD shocks. Those include patients with atrial fibrillation, tobacco use, diastolic hypertension and non-ischemic heart disease.9 Although ICD therapy provides life-saving strategies in the case of sudden cardiac arrest, ICD shocks, both appropriate and inappropriate, are associated with increased mortality.9,24 ICD shocks adversely affect mental and physical well-being. Strategies developed to reduce incidence of ICD shock involve:9

  • Antitachycardia pacing, replacing shock as safe and effective therapy for fast ventricular tachycardia (>200 bpm)25
  • ICD programming
  • More stringent arrhythmia detection algorithms
  • Dual-chamber ICDs with detection of atrial and ventricular arrhythmias: in theory, but no data exists to support that dual-chamber ICDs reduce rate of inappropriate shocks
  • Antiarrhythmic medication
  • Catheter ablation


1. Al-Khatib SM, Stevenson WG, Ackerman MJ, et al. 2017 AHA/ACC/HRS Guideline for Management of Patients With Ventricular Arrhythmias and the Prevention of Sudden Cardiac Death. Circulation. 2018;138(13):e210-e271.

2. Wilkoff BL, Fauchier L, Stiles MK, et al. 2015 HRS/EHRA/APHRS/SOLAECE expert consensus statement on optimal implantable cardioverter-defibrillator programming and testing. Ep Europace. 2016;18(2):159-183.

3. Raj LM, USA U of SC USC Center for Body Computing, Keck School of Medicine, Los Angeles, CA, Saxon LA. Haemodynamic Monitoring Devices in Heart Failure: Maximising Benefit with Digitally Enabled Patient Centric Care. Arrhythmia Electrophysiol Rev. 2018;7(4):1.

4. Gill J. Implantable cardiovascular devices: current and emerging technologies for remote heart failure monitoring. Cardiol Rev. 2022;Publish Ahead of Print.

5. Moss AJ, Hall WJ, Cannom DS, et al. Improved Survival with an Implanted Defibrillator in Patients with Coronary Disease at High Risk for Ventricular Arrhythmia. New Engl J Medicine. 1996;335(26):1933-1940.

6. Moss AJ, Zareba W, Hall WJ, et al. Prophylactic Implantation of a Defibrillator in Patients with Myocardial Infarction and Reduced Ejection Fraction. New Engl J Medicine. 2002;346(12):877-883.

7. Buxton AE, Lee KL, Fisher JD, Josephson ME, Prystowsky EN, Hafley G. A Randomized Study of the Prevention of Sudden Death in Patients with Coronary Artery Disease. New Engl J Medicine. 1999;341(25):1882-1890.

8. Investigators A versus ID (AVID). A Comparison of Antiarrhythmic-Drug Therapy with Implantable Defibrillators in Patients Resuscitated from Near-Fatal Ventricular Arrhythmias. New Engl J Medicine. 1997;337(22):1576-1584.

9. Borne RT, Varosy PD, Masoudi FA. Implantable Cardioverter-Defibrillator Shocks: Epidemiology, Outcomes, and Therapeutic Approaches. Jama Intern Med. 2013;173(10):859-865.

10. Kadish A, Dyer A, Daubert JP, et al. Prophylactic Defibrillator Implantation in Patients with Nonischemic Dilated Cardiomyopathy. New Engl J Medicine. 2004;350(21):2151-2158.

11. Bardy GH, Lee KL, Mark DB, et al. Amiodarone or an Implantable Cardioverter–Defibrillator for Congestive Heart Failure. New Engl J Medicine. 2005;352(3):225-237.

12. Schuller JL, Zipse M, Crawford T, et al. Implantable Cardioverter Defibrillator Therapy in Patients with Cardiac Sarcoidosis. J Cardiovasc Electr. 2012;23(9):925-929.

13. Anselme F, Moubarak G, Savouré A, et al. Implantable cardioverter-defibrillators in lamin A/C mutation carriers with cardiac conduction disorders. Heart Rhythm. 2013;10(10):1492-1498.

14. Corrado D, Link MS, Schwartz PJ. Implantable defibrillators in primary prevention of genetic arrhythmias. A shocking choice? Eur Heart J. Published online 2022.

15. Gehi AK, Mehta D, Gomes JA. Evaluation and Management of Patients After Implantable Cardioverter-Defibrillator Shock. Jama. 2006;296(23):2839-2847.

16. Rhyner J, Knight BP. The Totally Subcutaneous Implantable Defibrillator. Cardiol Clin. 2014;32(2):225-237.

17. Gold MR, Theuns DA, Knight BP, et al. Head‐To‐Head Comparison of Arrhythmia Discrimination Performance of Subcutaneous and Transvenous ICD Arrhythmia Detection Algorithms: The START Study. J Cardiovasc Electr. 2012;23(4):359-366.

18. Raja J, Guice K, Oberoi M, et al. Shock without wires: A look at subcutaneous implantable cardioverter-defibrillator (ICD) compared to transvenous ICD for ventricular arrhythmias. Curr Prob Cardiology. Published online 2021:100927.

19. Lim WY, Prabhu S, Schilling RJ. Implantable Cardiac Electronic Devices in the Elderly Population. Arrhythmia Electrophysiol Rev. 2019;8(2):143-146.

20. Alvarez PA, Sperry BW, Pérez AL, et al. Implantable Cardioverter Defibrillators in Patients With Continuous Flow Left Ventricular Assist Devices: Utilization Patterns, Related Procedures, and Complications. J Am Hear Assoc Cardiovasc Cerebrovasc Dis. 2019;8(14):e011813.

21. Friedman DJ, Tully AS, Zeitler EP. Subcutaneous and Transvenous ICDs: an Update on Contemporary Questions and Controversies. Curr Cardiol Rep. 2022;24(8):947-958.

22. Varma N, Love CJ, Michalski J, Epstein AE, TRUST Investigators. Alert-Based ICD Follow-Up A Model of Digitally Driven Remote Patient Monitoring. Jacc Clin Electrophysiol. 2021;7(8):976-987.

23. Daubert JP, Zareba W, Cannom DS, et al. Inappropriate Implantable Cardioverter-Defibrillator Shocks in MADIT II Frequency, Mechanisms, Predictors, and Survival Impact. J Am Coll Cardiol. 2008;51(14):1357-1365.

24. Moss AJ, Greenberg H, Case RB, et al. Long-Term Clinical Course of Patients After Termination of Ventricular Tachyarrhythmia by an Implanted Defibrillator. Circulation. 2004;110(25):3760-3765.

25. Wathen MS, DeGroot PJ, Sweeney MO, et al. Prospective Randomized Multicenter Trial of Empirical Antitachycardia Pacing Versus Shocks for Spontaneous Rapid Ventricular Tachycardia in Patients With Implantable Cardioverter-Defibrillators. Circulation. 2004;110(17):2591-2596.

Author Bio

Ivana Celic, PhD, is a biomedical scientist and freelance medical and scientific writer. Her research interests include genome plasticity, cancer, aging, neurodegenerative disease and infertility. She actively participates in laboratory research and scientific writing and presentations.