Understanding the role of Implantable Cardioverter defibrillator (ICD) and its implications.

 Dr. Zayed | Published : 10, AUGUST 2025.

Introduction

An Implantable Cardioverter Defibrillator (ICD) is a small, pocket-sized pulse and/or shock generator designed to monitor & correct life-threatening arrhythmias. It is typically implanted under the skin, either below the clavicle, along the ribs, or in the abdomen (especially in infants). The ICD’s leads travel via venous blood vessels to the Rt side of the heart, where they lodge at the apex or septum of the Rt ventricle.

Mechanism of Action

The ICD continuously monitors heart rate and rhythm. When an arrhythmia is detected, it delivers therapy in one of several forms:

  1. Anti-bradycardia pacing – In cases of bradycardia, the ICD can deliver low-energy pacing impulses synchronized to restore normal heart rate.

  2. Anti-tachycardia pacing (ATP) – In ventricular tachycardia (VT), the ICD may deliver rapid pacing stimuli to interrupt re-entrant circuits and restore sinus rhythm.

    • Important: ATP is not delivered during ventricular fibrillation (VF); in such cases, a direct shock is required.

  3. Defibrillation – For ventricular fibrillation or pulseless VT, the ICD delivers a high-energy shock to terminate the arrhythmia.

The system operates by detecting a rate and rhythm that exceed preset thresholds, then initiating the appropriate therapy.

Key Clinical Trials Supporting ICD Use

  • SCD-HeFT Trial (Sudden Cardiac Death in Heart Failure Trial): Demonstrated a 23% reduction in all-cause mortality in patients with congestive heart failure receiving ICD therapy.
  • AVID Trial (Antiarrhythmics Versus Implantable Defibrillators): Showed ICD therapy was superior to antiarrhythmic drugs for preventing sudden cardiac death.
  • MADIT-II Trial (Multicenter Automatic Defibrillator Implantation Trial): Supported ICD use in post-MI patients with an LVEF < 30%, showing significant mortality reduction.

Indications for ICD Implantation

Primary Prevention

  1. LVEF ≤ 35% due to prior MI, ≥ 40 days post-MI.
  2. LVEF ≤ 30% due to prior MI, ≥ 40 days post-MI.
  3. Non-ischemic dilated cardiomyopathy (DCM) with LVEF ≤ 35%.

Secondary Prevention

  1. Nonsustained VT post-MI with LVEF < 40% and inducible sustained VT or VF on electrophysiological study.
  2. Structural heart disease with spontaneous sustained VT (hemodynamically stable or unstable).
  3. Unexplained syncope with clinically significant sustained VT or VF induced during electrophysiological testing.

REFERENCES:

1. Tracy, Cynthia M.; Epstein, Andrew E.; Darbar, Dawood; DiMarco, John P.; Dunbar, Sandra B.; Estes, N.A. Mark; Ferguson, T. Bruce; Hammill, Stephen C.; Karasik, Pamela E.; Link, Mark S.; Marine, Joseph E.; Schoenfeld, Mark H.; Shanker, Amit J.; Silka, Michael J.; Stevenson, Lynne Warner; Stevenson, William G.; Varosy, Paul D. (January 2013). "2012 ACCF/AHA/HRS Focused Update Incorporated Into the ACCF/AHA/HRS 2008 Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities". Journal of the American College of Cardiology. 61 (3): e6 – e75. doi:10.1016/j.jacc.2012.11.007. PMID 23265327.

2. Bardy GH, Lee KL, Mark DB, Poole JE, Packer DL, Boineau R, Domanski M, Troutman C, Anderson J, Johnson G, McNulty SE, Clapp-Channing N, Davidson-Ray LD, Fraulo ES, Fishbein DP, Luceri RM, Ip JH; Sudden Cardiac Death in Heart Failure Trial (SCD-HeFT) Investigators. Amiodarone or an implantable cardioverter-defibrillator for congestive heart failure. N Engl J Med. 2005 Jan 20; 352(3):225–37.

3.Causes of death in the Antiarrhythmics Versus Implantable Defibrillators (AVID) Trial". Journal of the American College of Cardiology. 34 (5): 1552–59. November 1999. doi:10.1016/s0735-1097(99)00376-9. PMID 10551706.


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