Implantable Cardioverter-Defibrillator complication



  • All ICDs are also pacemakers (i.e. have pacing functionality) but pacemakers are not ICDs (i.e. do not have shocking functionality)


  • Pulse Generator
    • Typically implanted in L upper chest wall
    • Contains device hardware and battery
    • Acts as a shocking electrode in defibrillation
  • Leads
    • Insulated and antimicrobial-coated
    • Atrial lead
      • Pacing and sensing functionality
      • Typical bipolar (pacing and sensing are performed by two electrodes, located several millimeters apart at the tip of the lead)
    • Right ventricular lead
      • Pacing, sensing, and defibrillation functionality
      • Has one or two coils to facilitate defibrillation
        • Easily identified on Chest X-Ray and can hep differentiate an ICD from a pacemaker
      • Typical bipolar (pacing and sensing are performed by two electrodes, located several millimeters apart at the tip of the lead)
    • Left Ventricular Lead
      • Pacing and sensing functionality
      • Bipolar or unipolar (pacing and sensing are performed by one electrode at the tip of the LV lead and a second remote electrode, typically either one of the RV electrodes or the pulse generator)

Indications for ICD implantation

  • Secondary Prevention[1]
    • Previous episode of unstable VT or VF
    • Sustained VT in setting of cardiomyopathy or channelopathy
  • Primary Prevention[2]
  • Biventricular ICD (i.e. Cardiac Resynchronization Therapy)
    • Any of the above with intraventricular conduction delay of >120ms


  • Can perform all functions of pacemakers
  • Anti-tachycardia pacing
    • In response to a ventricular rate within a pre-set range (typically ~150-220 bpm), the device will compare the ongoing QRS morphology to a saved image of a QRS complex that is known to be a sinus beat; if the ICD algorithm determines that the ongoing morphology is significantly different (i.e. is unlikely to be a sinus tachycardia), it will deliver a series of paced beats at a rate slightly faster than the ongoing rate in an attempt to break the re-entrant cycle
    • A device will typically be programmed to attempt anti-tachycardia pacing several times; if unsuccessful, it will move on to defibrillation
  • Defibrillation
    • Delivery of a large electrical shock (up to 42 joules) from one electrode to the other in response to VT or VF
  • Magnet Mode
    • Placement of a magnet over the generator will deactivate anti-tachycardia pacing and defibrillation modes as long as the magnet is in place.
  • Diagnostics
    • Records rhythm strips of AF, VT, and VF episodes for later review
      • If anti-tachycardia pacing or defibrillation has occurred, there will be a large alert on the device's interrogation home screen
    • Keeps track of % of paced vs intrinsic beats
    • Keeps a rate histogram, as well as % of time spent in AF

Methods to Identify Manufacturer

  • Patient should carry a pocket card indicating manufacturer
  • Manufacturer Hotline has patient database
    • Medtronic Inc. (1-800-328-2518)
    • St. Jude Medical Inc. (recently acquired by Abbott, Inc.) (1-800-722-3774)
    • Boston Scientific Inc. (1-800-227-3422)
  • Magnet mode
    • Does not help differentiate ICDs (different than pacemakers)

Electromagnetic Interference

  • Nonmedical
    • Cell phones: do not interact with device
    • Airport security: may trigger alarm, no alteration of activity
  • Medical Sources
    • MRI: mostly safe, consult cards on device specific recs
    • Cardioversion: Use AP pads >8cm from device to minimize adverse effects
    • Unipolar Cautery - can cause sensing and pacing malfunction as well as reprogramming


Defibrillator Shocks

  • Approximately 1/3 - 1/2 of ICD patients receive a shock within one year of placement[3]
    • Generally described by patients as a powerful "kick to the chest"
    • Can be very distressing and a source of anxiety for patients
  • Inappropriate Shocks
    • Causes
      • Misclassification by device of SVT or AF as VT or VF
      • Electrical Noise
      • Fractured Lead
    • Can be diagnosed by interrogation of device after a reported shock
  • Phantom Shocks
    • The perception of a shock without a delivered shock
    • Generally described by patients as a mild electrical pain or pain around the defibrillator site
    • More common in patients who have never had an actual shock
    • Occurs in 9% of patients each year[4]

Pacemaker Malfunctions

Problems with pocket

  • Infection
    • Most commonly S. aureus or S. epidermidis
    • 2% local wound infection; 1% sepsis/bacteremia
  • Hematoma
    • Typically occurs shortly after placement

Problems with leads

  • Dislodgment
  • Perforation (most commonly at RV apex)
  • Infection can cause severe sepsis
  • Tricuspid regurgitation
  • Coiling (ie: Twiddler's Syndrome)
    Twiddler Syndrome after large pocket and defibrillator wires coiled around the generator

Failure to Capture

  • Delivery of pacing stimulus without depolarization
    • Functional - refractory myocardium, desensitized local tissue around the lead
    • Medical - drugs, myocardial disease, electrolytes
    • Technical - insufficient device output, lead dislodgment, fracture, insulation defect, ventricular wall perforation
  • Battery of End of Life (EOL)

Failure to Pace

  • Failure to deliver a stimulus to the heart
    • Oversensing - most common cause: retrograde P’s, T’s, skeletal muscle myopotentials,
    • Crosstalk - type of oversensing where the ventricular lead senses atrial pacing stimulus, and ventilator output inhibited

Failure to Sense

  • Normal function: a sensed myocardial depolarization greater than the programmed threshold causes inhibition of pacing
  • Failure to sense results in a paced beat on top of an intrinsic beat (as the device is "unaware" of the intrinsic beat")
    • Voltages of patient's intrinsic QRS complex is too low to be detected
    • New intrinsic arrhythmia (AF has a smaller depolarization than sinus beat), AMI, electrolyte abnormalities, lead separation, battery depletion

Runaway Pacing

  • Physiologic electrical activity (T waves, muscle potentials)
  • External electromagnetic interference
  • Signals generated by interaction of different portions of the pacing system
  • Potentially life-threatening as it can cause V-Fib or (paradoxically) bradycardia due to failure to capture

Pacemaker Mediated Tachycardia

  • Also known as Endless Loop Tachycardia
  • Formation of a re-entrant circuit causing inappropriate tachycardia
    • Most commonly: paced ventricular beat -> retrograde AV node conduction -> intrinsic P wave -> device reacts to intrinsic P wave by looking for intrinsic QRS, but since AV node is now refractory it delivers a paced beat at the programmed P-R interval (typically ~200-250ms), starting the process anew
  • Tachycardia does not exceed programmed upper limit rate on pacemaker



  • BMP and Mg
  • CXR
  • ECG
  • Troponin
  • Interrogation
    • Each company has on-call representatives who will come interrogate a device 24/7 (phone numbers above)
    • Most cath labs will have machines capable of interrogating each brand

Expected ECG Patterns

  • Absence of pacer artifact indicates intrinsic depolarization
    • With newer pacemakers, pacer spikes may not appear on some or all paced beats, depending on EKG machine
  • Pacing artifacts preceding depolarizations indicate successful pacing and capture
  • Leads in RV apex produce LBBB pattern with appropriate discordance
  • New RBBB pattern may indicate lead in LV
  • Bi-ventricular devices can produce paced QRS complexes that are either narrow or wide and bizarre, depending on device programming
  • Simultaneous depolarization of ventricles produces dominant R wave in V1

Plain Film Findings

  • Obtain PA/Lateral Films to confirm pulse generator, manufacturer, lead placement/number/integrity
  • R atrial lead J shaped(tip medially on AP) entering right atrial appendage
  • RV leads point downward with tip between left spine and cardiac apex--lateral XR shows inferior and anterior
  • Coronary sinus lead- courses posteriorly on lateral XR
  • Extra leads may be appropriately abandoned and capped
  • ICD component appears as thickened shock coil


  • Reported Shock
    • If unable to prove a phantom shock, electrophysiology or cardiology consult is often needed
    • Interrogation is mandatory, either by an experienced clinician or by device representative
      • Some devices can be remotely interrogated; contact the respective company representative at the above phone numbers to discuss this possibility
  • Repeated Shocks
    • Before placement of a magnet or changing the device parameters, you must determine if the shocks are appropriate (i.e. the patient is in VT storm or having recurrent episodes of VF) or inappropriate
      • If it is determined that the shocks are inappropriate, a magnet will deactivate the shocking function while placed over the device
    • Electrophysiology consult
  • Pacemaker Mediated Tachycardia
    • Break with adenosine - magnet will not fix PMT in a patient with a defibrillator


  • Consider discharge if:
    • Proven Phantom Shock
    • Resolved Pacemaker Mediated Tachycardia
    • Single shock only AND Currently asymptomatic AND No concerning associated symptoms (e.g. chest pain, dyspnea, syncope)
      • Ensure patient has Electrophysiology/Cardiology follow-up
      • Consider Electrophysiology/Cardiology consult
  • Admit all others

See Also

External Links


  1. Al-Khatib SM et al. 2017 AHA/ACC/HRS Guideline for Management of Patients With Ventricular Arrhythmias and the Prevention of Sudden Cardiac Death: A Report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines and the Heart Rhythm Society. J Am Coll Cardiol. 2018;72(14):e91. Epub 2018 Aug 16.
  2. Al-Khatib SM et al. 2017 AHA/ACC/HRS Guideline for Management of Patients With Ventricular Arrhythmias and the Prevention of Sudden Cardiac Death: A Report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines and the Heart Rhythm Society. J Am Coll Cardiol. 2018;72(14):e91. Epub 2018 Aug 16.
  3. Sears SF Jr., Shea JB, Conti JB. Cardiology patient page. How to respond to an implantable cardioverter-defibrillator shock. Circulation. 2005;111:e380–e382
  4. Berg SK et al. Phantom shocks in patients with implantable cardioverter defibrillator: results from a randomized rehabilitation trial (COPE-ICD). Europace. 2013 Oct;15(10):1463-7. Epub 2013 Apr 18.