Implantable Cardioverter-Defibrillator complication

Background

Nomenclature

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

Parts

• 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
  • Previous episode of unstable VT or VF
    • Not indicated if VT/VF occurs <48 hours after myocardial infarction
  • Sustained VT in setting of cardiomyopathy or channelopathy
• Primary Prevention
  • Myocardial infarction >40 days ago with LVEF <30%
  • Cardiomyopathy + NYHA Functional Class II or III, and LVEF <35%
    • Controversy exists over applying the above criteria to non-ischemic cardiomyopathy
  • Patients with underlying disorders placing them at high risk for unstable VT or VF
    • Congenital long QT syndrome
    • High-risk HOCM
    • Brugada syndrome
    • ARVD
    • Other channelopathies
• Biventricular ICD (i.e. Cardiac Resynchronization Therapy)
  • Any of the above with intraventricular conduction delay of >120ms

Functions

• 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

Shocks (i.e. Defibrillation)

• Approximately 1/3 - 1/2 of ICD patients receive a shock within one year of placement
  • 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

Disposition

• Consider discharge if:
  • Single shock only
  • Currently asymptomatic
  • No concerning associated symptoms (e.g. chest pain, dyspnea, syncope)
• Admit all others

See Also

• Pacemaker complication
• Medical device complications