Pacemaker Summary

Device Types

  • Conventional Pacemakers: Brady-arrhythmias
  • ICDs: Tachy-arrhythmias
  • CRT: Heart failure (biventricular pacing)

Pacemaker Nomenclature

  • 1st Letter: Chamber paced (A, V, D)
  • 2nd Letter: Chamber sensed (A, V, D, O)
  • 3rd Letter: Response mode (I, T, D)

Key Terms

  • Pacing: Delivery of electrical stimulus
  • Capture: Successful myocardial depolarization
  • Threshold: Minimum voltage for reliable capture
  • LRL/URL: Lower/Upper Rate Limits

Common Failure Modes

  • Device failure: Sensing/pacing issues
  • Lead failure: Displacement/fracture
  • Pocket problems: Infection/erosion

ICD Functions

  • Anti-bradycardia pacing
  • Anti-tachycardia pacing (ATP)
  • Defibrillation shocks (26-36J)

ED Evaluation Approach

  • Identify device type and settings
  • Assess intrinsic rhythm
  • Check pacing/sensing on ECG
  • Look for mechanical complications
  • Check for local complications

CRT Considerations

  • 60-70% response rate in selected patients
  • LV lead via coronary sinus branch
  • Vulnerable to displacement
  • Complex interpretation - seek expert advice

Pacemaker Function and Failure

Implantable devices in cardiology covering diagnostic and therapeutic modalities

Device Therapy Overview

Conventional Pacemaker Function

Device Anatomy

Various forms of conventional pacing are available, and the choice depends predominantly upon the electro-anatomical site and nature of the conduction problem.

These devices are delivered transvenously, so pacing activity originates from the right heart. Right atrial leads are usually positioned with the tip in the RA appendage. Traditionally, the RV lead is positioned at the RV apex.

Pacemaker Nomenclature

Term Definition
Pacing Delivery of a programmed electrical stimulus from the pacemaker to the myocardium via the implanted lead
Capture Successful depolarisation of the myocardium from a pacemaker stimulus
Threshold The lowest output voltage from the device that will reliably capture
Oversensing/Undersensing Device sensitivity to electrical activity (cardiac vs. non-cardiac sources)
Lower Rate Limit (LRL) The slowest rate the device will allow the heart to drop to
Upper Rate Limit (URL) The fastest heart rate that the device will pace at
Battery End Of Life (EoL) Depletion indicator for battery replacement planning
Hysteresis Separation of lower pacing rate from lower sensing rate to preserve intrinsic activity

Pacemaker Classification (NBG Codes)

1st Letter: Chamber Paced

  • A (atrium)
  • V (ventricle)
  • D (dual)

2nd Letter: Chamber Sensed

  • A (atrium)
  • V (ventricle)
  • D (dual)
  • O (neither)

3rd Letter: Response Mode

  • I (inhibited)
  • T (triggered)
  • D (dual)

Common Pacing Modes

  • VVI: Single lead to RV provides both sensing and pacing
  • DDD: Leads in both RA and RV that can both sense and pace

Pacing Leads

Passive Fixation

  • Uses small tines to hook into trabeculations
  • Provides resistance to displacement

Active Fixation

  • Uses corkscrew mechanism into myocardium
  • Less likely to displace
  • Common in ventricular leads of ICDs

Conventional Pacemaker Failure

Pacemakers are amongst the most reliable medical devices, but problems do occur.

Device Failure

  • Failure to sense and/or pace appropriately
  • May require programming adjustments or device replacement

Lead Failure

  • Immediate post-implant displacement
  • Subtle parameter changes or obvious position shifts
  • Late displacement rare without trauma/surgery

Pocket Problems

  • Infection (incision, pocket, device/leads)
  • Device discomfort or erosion
  • Often requires revision/extraction

ED Evaluation Approach

  1. What device is implanted, and when?
  2. How is the device set up (mode, rate etc.)?
  3. What is the patient's intrinsic rhythm and rate?
  4. Evidence of appropriate pacing on ECG?
  5. Evidence of appropriate sensing on ECG?
  6. If no to 4/5, evidence of inappropriate pacing/sensing?
  7. Evidence of mechanical complication (lead displacement/fracture)?
  8. Evidence of local complication (infection/erosion)?

Useful information can be gathered from history, clinical examination, surface ECG, chest x-ray and blood tests.

Implantable Cardioverter Defibrillator (ICD) Function

Components

Functions

Function Description
Anti-bradycardia pacing Conventional pacing function
Anti-tachycardia pacing (ATP) Pacing bursts to terminate VT (>95% success rate)
Defibrillation shocks High-energy shocks (26-36J) for persistent VT/VF

ICD Failure

Common Presentations

Delivered Shock Therapy While Conscious

  • Evaluate for appropriate vs. inappropriate therapy
  • Assess for ICD storm (repeated defibrillation)
  • Check electrolytes (K+, Mg++)
  • Evaluate for acute ischemia
  • Assess for lead fracture

Syncope

  • May not be cardiac-related
  • Consider sustained bradycardia/VT/VF
  • Device interrogation required
  • Usually requires admission

Associated Cardiac Symptoms

  • Chest pain, palpitations, dyspnea
  • Consider pneumothorax (post-implant)
  • Consider device infection/endocarditis

Special Considerations

  • MRI: Contraindicated without cardiology consultation
  • Surgery: Risk of diathermy interference with ICD
  • End-of-life care: Should disable tachyarrhythmia therapy

Cardiac Resynchronization Therapy (CRT)

Response rate to CRT is 60-70% in properly selected patients, with another 10% improving after parameter optimization.

CRT Troubleshooting

CRT evaluation requires CXR and ECG, but interpretation is complex - seek expert advice.