Pacemaker Basics

Have you ever been confused by the alphabet soup of pacemakers? This post will serve as a pacemaker basics reference.

Pacemaker Terminology

• Five letter code system that describes the different pacemaker settings
• First three letters are most useful in ED management

• 

  Sensing Response

  • Triggered (T): Sensed intrinsic depolarization will result in the pacemaker discharging (this setting not used in current generation pacemakers)
  • Inhibited (I): Sensed intrinsic depolarization will result in inhibition of the pacemaker
  • Dual (D): Dual inhibition of both atrial and ventricular pacing in response to intrinsic ventricular depolarization
  • None (O): Does not trigger or inhibit regardless of the native activity
• Example #1: DDD
  • Chamber paced: Atrium and ventricle can be paced
  • Chamber sensed: Intrinsic depolarization of the atrium and ventricle is sensed
  • Sensory response: Can inhibit pacemaker in response to an intrinsic ventricular depolarization or trigger in response to an atrial intrinsic depolarization without a ventricular response
• Example #2: VVI
  • Device has a single lead in the ventricle that senses ventricular activity and can pace the ventricle
  • A ventricular event outside the refractory period will be inhibited

Common Indications for Permanent Pacemaker Placement

• 3rd degree or advanced 2nd degree (Mobitz II) AV block with any of the following
  • Symptomatic bradycardia
  • Ventricular dysrhythmia resulting from block
  • Symptomatic bradycardia secondary to necessary drugs for dysrhythmia management
• Symptomatic bradycardia resulting from 2nd degree AV block
• Chronic bifascicular or trifascicular block with intermittent 3rd degree or 2nd degree AV block
• Exertional 2nd or 3rd degree AV block

Pacemaker Components

• Pulse Generator
  • Houses power source (almost always lithium battery)
  • Battery duration: 4-10 years (depends on features of device)
  • Power decreases gradually with lithium battery; not abrupt cessation of activity
• Electronic Circuitry
• Leads
  • Single lead: endocardial lead positioned in contact with right ventricle
  • Dual lead: endocardial lead positioned in contact with right atrium and ventricle

Sample Normal EKGs

• EKG appearance depends on type of pacemaker in use
• Single Ventricular Lead
  • Narrow “spike” represents pacemaker generated electrical stimulus
  • Followed by LBBB formation QRS complex
  • Note: RBBB formation signifies lead displacement
• Dual Chamber Pacing
  • Two narrow “spikes” seen representing an atrial stimulus (1st spike) and ventricular stimulus (2nd spike)
  • First spike results in atrial depolarization. Second spike results in ventricular depolarization
  • Ventricular response variable
    • Narrow QRS
      • Conducted from intrinsic atrial activity
      • Atrial spike conducted through AV node and generates QRS
    • Wide QRS
      • Generated by ventricular lead stimulus
  • Absence of paced complexes in a patient with a pacemaker does not always indicate pacemaker dysfunction. It may reflect adequate native conduction.

Complications

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  Infectious Complications

  • Rare
    • Pocket infection: 2%
    • Bacteremia: 1%
  • Signs/Symptoms: warmth, swelling, erythema, pain, fever
  • Blood cultures: (+) in 20-25% of patients (commonly S. Aureus, S. epidermidis)
  • Can result in pacemaker pocket erosion
  • Treatment
    • IV antibiotics (i.e.vancomycin)
    • Needle aspiration: should only be performed under fluoroscopy to avoid cutting components of the device
      • Cardiology consultation for removal and replacement
• Thrombophelbitis
  • Common complication (30-50% with some venous obstruction) (Kucher 2011)
  • Symptoms
    • Occur in only 0.5 -3.5% of patients due to collateralization
    • Pain, swelling, venous engorgement of the ipsilateral arm
  • Diagnostic Testing: Duplex sonography
  • Treatment: Anticoagulation
• Pacemaker Syndrome
  • Definition
    • Loss of AV synchrony and loss of atrial “kick”
    • Occurs when there is intact sinus node function but atrium contracts with tricuspid/mitral valves closed (i.e. during ventricular systole)
  • Symptoms: Syncope/Near-syncope, orthostatic dizziness, exercise intolerance, generalized weakness, palpitations
  • Treatment: replacement of single chamber pacemaker to dual chamber pacemaker

Pacemaker Manufacturer

• In order to interrogate a pacemaker, the device manufacturer must be known
• The majority of the time, this information can be determined from the medical record or, the patient will carry a device manufacturer card with all of the information
• Algorithm to determine manufacturer using X-ray (Jacob 2011)

Effect of Magnet on Pacemaker (Core EM Video Link)

• Magnet application over a pacemaker will lead the pacemaker to default to the “magnet mode”
• Typically will lead to asynchronous pacing mode
  • Results in a constant paced rate regardless of the native rate or rhythm
  • Either AOO, VOO or DOO
  • The pacemaker will pace either the atrium, ventricle or both but will not sense or have a response to sensing
• Risk: Asynchronous ventricular pacing can lead to pacemaker-induced ventricular tachycardia

Pacemaker Malfunction

• Problems with Sensing
  • Undersensing
    • Definition: Pacemaker fails to sense native cardiac activity resulting in asynchronous pacing
    • Can be complete or intermittent
    • Commonly results from exit block
      • Exit block: Failure of an adequate stimulus to depolarize the chamber. Endocardium in contact with pacing lead requires higher stimulation threshold for conduction
      • Exit block results from changes in cardiac tissue over time (i.e. an RV Infarction, progressive cardiomyopathy)
    • EKG Findings
      • Pacing spikes occurring earlier than the programmed rate
      • Spike may not be followed by QRS complex if within the refractory period
      • Presence of spikes within QRS complexes also suggestive
  • Oversensing
    • Definition: Pacemaker mistakes electrical signals as native cardiac activity and thus, pacemaker function is inhibited
    • Sources: Large P or T wave, pectoralis muscle contraction, cell phone signal (typically when on ipsilateral ear)
• Problems with Pacing
  • Output Failure
    • Definition: No paced stimulus is generated from the device resulting in decreased or absent pacemaker function
    • Causes: Oversensing, Lead displacement, Fracture of pacing wires
  • Failure to Capture
    • Definition: Electrical stimulus does not result in depolarization of the myocardium (no QRS complex generated)
    • Causes: Exit block, myocardial infarction, lead displacement, wire fracture, electrolyte abnormalities

• Pacemaker Associated Dysrhythmias
  • Sensor-induced tachycardia
    • Pacemakers designed to respond to physiologic stress by increasing heart rate (i.e. during exercise, hypercapnia, tachypnea)
    • Pacemaker reacts to stimuli not intended to increse heart rate (vibrations, electrocautery, muscle contractions) resulting in inappropriate rate (rate will not exceed pacemaker’s upper rate limit)
    • Treatment: Application of a magnet will terminate
  • Pacemaker mediated tachycardia (PMT) (aka endless loop tachycardia)
    • Re-entry Tachycardia: Ventricular depolarization conducts retrograde into the atria leading atrial lead to detect activity as incoming P wave resulting in ventricular depolarization (viscous cycle develops)
    • Treatment
      • Administer AV blocker (adenosine, beta blocker, calcium channel blocker)
      • Apply magnet over pacemaker

• Runaway pacemaker (Ortega 2005)
  • Life-threatening malfunction seen in older-generation pacemakers resulting from low battery voltage
  • Pacemaker delivers runs of pacing spikes in excess of 2000 bp
    • Can provoke ventricular fibrillation
    • Can also cause failure to capture as spikes low amplitude
    • Treatment
      • Apply magnet over pacemaker
      • Replace pacemaker

• Lead displacement dysrhythmia
  • Results from a dislodged pacing wire “floating” around the right ventricle
  • EKG: May demonstrate changing QRS morphology
  • CXR: May aid in diagnosis

• Twiddler’s Syndrome
  • Dysfunction of the pacemaker resulting from patient manipulation of the pulse generator
  • Repeated manipulation results in the pacemaker rotating on it’s axis and dislodgment of the pacing leads
  • Can cause pacing of the diaphragm or brachial plexus (arm twitching)

For More on This Topic Checkout:

• LITFL: Pacemaker Rhythms – Normal Patterns
• LITFL: Pacemaker Malfunction
• Canadiem: Pacemaker Essentials: What We Need to Know in the ED
• Core EM: How to Use a Magnet for Pacemakers + ICDs

References:

1. Riemann JT, Squire B: Implantable Cardiac Devices, in Marx JA, Hockberger RS, Walls RM, et al (eds): Rosen’s Emergency Medicine: Concepts and Clinical Practice, ed 8. St. Louis, Mosby, Inc., 2010, (Ch) 80: p 1064-75.
2. Kucher N. Deep-vein thrombosis of the upper extremities. NEJM 2011; 364:861. PMID: 21366477
3. Jacob S et al. Cardiac rhtyhm device identification algorithm using X-Rays: CaRDIA-X. Heart Rhythm 2011; 8(6): 915-22. PMID: 21220049
4. Ortega DF. Runaway pacemaker: A forgotten phenomenon? Europace 2005; 7(6): 592-7. PMID: 16216762

Post Peer Reviewed By: Salim R. Rezaie (Twitter: @srrezaie)