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

  • Biventricular Pacing (

    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.



  • Pacemaker Pocket Infection (

    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)
CaRDIA-X Algorithm

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
Failure to Capture (LITFL)
  • 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
Pacemaker Mediated Tachycardia (


Pacemaker Mediated Tachycardia (LITFL)
  • 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
Runaway Pacemaker (LITFL)
  • 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)
Twiddler’s Syndrome (NEJM)

For More on This Topic Checkout:


  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)

Cite this article as: Anand Swaminathan, "Pacemaker Basics", REBEL EM blog, August 24, 2017. Available at:
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Anand Swaminathan

Clinical Assistant Professor of Emergency Medicine at St. Joe's Regional Medical Center (Paterson, NJ)
REBEL EM Associate Editor and Author

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10 thoughts on “Pacemaker Basics”

  1. Example #2: VVI
    Device has a single lead in the ventricle that senses ventricular activity and can pace the ventricle
    An atrial event outside the refractory period will be inhibited.
    I do not understand that last because it is a VVI pacemaker

  2. We can help with pacemaker / ICD identification if you have an x-ray. Just upload a screenshot to our site or mobile phone app and you can instantaneously find out which device it is!


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