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

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:

References:

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. Kucher N. Deep-vein thrombosis of the upper extremities. NEJM 2011; 364:861. PMID: 21366477 Jacob S et al. Cardiac rhtyhm device identification algorithm using X-Rays: CaRDIA-X. Heart Rhythm 2011; 8(6): 915-22. PMID: 21220049 Ortega DF. Runaway pacemaker: A forgotten phenomenon? Europace 2005; 7(6): 592-7. PMID: 16216762

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