Pacemaker-mediated tachycardia

Pacemaker Essentials: What we need to know in the ED

In Medical Concepts by Lorne Costello11 Comments

This post was the winning entry into the 2016 CanadiEM/SGEM Essentials of EM Fellow competition. On behalf of the CanadiEM team, I’d like to thank him for his excellent contribution and wish him the best of luck with the Fellowship. -Brent Thoma, CanadiEM Editor

Who knew a pacemaker could cause so much grief (to the physician, not the patient)? Join me on my journey to understand this troublesome little box and what pacemaker essentials emergency physicians ought to know.

The Case

It’s Sunday and you are working in a community ED. A 61 year old female presents with palpitations on and off for 1 day lasting minutes each time. She had a permanent pacemaker placed in 2002 for “brady-something” which was replaced 3 days ago. There is no other past medical history. All vitals are stable until you hear, “Doc, I feel it again.” On the monitor you see:

Paced tachycardia

Figure 1. Wide regular complex tachycardia following pacer spikes at ~130 bpm

The Quick & Dirty: Pacemaker Essentials 

Before getting into the case, we need to discuss some pacemaker essentials. Permanent pacemakers are named using the NBG code [1,2]. It has 5 positions (I to V) and tells you a pacemaker’s basic function. The last 2 positions are IV rate modulation or programmability (e.g. a spring or sensor that allows your HR to increase with exercise) and Vmultisite pacing (e.g. 2 places in the same chamber that are being paced). These 2 positions are not really relevant to the emergency physician so let’s focus on the first 3:

Pacemaker Essentials

Figure 2. The first 3 letters of the NBG Pacemaker Code

I – Where is the lead located and what it will pace?

  • O – There is no pacing
  • D – Means ‘dual’ so both the atria (A) and ventricle (V) can be paced

II – Which chamber is being sensed?

  • O – No chambers are being sensed
  • D – Both A and V are being sensed

III – What does the pacemaker do when it sees the native rhythm?

  • O – Does not trigger or inhibit pacing spikes regardless of native rhythm
  • I – Sensed activity results in inhibition of a pacing spike. So let’s say there is a native beat then the PPM won’t pace on top of it. We don’t want to cause Torsades de Pointes with a misplaced R on T!
  • T – Sensed activity results in triggering a pacing spike
  • D – Can inhibit or trigger pacing spikes depending on native activity

Some common examples

Our first example, a VVI pacemaker, is commonly seen in elderly patients and those with atrial fibrillation. It has a single lead in the ventricle that can be paced and sensed.VVI Pacemaker

Our second example is the wonderkid of pacemakers. It can pace and sense both the atria and ventricle. So if you have a normal P and QRS it just sits back and relaxes. If your P is normal but your QRS is lazy it will kick in to pace the ventricle. If your P is slow but always followed by a normal QRS it will pace the atria but not the ventricle. Finally, if both the atria and ventricle are slow it will pace them both consecutively.

DDD Pacemaker

Back to the Case

There are several common causes for a rapid pacemaker. It can be a normal response to a patient’s intrinsic tachycardia, an atrial arrhythmia (e.g. atrial flutter), a pacemaker-mediated tachycardia, or a sensor-induced tachycardia (e.g. when the sensor indicated by position IV of the pacemaker code misfires due to other stimulation) [1,2].

This case turned out to be a pacemaker-mediated or “endless loop tachycardia” where atrial to ventricular depolarization occurs normally, then the ventricular depolarization conducts retrograde into the atria, and the atrial lead detects this activity as an incoming P wave resulting in a vicious cycle.

Pacemaker-mediated tachycardia

Figure 3: Pacemaker Mediated Tachycardia (PMT) mechanism – Orange arrows follow a normal permanent pacemaker (PPM) paced spike. The purple arrows follow the retrograde conduction in a PMT circuit that goes on and on.

One pearl in the diagnosis of Pacemaker-Mediated Tachycardia is that it can only run as fast as the pacemaker’s maximal rate!

Treatment of Pacemaker-Induced Tachycardia

Occasionally pacemaker-induced tachycardia can self-resolve. Additionally, some permanent pacemaker algorithms can sense and terminate this rhythm. However, if that does not occur we can keep it pretty basic: the cause is a circuit that needs to be disrupted. Options for this include:

Block the AV node, block the circuit

  • Vagal maneuvers (e.g. modified valsalva [3])
  • Adenosine (other AV nodal blocking agents such as beta blockers and calcium channel blockers may also work)

Use a Magnet  

  • Applying a magnet to the pacemaker converts it into asynchronous mode with no sensing (e.g. DOO = pacing both A and V). This is more complicated if the patient’s pacemaker is combined with an implantable defibrillator (ICD).

Electrophysiology/Cardiology can be consulted to interrogate the pacemaker if the diagnosis is in doubt or it does not resolve with standard treatment.

Case Resolution

The patient’s palpitations stop abruptly. Cardiology is called to interrogate the pacemaker and confirm that the patient’s tachycardia was, in fact, pacemaker-mediated tachycardia and the PMT function is turned on. This function is able to break PMT by prolonging the refractory interval between V to A; thereby breaking the circuit. However, it isn’t always turned on in newly inserted pacemakers. The patient was subsequently discharged to follow up with her regular cardiologist.

Thanks for reading! If you found this useful, then stay tuned for follow-up posts investigating “How to interpret Pacemaker ECGs” and “Okay enough on Pacemakers, let’s talk ICDs and CRT.” Special thanks to Dr. Sheldon Singh, cardiologist and electrophysiologist at Sunnybrook Health Sciences Centre, for reviewing this piece for accuracy!


  1. Martindale J, deSouza IS. Managing pacemaker-related complications and malfunctions in the emergency department. Emerg Med Pract. 2014; 16(9): 1-21; quiz 21-2. PMID: 25436255
  2. Marx, J. (2013). Rosen’s Emergency Medicine: Concepts and Clinical Practice 8th edition (pp. 1064-1074). Philadelphia, PA: Mosby/Elsevier.
  3. Appelboam A, Reuben A, Mann C, et al. Postural modification to the standard Valsalva manoeuvre for emergency treatment of supraventricular tachycardias (REVERT): a randomised controlled trial. Lancet. 2015; 386(10005): 1747-53. PMID: 26314489

Lorne Costello

Dr. Lorne Costello is an emergency physician at Sunnybrook Health Sciences Centre and Michael Garron Hospital and an assistant professor at the University of Toronto. He is interested in pacemakers (surprise), medical education and all things simulation.