‍Introduction

Pacemaker mediated tachycardia (PMT), also known as endless-loop tachycardia, generally refers to all tachyarrhythmias in which the pacing device plays a major role. 

Classically PMT usually denotes a form of reentrant tachycardia that is encountered in patients with dual-chamber pacemakers in which:

• pacemaker forms the antegrade limb
• AV node (or an accessory pathway) forms the retrograde limb of the reentrant circuit

Etiology

• The pacemaker has to be in a pacing configuration where it senses the atrium and paces the ventricle (DDD or VAT mode). This is why this form of tachyarrhythmia does not happen in single-chamber pacemakers or with other pacing modes like VVI or AAI

• There needs to be an event that dissociates the P wave from the QRS complex and triggers the tachycardia. In most instances, this is a premature ventricular contraction or premature atrial contractions, but it can also occur when there is an intermittent loss of atrial sensing or loss of atrial capture

• There has to be a retrograde conduction through either the AV node or an accessory pathway

• The ventriculoatrial (VA) conduction needs to be long enough that it is able to be sensed by the atrial lead, meaning that it lasts beyond the atrial refractory period

Figure 1 Mechanism of PMT initiation

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Treatment

• Acute termination of PMT is achieved by applying a magnet to the pacemaker pocket  (switching to asynchronous pacing mode), meaning that the pacemaker will only pace at a set rate and will not track atrial activity. This will terminate the tachycardia by removing the antegrade limb of the reentrant circuit.

• Other temporizing measures include carotid massage or medications that act as AV nodal blockers (digoxin, beta-blockers, and non-dihydropyridine calcium channel blockers), which can prevent the tachycardia by slowing VA conduction and thus blocking the retrograde limb of the circuit

• However, the most definitive management is the reprogramming of the pacemaker to prevent the recurrence of PMT. One easy way of doing so is to lengthen the PVARP  - by increasing the duration of PVARP, there is less likelihood that retrograde conduction will be sensed, and PMT will be triggered

• Another option would be changing the sensitivity of the atrial lead, so that sinus P waves are sensed, but not retrograde P waves.

Modern pacemakers have certain manufacturer-specific algorithms that allow detection and early termination of PMT. Including the use of adaptive PVARP in pacemakers with rate response, automatically prolonging the PVARP after an earlier than normal ventricular activation (PVC) is sensed, or dropping a ventricular paced beat after the pacemaker had been pacing at the maximum tracking rate for a specific period of time.

ECG features

Suspicion on PMT should be raised in all paced tachycardias that run at the upper tracking rate limit of the device (usually 130/min). 

ECG 1 Pacemaker mediated tachycardia 130/min (upper tracking rate limit of the PM) - PVARP was subsequently increased to 275 ms 

References

1. Abu-haniyeh A, Hajouli S. Pacemaker Mediated Tachycardia. [Updated 2020 Jul 26]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK560831/
2. https://www.uptodate.com/contents/unexpected-rhythms-with-normally-functioning-dual-chamber-pacing-systems
3. https://litfl.com/pacemaker-malfunction-ecg-library/
4. https://rebelem.com/pacemaker-basics/pacemaker-mediated-tachycardia-learningecg-blogspot-com/