What does PMT (Pacemaker-Mediated Tachycardia) refer to on a pacemaker?

What is PMT (Pacemaker-Mediated Tachycardia)?

PMT is a reentrant tachycardia caused by the pacemaker itself, where the device senses retrograde atrial activity (from ventricle back to atrium) and then paces the ventricle in response, creating a repetitive loop that drives the heart rate up to the maximum tracking rate. 1, 2, 3

Mechanism of PMT

PMT occurs specifically in dual-chamber pacemakers (DDD, VDD modes) that can track atrial activity and requires two essential components 3, 4:

• Intact ventriculoatrial (VA) conduction: The ability for electrical impulses to conduct backward from ventricle to atrium
• Atrial tracking capability: The pacemaker must be able to sense atrial activity and trigger ventricular pacing in response

The circuit works as follows 2, 3:

1. A ventricular paced or premature beat conducts retrograde to the atrium
2. The pacemaker senses this retrograde P wave (after the post-ventricular atrial refractory period ends)
3. The pacemaker interprets this as normal atrial activity and paces the ventricle
4. This ventricular pacing again conducts retrograde to the atrium
5. The cycle repeats at or near the maximum tracking rate

Common Triggers

PMT is typically initiated by events that cause AV dissociation 3, 4:

• Premature ventricular contractions (PVCs) - most common trigger
• Premature atrial contractions
• Loss of atrial capture or sensing
• External interference or myopotentials sensed by the atrial channel
• Excessively long programmed AV delay
• Magnet removal without appropriate refractory period extension
• Mode switching from non-tracking to tracking modes

Clinical Recognition

Suspect PMT when 1, 5:

• A patient with a dual-chamber pacemaker develops sudden tachycardia at or near the programmed upper tracking rate
• Magnet application temporarily terminates the tachycardia by switching the pacemaker to asynchronous mode (VOO/DOO)
• ECG shows ventricular paced beats with a short RP interval and retrograde P waves (often inverted in inferior leads)

Prevention Strategies

Modern pacemakers incorporate multiple protective algorithms 1, 2, 4:

• Post-ventricular atrial refractory period (PVARP) programming: Extending PVARP prevents sensing of retrograde P waves, though this limits maximum tracking rate 2, 4
• PVARP extension after PVCs: Automatic prolongation for one cycle after premature beats 2, 4
• Automatic PMT termination algorithms: Device detects repetitive VA patterns and extends PVARP to break the circuit 2, 3
• AV delay optimization: Shortening AV delay before extending PVARP preserves exercise capacity 2
• Mode-switching algorithms: Prevent tracking of pathologic atrial rhythms 1

Management When PMT Occurs

Acute termination 5, 4:

• Apply magnet to temporarily convert to asynchronous pacing and break the circuit
• Pacemaker interrogation with intracardiac ECG confirms the diagnosis more accurately than surface ECG
• Reprogram PVARP to extend beyond the VA conduction time
• Shorten AV delay if PVARP extension alone is insufficient

Important Contraindications

Patients with accessory pathways capable of rapid anterograde conduction should NOT receive atrial-tracking pacemakers (DDD, VDD) due to risk of life-threatening tachyarrhythmias 6, 1

Clinical Significance

PMT is largely a solved problem with modern dual-chamber pacemakers that incorporate sophisticated detection and termination algorithms 4. However, it can still occur with newer technologies like AV-synchronous leadless pacemakers, where it may be underrecognized due to lower rates 7. The key is appropriate programming of refractory periods balanced against maintaining adequate upper rate response for exercise capacity 2.