Pacemaker Mediated Tachycardia

Pacemaker mediated tachycardia (PMT, endless loop tachycardia) is a re-entrant tachycardia in a dual chamber device that tracks atrial activity. The device forms the antegrade limb and native retrograde ventriculoatrial conduction (through the atrioventricular node or an accessory connection) forms the retrograde limb.

A premature ventricular beat is the usual trigger. Others are loss of atrioventricular synchrony, atrial undersensing or oversensing, a change in programming, or removal of a magnet. Retrograde conduction produces an atrial deflection that falls outside the post-ventricular atrial refractory period (PVARP).

The device senses it as a P wave and paces the ventricle after the programmed atrioventricular delay, and that paced beat conducts retrogradely again. The rate settles at or near the upper tracking rate (usually 120 to 150/min).

PMT is uncommon with modern devices that have termination algorithms enabled, and needs intact retrograde conduction, which is present in a substantial minority of patients with sick sinus syndrome or atrioventricular block.

Pathophysiology. The loop needs three elements: retrograde conduction that arrives after the PVARP ends, an atrial sensing channel, and tracking of sensed atrial events. A longer PVARP, a PVC response that extends PVARP, or termination algorithms interrupt the loop.

The tachycardia can be mistaken for tracking of an atrial tachyarrhythmia, but the VA interval in PMT is fixed and the rate is regular.

• Sudden or recurrent palpitations, dyspnoea, dizziness, chest pain, presyncope or syncope in a patient with a dual chamber pacemaker or defibrillator.
• Hypotension, heart failure or angina when sustained.
• Pulsations in the neck from retrograde atrial contraction against closed valves.
• Examination: regular tachycardia at 110 to 150/min. A history of a premature beat or a change in programming or device interrogation just before onset.

Red flags.

• Syncope, hypotension or ischaemic chest pain
• Incessant or recurrent PMT
• Delivery of inappropriate defibrillator therapy
• Recent lead revision, generator change or reprogramming
• Heart failure, or a patient who is pacemaker dependent

• 12-lead ECG. Ventricular paced beats at a regular rate near the upper tracking rate, with a retrograde P wave after each QRS, usually in the ST segment or T wave, at a constant VA interval.
• Device interrogation. Marker channels show an atrial sensed event after each paced beat. Stored electrograms show the onset (often a premature ventricular beat). Measure VA conduction time, PVARP, upper tracking rate, atrioventricular delay, PMT algorithm status, atrial sensitivity, lead impedance and thresholds, and battery status.
• Magnet test (pacemaker only). Placing a magnet over a pacemaker converts it to asynchronous pacing and terminates PMT. A magnet over a defibrillator suspends tachycardia therapy without changing pacing, so it is not a test for PMT in a defibrillator.
• Adenosine response. Termination by blocking retrograde atrioventricular node conduction supports the diagnosis.
• Chest X-ray for lead position, dislodgement or fracture. Laboratory: electrolytes, thyroid function, troponin when ischaemia is suspected.

Differential diagnosis.

• Sinus tachycardia with atrial tracking (P wave before the QRS, gradual rate change)
• Tracking of atrial fibrillation, flutter or atrial tachycardia (irregular rate, upper rate behaviour with Wenckebach pattern)
• Sensor driven tachycardia from the rate adaptive sensor
• Tracking of myopotentials or far field R wave oversensing on the atrial channel
• Crosstalk
• Runaway pacemaker (older devices)
• Atrioventricular reentrant or nodal reentrant tachycardia in a patient with a pacemaker
• Ventricular tachycardia

Acute termination.

• Apply a magnet over a pacemaker. Remove it after interrogation and reprogramming, because the loop restarts when tracking resumes.
• Adenosine 6 mg rapid intravenous bolus with a 20 mL saline flush, then 12 mg after 1 to 2 minutes if needed, then 12 mg once more, maximum total 30 mg. Use 3 mg through a central line, with dipyridamole or carbamazepine, and in heart transplant recipients. Contraindicated in asthma. Warn of chest tightness lasting under a minute.
• For haemodynamic instability, switch the device to asynchronous or non-tracking mode by programmer, or perform synchronised cardioversion at 70 to 120 J biphasic and then reprogramme.

Reprogramming to prevent recurrence.

• Lengthen PVARP to exceed the measured VA conduction time (usually 300 to 400 ms).
• Enable the PMT termination algorithm and the PVC response, which extend PVARP after a ventricular event not preceded by an atrial event.
• Reduce atrial sensitivity if oversensing is the trigger, check atrial lead position, and replace or reposition a faulty lead.
• Lower the upper tracking rate when PMT is poorly tolerated. Use a non-tracking mode (DDI or VVI) as a temporary measure.
• Treat frequent premature ventricular beats: correct potassium to 4.0 to 5.0 mmol/L and magnesium to the normal range, and consider bisoprolol 2.5 mg orally once daily, increased to 5 mg once daily, maximum 10 mg daily at creatinine clearance under 20 mL/min or in severe liver impairment, long term, with heart rate and blood pressure monitoring. Contraindicated in severe asthma and sick sinus syndrome without pacing.

Definitive. Catheter ablation of an accessory connection when it is the retrograde limb. Device upgrade or lead revision when oversensing or crosstalk is structural.

Defibrillators. Check that PMT does not reach the tachycardia detection zone. Adjust detection and discrimination so PMT is not treated with shocks.

Monitoring. Interrogate at 2 to 4 weeks after reprogramming, then at routine device checks every 3 to 12 months with remote monitoring. Review the episode log for PMT and PVC burden. Ambulatory ECG with symptoms.

Referral and escalation. Refer to the device clinic or electrophysiology for any PMT. Urgent for syncope, ischaemia, hypotension, inappropriate shocks or recurrent episodes after reprogramming.