Pacemaker Mediated Tachycardia

When interpreting dual chamber pacemaker Holter monitor reports, brief rapid ‘‘inappropriate” ventricular pacing is frequently seen and difficult to interpret. Most of these are probably short runs of a supraventricular tachycardia, which the ventricular channel follows until the programmed upper limit.

A similar appearance is a pacemaker mediated tachycardia which is an endless loop re-entry tachycardia that occurs in patients with dual chamber pacemakers who have retrograde conduction. The pacemaker acts as the anterograde limb of an accessory pathway and, in order to create the substrate for a pacemaker mediated tachycardia, retrograde conduction must be sensed outside the PVARP (post ventricular atrial refractory period) which triggers ventricular pacing, close to or at the upper rate.

Historically, this was first seen with minimally programmable VDD models, called ASVIP (atrial sensing ventricular inhibited pacemaker). They were predominantly used in young patients with congenital high degree AV block and despite the anterograde block, retrograde conduction could still occur, thereby creating the milieu for an endless loop tachycardia.

There is atrial sensing (As),ventricular pacing (Vp) (red highlight) and sudden onset endless loop tachycardia (yellow highlight).

The Holter monitor rate profile was frightening with upper rate pacing, most of the time. The low rate was 70 bpm (red stippled horizontal line) and upper rate 150 bpm (blue stippled horizontal line).

Because, there was only one mode of pacing (no VVI), correction was very difficult and on occasion, the pulse generator was replaced with a VVI model.

The problem continued with early DDD models, but now there were other programmable options.

Palpitations during recording of ECG. There is atrial sensing (As), ventricular pacing (Vp) (red highlight) and frequent ventricular ectopics (yellow highlight). There is an endless loop tachycardia during the recording of V1 (blue highlight).

Here are some of the precipitating causes of an endless loop tachycardia in the presence of retrograde conduction?

• Atrial and ventricular ectopics
• Intermittent loss of atrial sensing or pacing
• Magnet removal
• Algorithms that extend AV delay
• Short post ventricular atrial refractory period (PVARP)
• Long AV delay

Here are two examples of atrial ectopics initiating an endless loop tachycardia.

There is atrial pacing (Ap) or sensing (As) and ventricular pacing (Vp) (red highlight). An atrial ectopic (yellow highlight) allows retrograde conduction and an endless loop tachycardia follows (blue highlight). Some may argue that the ectopics are actually retrograde conduction, but the timing of the ectopics are later.

In the pre-programmable era, the following were used to treat the tachycardia:

• Magnet
• Drugs to block retrograde conduction – ß blockers, verapamil, adenosine
• His bundle ablation (prevent retrograde conduction)
• Last resort – replace VDD with VVI

Today, reprogramming options are available:

• Correct the cause – atrial sensing or pacing
• DDI, AAI, or VVI
• Prolong PVARP
• Shorten AV delay
• Algorithms to terminate the tachycardia

All companies have programmable algorithms for the successful treatment of endless loop tachycardia. As they are now no longer endless loops, I now prefer the term pacemaker mediated tachycardia.

There are two types of programmable algorithms available:

• Prevention
• Detection and intervention

Prevention involves detection of ventricular ectopics and extension of the ectopic PVARP, to allow retrograde P waves to fall within it.

Detection and intervention. Although the algorithms are all different, once an abrupt tachycardia is detected, it must be determined over a set number of cycles, whether this is pacemaker mediated; Medtronic 8, Abbott 10, and Boston 16 cycles. Once confirmed, the PVARP is extended over one cycle to allow the p wave to fall within the PVARP.

Short pacemaker tachycardia runs can now be visualized on the ECG.

There is atrial (Ap) and ventricular pacing (Vp) (red highlight). Following an atrial exit block (no P wave, yellow highlight), there is retrograde conduction (red vertical arrow) and a pacemaker mediated tachycardia follows. Once detected and confirmed over 12 cycles, the PVARP is extended and the retrograde P wave is now refractory aborting the tachycardia. Because it may take a number of cycles to detect a tachycardia before confirmation commences, I have not been able to determine the manufacturer by the number of tachycardia cycles in the sequence.

Unfortunately, the precipitating cause for the pacemaker mediated tachycardia is not usually seen on the ECG and differentiating it from a supraventricular tachycardia with ventricular conduction is often guesswork.

Harry Mond