Endless loop tachycardia

Endless loop tachycardia

Endless loop tachycardia (ELT) is the commoner of the two types of pacemaker mediated tachycardias, the latter being repetitive non-reentrant ventriculoatrial synchrony (RNRVAS) [1]. Endless loop tachycardia is repetitive reentrant ventriculoatrial synchrony.

In RNRVAS, pacemaker does not sense the retrograde P waves. In this form of ventriculoatrial synchrony, the atrial stimulus is ineffectual because it falls within the atrial myocardial refractory period generated by the preceding unsensed retrograde P wave. This leads to a long atrioventricular interval and when there is a relatively fast lower rate or sensor driven rate in DDDR pacing, leads to atrioventricular desynchronization arrhythmia with unfavourable hemodynamics [2].

Here is an ECG with paced rhythm and retrograde P waves:

It can be seen that VA interval is short and AV interval is long which gives rise to unfavourable hemodynamics. In fact there will be regular cannon waves in this case of VVI pacing.

In pacemaker mediated ELT, the pacemaker actively participates in the tachycardia circuit. This occurs only in dual chamber pacemakers. The ventricular depolarization is conducted retrogradely and is sensed by the pacemaker circuitry. This triggers a ventricular pacing stimulus at the end of the programmed AV delay. The ensuing ventricular beat is again conducted back into the atrium and the vicious cycle continues. Final result is a tachycardia at the upper rate interval of the pacemaker. The tachycardia is usually triggered by a ventricular ectopic beat which fortuitously has an appropriate timing for initiating the vicious cycle. Sometimes ELT can also be initiated by an atrial ectopic beat occurring within the post ventricular atrial refractory period (PVARP). Pacemaker mediated ELT is usually terminated by magnet application and it can be prevented by programming the PVARP appropriately.

Though usually, the tachycardia is terminated by application of a magnet one unusual case in which it was induced by magnet application and persisted despite repeated removal and reapplication of magnet has also been reported [3]. If a pacemaker programmer device is not available, pacemaker mediated endless loop tachycardia can also be terminated by chest wall stimulation to inhibit the ventricular channel of the dual chamber pacemaker.

A study evaluated chest wall stimulation by an external pulse generator used for transcutaneous pacing, provocation of myopotential oversensing and chest thumping for termination of endless loop tachycardia [4]. Chest wall stimulation was applied in 10 patients with unipolar and 10 patients with bipolar DDD devices during endless loop tachycardia and was successful in terminating all cases. Myopotential oversensing could terminate endless loop tachycardia in 6 out of 10 cases and chest thumping terminated 4 of 6 cases.

Occasionally pacemaker mediated endless loop tachycardia gets converted to RNRVAS and is one of the reasons for magnet unresponsiveness [5].

References

1. Francis J. Repetitive Nonreentrant Ventriculoatrial Synchrony (RNRVAS). Indian Pacing Electrophysiol J. 2010 May 5;10(5):203-4.
2. Barold SS. Repetitive reentrant and non-reentrant ventriculoatrial synchrony in dual chamber pacing. Clin Cardiol. 1991 Sep;14(9):754-63.
3. Barold SS, Falkoff MD, Ong LS, Heinle RA. Paradoxical induction of endless loop tachycardia by magnet application over a DDD pacemaker. Pacing Clin Electrophysiol. 1986 Jul;9(4):503-10.
4. Barold SS, Falkoff MD, Ong LS, Heinle RA. Pacemaker endless loop tachycardia: termination by simple techniques other than magnet application. Am J Med. 1988 Dec;85(6):817-22.
5. Barold SS, Falkoff MD, Ong LS, Heinle RA. Magnet unresponsive pacemaker endless loop tachycardia. Am Heart J. 1988 Sep;116(3):726-32.