Endless loop tachycardia

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Endless loop tachycardia is the commoner of the two types of pacemaker mediated tachycardia, the latter being repetitive non-rentrant ventriculoatrial synchrony (RNRVAS). In pacemaker mediated endless loop tachycardia, the pacemaker actively participates in the tachycardia circuit. This occurs only in dual chamber pacemakers. The ventricular depolarisation 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 endless loop tachycardia can also be initiated by an atrial ectopic beat occurring within the post ventricular atrial refractory period (PVARP). Pacemaker mediated endless loop tachycardia 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. 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. Occasionally pacemaker mediated endless loop tachycardia gets converted to RNRVAS and is one of the reasons for magnet unresponsiveness.

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