Termination of pacemaker-mediated tachycardia by a critically timed atrial extrasystole.

Pacemaker-mediated tachycardia (PMT) also called endless loop tachycardia is a macro-reentrant dual chamber pacemaker tachycardia conceptually similar to spontaneous reentrant supraventricular tachycardias (atrioventricular [AV] node reentrant tachycardia and orthodromic reciprocating tachycar-Figure 1. Termination of pacemaker-mediated tachycardia (PMT) by an atrial extrasystole. The tracing shows the simultaneously recorded atrial electrogram on top (A), and ventricular electrogram (V). The calibration for the atrial electrogram was 0.8 mm/mV. The device event markers are shown below the electrograms. In the marker recording, the vertical upward deflections represent atrial events, and the vertical downward deflections represent ventricular events. The beginning of this PMT was not captured in the stored memory. (Figure 2 shows an example of same PMT recorded at a different time). The device detects an atrial extrasystole in the postventricular refractory period (generating an unlabeled marker without symbolic designation). The atrial electrogram confirms the prematurity of the atrial extrasystole and its close resemblance to the configuration of the other atrial electrograms indicates that the signal or deflection is not artifactual. The prevailing sensor indicated interval is 1027 ms. The atrial paced event (AP) in the atrial electrogram are attenuated in this tracing and were mostly attenuated in the other stored recordings. The first atrial sensed event (AS) is associated with an attenuated atrial electrogram which occurred occasionally in the other stored recordings and they were probably caused by changes in body position. In any case, the interpretation of the recordings remains valid even if some kind of device malfunction interfered with the recording of the atrial electrogram. The two horizontal lines of varying thickness depict the duration of the refractory and blanking periods with the atrial intervals above the ventricular ones; VP — ventricular paced event; SIR — sensor-indicated rate. dia). These spontaneous and PMTs share common mechanisms for initiation and termination by means of single critically timed atrial and ventricular ex-trasystoles [1–3]. Although it is well-known that a critically timed ventricular extrasystole can terminate PMT, it is strange that termination by a criti