Tracing

At the beginning of the tracing, there is atrial sensing and ventricular sensing (SafeR mode, operating in ADI mode); acceleration of ventricular rhythm, onset of an AIVR with 1:1 retrograde conduction; long pseudo-PR pattern; after 6 consecutive cycles with a PR value greater than the programmed value, switching to DDD mode; on the first paced cycle, long AV delay (300 ms) favoring retrograde conduction and onset of a PMT; after a series of 8 PV cycles (AV delay 250 ms), suspicion of PMT and confirmation phase; prolongation of the AV delay by 50 ms (300 ms) over one cycle, delaying the subsequent ventricular pacing; analysis of the timing of the ensuing atrial cycle is indicative of a PMT; indeed, the As-As interval is prolonged by about 50 ms, demonstrating that it is a retrograde conduction and that the timing of the atrial occurrence is dependent on that of the paced ventricle; this As-As interval would have otherwise remained unchanged in the presence of atrial tachycardia, the differential diagnosis in this setting; the device applies a post-ventricular atrial refractory period of 500 ms on the next cycle; the retrograde atrial activity falls in this refractory period, is marked Ar and does not generate an AV delay which terminates the tachycardia and confirms the diagnosis of PMT;

Comments

The onset of a PMT involves the programming of an atrial tracking mode (DDD or VDD), the permeability of retrograde conduction, and a momentary loss of atrioventricular synchrony. In this patient, the occurrence of an IRVA with retrograde conduction favors the occurrence of a switch to DDD mode and the onset of a PMT.

The continuation of the PMT results from the sensing of a retrograde P’ wave outside of the refractory periods which causes the triggering of an often prolonged AV delay, which again promotes retrograde conduction after ventricular pacing. A PMT is therefore a repetitive sequence in which the pacemaker responds to each retrograde P’ wave by pacing the ventricle at a high rate which, in turn, generates a retrograde P’ wave.

For pacemakers from other manufacturers, the prevention of PMT occurrence is based on the programming of a PVARP longer than the retrograde conduction time. Conversely, this protection is based on the WARAD in Microport™ pacemakers which maintains a long atrial refractory period. In addition, following the sensing of a PVC, the atrial refractory period is 500 ms (RetroPwatch). This RetroPwatch applies to a maximum of 3 successive PVCs. The atrial events sensed in the RetroPwatch are symbolized by an As marker.

The «Anti-PMT» algorithm is always active in atrial tracking mode (non-deprogrammable for double-chamber devices of this manufacturer). The functioning of this PMT detection and termination algorithm can be broken down into 3 phases:

• initial detection phase over 8 consecutive Vp-As cycles (atrial sensing ventricular pacing outside of the WARAD); the device measures the retrograde conduction time (Vp-As) and suspects a PMT if the Vp-As intervals are stable (within 30 ms) and short (< 453 ms);
• confirmation phase over 2 cycles; the device modulates the AV delay (prolonging or decreasing by 50 ms per the programmed maximum rate) to verify the stability of the Vp-As interval; if stable, the device concludes to a PMT and attempts to terminate the latter; if not, sinus tachycardia or atrial tachycardia and As-Vp synchronization is pursued.
• termination phase; a relative atrial refractory period of 500 ms after ventricular pacing is applied for one cycle such so that the next atrial event is sensed in this refractory period (Ar-marker); this atrial activity is not synchronized to the ventricle for 1 cycle (no triggering of an AV delay); if it is indeed a PMT, the tachycardia is terminated.

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In the AIDA/PM tab, the number of PMT episodes is provided in the statistical data. The counter is saturated after reaching 255 episodes. On the other hand, it is not possible to visualize the EGMs corresponding to the PMT episodes diagnosed by the device.

The PMT detection and termination algorithm cannot be deprogrammed. If the anti-PMT option is set to « TERMIN », the device diagnoses and terminates the PMT. If the anti-PMT option is programmed to « REPROG » the device diagnoses and terminates the PMT. In addition, it automatically shortens the rest and exercise AV delays by 15 ms (limit of 125 ms for the rest AV delay and 80 ms for the exercise AV delay) if more than 10 PMT per day are detected in order to reduce the risk of occurrence of a new PMT (favored by long AV delays).