pacemaker-mediated tachycardia

Tracing
N° 29
Manufacturer Abbott Device PM Field AV delays, refractory periods management of tachycardia
Patient

68-year-old man, implanted with an AssurityTM + DR dual-chamber pacemaker for paroxysmal syncopal complete atrioventricular block; during the interrogation, highlighting of a PMT episode stored in the device memory.

Graph and trace

Initially, the VIP algorithm allows intrinsic atrioventricular conduction (AP-VS cycles with AP-VS interval time greater than the programmed AV delay); occurrence of a doublet of premature ventricular contractions with retrograde conduction; on the first cycle, the atrial signal falls within the PVARP (AR); on the second cycle, the retrograde atrial activity is classified as AS and thus triggers a prolonged delay (VIP) which favors the occurrence of a new retrograde conduction; onset of a PMT; over 3 cycles the AV delay is very long (VIP programmed for 3 cycles); maintenance of the PMT followed by return to the programmed AV delay (slightly prolonged so as not to exceed the maximum synchronous rate which is programmed at 130 beats/minute); after 8 consecutive VP-AS cycles greater than 110 beats/min without significant change in the VP-AS interval, suspicion of PMT; prolongation of the AV delay by 50 ms for one cycle (from 170 to 220 ms); on the next cycle, the VP-AS interval remains stable (50 ms prolongation of the AS-AS interval): diagnosis of PMT by the device; the following AS cycle does not trigger an AV delay; atrial pacing 330 ms after this non-followed AS, then ventricular pacing; termination of the PMT.

Comments

This tracing shows a PMT episode diagnosed, processed and stored in memory by the device. The tachycardia begins in a patient presenting retrograde conduction, premature ventricular contractions which dissociate atrial and ventricular activation, and in whom the VIP has been programmed with a temporary extension of the AV delay. The onset of a pacemaker-mediated tachycardia (PMT) involves the programming of an atrial tracking mode (DDD or VDD), the permeability of retrograde conduction, and a momentary or permanent loss of atrioventricular synchrony. Indeed, while ventricular activity is properly synchronized with that of the atrium, retrograde conduction is blocked. Maintenance 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 favors 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.

The cycle hence repeats itself indefinitely unless there is appearance of a retrograde block or the intervention of a specific algorithm. A prolonged PMT may be poorly tolerated with symptoms ranging from simple feeling of discomfort/uneasiness or palpitations up to cardiac decompensation in patients with an underlying heart disease. The rate of a PMT is contingent on the retrograde conduction time, the programmed maximum rate, and the current AV delay.

This tracing shows the specific features of AbbottTM devices in PMT management:
1) three settings are possible for the diagnosis and termination of PMTs:
- Off, no PMT detected;
- Passive: the PMTs are detected and counted in the diagnostics but the termination algorithm is not used;
- Atrial Pacing: the PMTs are detected and the specific termination algorithm is used;

2) it is possible to program a PMT detection rate which determines the low rate limit from which a tachycardia may correspond to a PMT; this parameter can be set between 90 and 180 beats/minute and cannot exceed the value of the maximum synchronous rate;
3) the device suspects the presence of a PMT after 8 consecutive VP-AS cycles with AS-AS intervals higher than the PMT detection rate and with stable VP-AS intervals (standard deviation ± 16 ms);

4) the device confirms the relationship between ventricular pacing and atrial sensing (ventricular pacing leading to retrograde atrial conduction), by modifying the AV delay by 50 ms on the ninth cycle (prolonging or shortening of the AV delay); if the next VP-AS interval (tenth cycle) is unchanged from the previous VP-AS interval, the device confirms the diagnosis of PMT; indeed the latter reflects the fact that activation of the atrium is dependent on ventricular pacing; if the VP-AS interval is modified (more than 16 ms difference), the diagnosis of PMT is reversed, no termination attempt is made and the search for PMT resumes only after 256 cycles so as not to multiply the interventions due to an episode of sinus tachycardia; it should be noted that on older platforms, the device did not compare the duration of the VP-AS interval of the tenth cycle with that of the ninth cycle but with an average of the VP-AS intervals of the first 8 cycles;

5) if the VP-AS interval is not impacted by the change in AV delay, the device concludes to PMT, atrial activity does not trigger an AV delay, ventricular pacing is inhibited, and atrial pacing is delivered 330 ms after this AS cycle with resumption of normal atrioventricular synchrony; this atrial pulse is inhibited if an atrial activity (AS) is sensed during a sensing period of 210 ms after the absolute atrial refractory period;

The various programmable parameters are therefore:
PMT Response: Off/Passive/Atrial Pace
PMT Detection Rate: 90, ..., 130, ..., 180 beats/minute

Message to remember

The following events can favor AV dissociation, retrograde conduction, and the onset of a PMT: premature ventricular contraction (the most common cause), premature atrial contraction with prolonged AV delay to maintain the programmed maximum heart rate,  AV delay programmed too long, external or myopotential interference sensed by the atrial chain, lack of atrial pacing, no extension of PVARP after removal of a magnet or at the end of measurement of the atrial threshold, application and removal of a magnet, programming of the VDD mode in a patient with sinus rhythm slower than the programmed minimum rate.

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