Tracing

At the beginning of the tracing, atrial pacing (A) and ventricular pacing (V); after 100 paced ventricular cycles, return to ADI mode; BAV III criterion fulfilled (2 consecutive blocked paced atrial complexes); new switching to DDD mode (vertical line);

Comments

In the preceding tracings, we described the 4 criteria allowing a switch from the ADI mode to the DDD mode. The pacemaker then episodically searches for the presence of the return of a normal atrioventricular conduction. A precise knowledge of the search mode for the return of atrioventricular conduction when the pacemaker has switched to DDD mode is essential to understanding the specificities of the different right ventricular pacing reduction algorithms. Indeed, the underlying philosophies differ according to the manufacturers leading to differences in effectiveness and presence of complications. Schematically, 2 options are opposed: 1) AV delay hysterisis (Boston Scientific, St Jude Medical and Biotronik) with a risk of triggering PMT episodes but without the possibility of observing blocked P waves, 2) switching to the ADI mode (Livanova ™, Medtronic) with a reduced risk of PMT induction but possibility of observing a ventricular pause if atrioventricular conduction is permanently impaired.

For Livanova™ pacemakers (SafeR algorithm), when the pacemaker has switched to DDD mode, the search for the return to normal atrioventricular conduction occurs:

• after sensing 12 consecutive ventricular events (operation in DDD mode but 12 consecutive PR or AR cycles);
• automatically after every 100 paced ventricular cycles;

These 2 parameters are not programmable. In order to search for a return to atrioventricular conduction, the pacemaker switches back to ADI mode. The 4 criteria of the algorithm described above are then applied to possibly switch back to the DDD mode. During this search, a ventricular pause and blocked P waves can therefore be observed as in the current example. In a patient with an atrioventricular block that has become permanent, it is therefore preferable to avoid the repetition of these search sequences in the presence of ventricular pauses which can be symptomatic. There are therefore a number of “durable” switching criteria in DDD mode when atrioventricular conduction is permanently impaired. The pacemaker switches to DDD mode overnight until 8 a.m. the next morning when the pacemaker has recorded:

• more than 45 episodes of ADI to DDD mode switches during the last 24 hours
• more than 15 episodes of ADI to DDD mode switches per 24 hours for 3 consecutive days
• more than 50% of the time spent in DDD mode during one hour

When the pacemaker has switched to DDD mode, the device searches for the return of normal atrioventricular conduction by switching back to ADI mode. The 4 criteria of the SafeR algorithm are then applied to possibly revert to DDD mode with a risk of episodic occurrence of ventricular pauses.