Operation of the Vp suppression algorithm

The AAI-DDD (Vp suppression) mode has been designed to allow pacing in ADI mode with permanent monitoring of atrioventricular conduction and switching to DDD mode when the pacemaker detects an alteration in conduction. There are 4 switching criteria corresponding to the different degrees of atrioventricular block (first-degree AV block, second-degree AV block, third-degree AV block and ventricular pause).

1. first-degree AV block criterion: the pacemaker switches to DDD mode when 2 consecutive atrial events (spontaneous or paced) are conducted with an AS-VS or AP-VS delay exceeding 450 ms (identical value whether the atrium is sensed or paced). This value cannot be changed. In this first tracing, the AP-VS delay is initially relatively prolonged (to the order of 350 ms) but remains below the threshold value, the pacemaker therefore continues to operate in ADIR mode. It is not possible to program this parameter solely in response to exertion. On the other hand, it should be noted that the threshold value required for switching decreases during exertion and during an increase in heart rate.
2. second-degree AV block criterion: the pacemaker switches to DDD mode when 3 (nominal) out of 8 atrial (spontaneous or paced) events are blocked (not followed by a spontaneous QRS). This latter value is modifiable: the more the value is decreased, the more frequent the switches; if this value is programmed to 5, this implies that there are at least 2 consecutive blocked atrial activities and the third-degree AV block is also fulfilled. A single blocked atrial activity as that observed in the first tracing therefore does not induce switching when the programming is set to nominal.
3. third-degree AV block criterion: the pacemaker switches to DDD mode when 2 consecutive spontaneous or paced atrial activities are blocked (not followed by a spontaneous QRS). This parameter cannot be changed.
4. pause criterion: the pacemaker switches to DDD mode when the interval between 2 sensed ventricular activations exceeds 2 seconds. This parameter is also non-modifiable (the duration of the pause cannot be prolonged).

No EGM is automatically recorded as a result of switching from ADI mode to DDD mode, thereby limiting the possibilities of assessing the quality of the algorithm.