Tracing

This tracing shows inappropriate switching to DDD mode in the absence of atrioventricular conduction disorder, the pause criterion being fulfilled following a premature ventricular contraction occurring during the post-atrial ventricular blanking; at the beginning of the tracing, atrial pacing at the minimum rate and intrinsic atrioventricular conduction (classic sinus dysfunction pattern); premature ventricular contraction occurring simultaneously with atrial pacing and undetected because occurring in the post-atrial ventricular blanking; the pause criterion (2 seconds) is fulfilled; switching to DDD mode (vertical line); atrial and ventricular pacing (likely unnecessary ventricular pacing);

Tracing 2

This tracing shows inappropriate switching to DDD mode in the absence of atrioventricular conduction disorder, the pause criterion being fulfilled following a premature ventricular contraction; at the beginning of the tracing, atrial pacing at the minimum rate and intrinsic atrioventricular conduction (classic sinus dysfunction pattern); premature ventricular contraction followed by atrial activity outside of any refractory period and classified as P (no triggered AV delay); ventricular pause of 2 seconds; switching to DDD mode (vertical line) and atrial and ventricular pacing (ventricular pacing likely unnecessary);

Comments

These 2 tracings illustrate episodes of inappropriate switching from AAI mode to DDD mode in conjunction with the occurrence of a premature ventricular contraction.

In the first tracing, the premature ventricular contraction is not sensed since occurring in the post-atrial ventricular blanking. It is typically considered that the pacemaker operates in AAI mode with a ventricular sensing channel functioning independently for diagnosing the occurrence of a paroxysmal atrioventricular conduction disorder. However, the 2 channels (atrial and ventricular) cannot function in completely independent manner. Indeed, it is essential to « protect » the ventricular channel against the crosstalk following atrial pacing, thereby involving the need to preserve, as for a conventional DDD mode, a post-atrial ventricular blanking followed by a safety window. If atrial pacing is blocked, a signal that falls in the ventricular blanking will not be sensed, thereby leading to appropriate switching. On the other hand, if the event is a premature ventricular contraction as in this example, this leads to inappropriate switching.

In the second tracing, the premature ventricular contraction occurs outside of any refractory period. The ensuing compensatory pause leads to inappropriate switching. The duration of the pause is programmable between 2, 3 and 4 seconds. In order to considerably reduce this type of switching, it may be useful to set a pause time of 4 seconds, its incidence being much higher for a duration of 2 seconds. This reduces the number of switches and the percentage of unnecessary ventricular pacing while preventing increasing the risk of symptoms in the presence of an atrioventricular block episode. Indeed, if the minimum rate is programmed at 60 bpm, and atrioventricular conduction is interrupted, the AVB III criterion will be fulfilled well before the occurrence of a 4-second pause.