Tracing

This tracing shows inappropriate switching to DDD mode in the absence of atrioventricular conduction disorder, the AVB II criterion being fulfilled on a non-sustained atrial arrhythmia episode; at the beginning of the tracing, atrial pacing at the minimum rate and intrinsic atrioventricular conduction (classic sick sinus syndrome pattern); initiation of an atrial arrhythmia; during this episode, atrial sensing occurs outside the refractory periods and the cycles are classified as As; after 3 atrial cycles (As) without conducted ventricular events (blocked « P waves »), the AVB II criterion is fulfilled (at least 3 blocked P waves out of 12); switching to DDD mode (vertical line); termination of arrhythmia and atrial and ventricular pacing (probably unnecessary ventricular pacing);

Comments

This tracing illustrate  mode switching to DDD in conjunction with the occurrence of atrial arrhythmia (non-sustained atrial tachycardia) and leading to unnecessary ventricular pacing, despite preserved atrioventricular conduction.

When the pacemaker operates in SafeR mode, after atrial sensing or pacing, a non-programmable relative refractory period is triggered at the atrial level. Its duration is dynamic and is calculated according to the atrial rate. Its purpose is to detect the acceleration of the atrial rhythm (hence its name: WARAD for Window of Atrial Rate Acceleration Detection). When the atrial rate is less than 80 bpm, the WARAD is 62.5% of the previous P-P (or A-A) interval. When the atrial rate is greater than (or equal to) 80 bpm, the WARAD represents 75% of the previous P-P interval. All atrial events sensed in the WARAD are noted by markers in refractory periods (« Ar »). When a premature atrial contraction occurs in the WARAD, the value of the WARAD is then fixed to that of the WARAD calculated on the sinus cycle prior to the very first PAC. It cannot exceed 500 ms.

On the tracing with blocked premature atrial contractions, the initial atrial rate is less than 80 bpm, the prematurity of the extrasystole is poor (the first PAC cycle is longer than 62.5% of the previous AA cycle), which explains that the extrasystoles are labeled P and not p because occurring outside the WARAD. Similarly, on the tracing with the non-sustained atrial arrhythmia, the initial atrial rate is less than 80 bpm and the prematurity of the first arrhythmic beat is also poor (the first PAC cycle is longer than 62.5% of the previous AA cycle), which also explains the marker P and not p because occurring outside the WARAD. The acceleration of the atrial rate is hence progressive, with the WARAD adapting cycle to cycle. No atrial cycle falls in the WARAD and all cycles are classified As. In SafeR mode, the switching to the DDD mode is based on the presence of intrinsic atrial activities outside refractory periods (As) or blocked paced atrial complexes (Ap) or the presence of long PR or AR intervals. The atrial cycles sensed in the WARAD (Ar) are excluded from this analysis since the device suspects an onset of atrial arrhythmia that can physiologically impair the quality of atrioventricular conduction. Given a suspicion of the onset of atrial arrhythmia (succession of cycles classified as Ar), the only valid remaining switching criterion is that of ventricular pause (not the AVB I, AVB II or AVB III criteria). After sensing of a PAC in the WARAD, the Pause criterion is temporarily forced to 2 seconds during the next 12 ventricular cycles.

In patients with frequent blocked premature atrial contractions undetected in the WARAD (at least 3 in 12), the number of mode switches according to the AVB II criterion will be substantial, thereby increasing the percentage of « unnecessary » ventricular pacing.

Example of inappropriate switch to DDD due to PACs

Similarly, in patients with atrial arrhythmia with non-sudden onset (first atrial cycle outside of the WARAD), it is common to observe switches 1) according to AVB 1: indeed, when the atrial rate is relatively high but below Wenckebach’s point, a 1:1  conduction is observed at the occasional cost of a prolongation of the PR interval which can sometimes exceed the duration limit of the AVB I criterion, especially since this value is adaptive according to the heart rate (the faster the rate, the smaller the value). To prevent such inappropriate switching and ensuing unnecessary ventricular pacing, it may be useful to program a relatively significantly long PR value with no variation as a function of rate (PR max = PR min); 2) according to AVB II or AVB III: indeed, when the atrial rate is relatively fast, a certain number of intermittent (AVB II criterion) or successive (AVB III criterion) atrial activities outside of the WARAD can be blocked. Since the WARAD is not programmable or modifiable, there are no programming modifications to propose to reduce the incidence of inappropriate switching, which is usually responsible for a modest increase in the percentage of ventricular pacing.

The interrogation of the pacemaker memory can highlight a number of inappropriate switching to the DDD mode (AVB I, AVB II or AVB III criteria) related to the occurrence of atrial arrhythmia episodes or blocked premature atrial contractions.