Window of Atrial Rate Acceleration Detection (WARAD)
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At the beginning of the tracing, sinus rhythm with intrinsic conduction; initiation of atrial arrhythmia by a premature atrial contraction sensed in the WARAD (Ar); the value of the WARAD for the remainder of the episode is fixed on this first cycle; certain atrial cycles fall in the WARAD and are classified as Ar, others are outside this refractory period and are classified as As;
Unlike other manufacturers, there is no PVARP after ventricular sensing or pacing. The WARAD is a very specific atrial refractory period triggered by atrial sensing outside of refractory periods or by atrial pacing.
In DDD mode, following atrial sensing, a refractory period of 80 ms is triggered at the atrial stage. The first 30 ms are an absolute refractory period while the next 50 ms are automatically retriggerable upon atrial sensing. The value of this period is not programmable. Following atrial pacing, the periods corresponding to the AV delay are absolute refractory periods at the atrial stage.
When the SafeR dual-chamber pacing mode is programmed and the pacemaker is operating in ADI mode, a 150 ms refractory period is triggered at the atrial stage after atrial pacing. The first 100 ms are an absolute refractory period while the next 50 ms are automatically retriggerable. The value of this period is not programmable.
Following ventricular sensing or pacing, there is an atrial blanking period to avoid crosstalk. On the other hand, as explained above, the great peculiarity of the pacemakers of this manufacturer is that there is no PVARP (refractory period triggered by post-ventricular (sensed or paced) atrial blanking to avoid the occurrence of a pacemaker-mediated tachycardia).
Following atrial sensing or pacing, a relative refractory period is triggered at the atrial stage. Its duration is dynamic and is calculated according to atrial rhythm. It is effective only after the end of the atrial refractory period (80 ms, 150 ms or at the end of the AV delay + ext). Its purpose is to detect the acceleration of the atrial rhythm (hence its name: WARAD). When the sinus rhythm is less than 80 min-1, the duration of the WARAD is 62.5% of the previous As-As interval. When the sinus rhythm is greater than (or equal to) 80 min-1, the duration of the WARAD is 75% of the previous As-As interval. At the onset of the fast atrial rhythm, a short AV delay may be triggered on certain sensed atrial events during the WARAD if and only if the applied Vs-Vp or Vp-Vp interval is greater than 500 ms. This allows preventing of unnecessarily rapid ventricular pacing at the beginning of an atrial arrhythmia episode before switching to asynchronous mode. During this suspicion phase of atrial rhythm disorder, all sensed atrial events are annotated by markers in the refractory periods (Ar). The value of the WARAD is then fixed to that of the WARAD having sensed the very first PAC and cannot exceed 500 ms. The short AV delay that can be triggered after sensing of certain atrial events in the WARAD is 30 ms (not programmable). In addition to reducing or eliminating the risk of pacemaker-mediated tachycardia, this short AV delay can also notably reduce the duration of atrial refractory periods in order to better detect fast atrial arrhythmias.
The WARAD is triggered on every atrial event (As, Ap, Ar). The WARAD duration depends on the atrial paced or sinus rhythm (here 62.5% of the previous As-As interval). An atrial event is considered as a PAC (Premature Atrial Contraction) if it is detected within the WARAD.
Following a sensed ventricular beat considered as a PVC, a relative refractory period (RetroPwatch) is triggered at the atrial channel. Its duration is 500 ms and is applied on a maximum of 3 consecutive PVCs. A PVC is defined as ventricular activity not preceded by atrial activity (within an interval of -31 and -300 ms relative to a PVC in DDD mode and a limitless interval in SafeR mode). The sensing of atrial activity in this refractory period is marked as p and does not trigger an AV delay. The objective of this refractory period is to reduce the risk of PMT following the occurrence of a PVC.
The WARAD is an atrial refractory period specific to Microport™ dual-chamber devices (triggered by atrial sensing or pacing) which function without a PVARP (atrial refractory period triggered by ventricular sensing or pacing).
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