Spatially discordant alternans due to periodic pacing site alternation.

Background: Bidirectional ventricular tachycardia (BVT) is a rare type of ventricular tachycardia that is characterized by a beat-to-beat alternation in the QRS axis. Previous studies have shown that it is caused by alternating focal activities from 2 locations.
Objective: This study proposes a novel mechanism for the formation of spatially discordant alternans (SDA) due to the periodic pacing site alternation that occurs in BVT.
Methods: We used mathematical models of cardiac tissue to understand the dynamic and physiologic mechanisms underlying SDA formation.
Results: We found that SDA was formed by periodic pacing site alternation. When tissue was paced from 2 locations alternately, the timing of pacing at distant locations varied, creating a long-short-long-short sequence of pacing periods and thus action potential durations. Importantly, the nodal lines were perpendicular to the wavefront, which is more arrhythmogenic than when nodal lines are parallel to the wavefront. A positive correlation was observed between the separation distance of the 2 sites and the alternans amplitude. SDA patterns can be predicted from the tissue geometry and pacing site locations.
Conclusion: Periodic pacing site alternation, which occurs in BVT, leads to arrhythmogenic SDA. The nodal lines associated with this phenomenon can be predicted on the basis of tissue geometry and focal locations.
Competing Interests: Disclosures The authors have no conflicts of interest to disclose.
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