Tachycardia cycle length alternans during a narrow QRS complex tachycardia with eccentric retrograde atrial activation sequence: What is the mechanism?

Differential diagnoses of regularly irregular narrow QRS tachycardias are AV re‐entrant tachycardia (AVRT) with alternating antegrade AVN pathways, atypical AV nodal re‐entrant tachycardia (AVNRT) with alternating antegrade AVN pathways, and atrial tachycardia (AT) with alternating antegrade AVN pathways or with Wenckebach periodicity.