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.