A regularly irregular tachycardia with variable QRS morphologies - What is the mechanism?

We present a complex ECG displaying a combination of atrial tachycardia with Mobitz type I AV block, and right bundle branch block (RBBB) alternans (RBBB alternating with normal QRS conduction) during Wenckebach cycle. A detailed explanation regarding the mechanism of this complex arrhythmia is provided, and the mechanism of bundle branch alternans is reviewed. A unique feature of our case is the added complexity given by the presence of atrial tachycardia and AV Wenckebach, which departs from the previous description of bundle branch alternans during 1:1 AV conduction.