QRS alternans with 2:1 atrioventricular conduction block: what is the mechanism?

ABSTRACT

An 81-year-old woman was admitted for symptomatic bradycardia. On admission, the ECG exhibited QRS alternans, narrow QRS complex and left bundle branch block with 21 AV block. The patient soon had complete AV block and underwent a pacemaker implantation. An appropriate mechanism for explaining those ECG findings might be 41 conduction over the left bundle branch and 21 conduction over the right bundle branch. An ECG pattern exhibiting QRS alternans with a narrow QRS complex and bundle branch block with 21 AV block may suggest the coexistence of both bundle branch blocks and a high risk of complete AV block.