Extremally rare cause of AV block during typical atrial flutter ablation not related to conduction tissue injury

51-year-old female with extensive prior atrial surgery involving myxoma resection and patch septum repair and prior typical atrial flutter as well as peripatch reentry underwent redo radiofrequency ablation of typical atrial flutter.

After high density mapping was performed, and gap in the prior typical flutter line was ablated.

During the ablation transient atrioventricular (AV) block was noted. Subsequent remapping of the right atrium revealed that there was a narrow strip of tissue between the cavo-tricuspid isthmus (CTI) and the coronary sinus (CS) os on which activation of the AV node was now depending. From all other directions, the AV node was surrounded by scar tissue.

The most likely explanation for the transient AV block during the ablation procedure is that there was reversible injury to the tissue strip between the CTI and the CS os, which is critical for the activation of the AV node.