Irregular orthodromic atrioventricular reentrant tachycardia using the left lateral accessory pathway due to intermittent mitral isthmus block.

Mitral isthmus block during left lateral accessory pathway ablation has been reported in the past. Here, we document for the first time an irregular atrioventricular reciprocating tachycardia due to alternating mitral isthmus block and mitral isthmus delay, resulting in different atrioventricular node conduction times and tachycardia cycle length.