Atrial flutter masking complete atrioventricular block in decompensated heart failure: a case report.

A slow, regular ventricular response during typical atrial flutter, especially with alternating bundle branch block, should raise suspicion of concealed complete atrioventricular block. We report a 73-year-old man with decompensated heart failure (ejection fraction 25%) in whom typical flutter masked advanced infra-Hisian block. Cavotricuspid isthmus ablation terminated the flutter and unmasked complete atrioventricular block (QRS 160 ms). Left bundle branch area pacing-optimized cardiac resynchronization therapy with intentional anodal capture narrowed the QRS to 90 ms, with early clinical improvement.