The mystery of the dark tracing: a strange apparent intraventricular conduction alternance—case report

We present the electrocardiogram (ECG) of an elderly woman with Mobitz II atrioventricular (AV) block, left bundle branch block (LBBB), and ventricular ectopic activity. At first glance, the ECG may give the misleading impression of Wenckebach periodicity and raise the suspicion of intermittent left anterior fascicular block (LAFB) and left posterior fascicular block (LPFB), suggesting an apparent alternating conduction block in the main divisions of the LBB. This intriguing appearance prompted us to present the case.