Case report A 74-year-old man presented with frequent palpitations, described as “the heart beating on the neck.” He also had a history of two syncope episodes; the most recent was more than six months before his admission. By careful history taking, we found that the syncope episodes did not seem to occur as a result of vasovagal reflex. The patient’s resting electrocardiogram (ECG) showed a right bundle branch block (RBBB) pattern (Figure 1 ). Holter monitoring and exercise tests revealed a bundle branch block alternating between the right and left bundles. His echocardiogram was normal with preserved ejection fraction (EF) (66%).After this initial evaluation, he was subjected to an electrophysiological study (EPS) and the basic intervals measured were as follows: PR, 186 ms; QRS, 153 ms (RBBB); AH, 86 ms (basal); and HV, 60 ms. Atrial electrical stimulation (AES) induced a wide QRS complex tachycardia (Figure 2A ), with predominantly RBBB morphology, some of the QRS having a left bundle branch block (LBBB) pattern, and some atrial beats being blocked to the ventricles below the bundle of His (Figure 2B ).