Alternating right bundle branch block or intermittent preexcitation?

*Correspondence: Angel Cueva-Parra E-mail: angel.cueva.parra@gmail.com Available online: 04-04-2023 Arch Cardiol Mex. 2023;93(2):240-242 www.archivoscardiologia.com Date of reception: 29-08-2021 Date of acceptance: 18-04-2022 DOI: 10.24875/ACM.21000279 Right bundle branch block (RBBB) is a very common electrocardiographic feature in patients with Ebstein anomaly (EA) and it is present in more than 94% of these patients1. We present the case of a 33-year-old woman who was admitted to the emergency room with palpitations, with BP 95/55 mmHg and HR 155 bpm, her heart rhythm was irregular and she had a systolic murmur in the tricuspid focus. In the electrocardiogram (ECG), there was an irregular wide QRS tachycardia, which was negative in leads V1 and DIII, compatible with preexcited AF by a right postero-septal accessory pathway (AP) (Fig. 1). Electrical cardioversion was performed going into sinus rhythm with intermittent RBBB (Fig. 2). The echocardiogram revealed findings consistent with EA with a tricuspid septal leaflet attachment index of 10 mm/m2 (Fig. 3). The patient was discharged with propafenone 150 mg QID pending ablation. In 2006, Iturralde et al. reported that the absence of RBBB in patients with EA is strongly associated with the presence of an AP2, this is explained because anterograde conduction through a right AP during sinus rhythm masked RBBB. In this case, the preexcitation appeared only in some QRS complexes, causing the RBBB to be masked when it was present; therefore, it is not an alternating RBBB itself, but rather an intermittent preexcitation. In these patients, the pre-excitation pattern is