Transvenous right ventricular pacing usually demonstrates left BBB pattern. Paced right BBB pattern may represent coronary venous pacing, septal or free wall perforation and left ventricular pacing through retrograde transarterial route or intracardiac defects such as patent foramen ovale, ventricular septal defect [^1-3]. However, uncomplicated pacing even in the right ventricle may sometimes produce right BBB type paced QRS complexes [⁴]. Therefore, it is clinically important to determine whether a right BBB pattern induced by ventricular pacing is the result of a malpositioned lead or uncomplicated transvenous right ventricular pacing. Positioning the ventricular lead inadvertently in the left ventricle may create serious clinical problems and require life long anticoagulation or lead extraction. Fortunately, not all right BBB configuration is associated with left ventricular pacing. Most will have right ventricular leads in appropriate positions. Klein et al. [⁵] reported eight patients with right BBB patterns in leads V1-2, left BBB pattern in lead I, and pacing leads located in the right ventricular apex. They recognized that placement of leads V1-2 one interspace lower than standard could eliminate right BBB appearance. Coman et al. [⁶] reported seven cases with right BBB pattern during permanent right ventricular pacing. Placing the leads V1-2 one interspace lower than standard resulted in disappearance of right BBB morphology and inscription of QS or rS complexes in V1-2. Each patient had leads located in the distal right ventricular septum or apex. However, four patients whose right BBB pattern could not be eliminated by moving of leads V1-2 had their pacing leads located in the mid septum. Hence, they suggested that this technique reliably distinguished patients with midseptal leads from those with leads in the distal septum and apex. In our patient moving the leads V1-2 one interspace lower than standard also eliminated right BBB pattern and resulted in QS complexes in V1-2. We also confirmed with the aid of echocardiography and fluoroscopy that the tip of the ventricular lead was located in the right ventricular apex. Coman et al. [⁶] developed an algorithm to differentiate right and left ventricular right BBB pacing morphologies using frontal axis and precordial transition. They suggested that after excluding left ventricular pacing from the proximal and mid septum, a frontal axis of 0º to -90º and precordial transition by V3 may be able to differentiate uncomplicated right ventricular septal or apical pacing from all other forms of left ventricular pacing with 86% sensitivity, 99% specificity, and 95% positive predictive value. The frontal plane axis found in our patient was around -60º and precordial transition occurred by V3 which was suggestive of uncomplicated right ventricular apical pacing according to the above mentioned algorithm. Together with this finding and elimination of right BBB pattern by moving the leads V1-2, as previously described, we satisfactorily determined safe right ventricular pacing in our patient. We also confirmed the correct lead position by echocardiography and fluoroscopy. Several hypotheses were proposed, as to the mechanism why right BBB configuration occurs in normally placed right ventricular leads. One plausible mechanism proposed by Mower et al. [⁷] was that portions of the interventricular septum which are anatomically right ventricle may behave functionally and electrically as left ventricle. Another suggestion made by Barold et al [⁸] was that the right BBB pattern could be the result of a combination of right ventricular activation delay due to severe disease of the right ventricular conduction system and early penetration of the electrical impulse into the left ventricular conduction system.

Right BBB pattern during ventricular pacing does not always point out left ventricular pacing due to a malpositioned ventricular lead. Safe uncomplicated right BBB pattern may be seen with correctly positioned right ventricular leads in the distal septum or apex. Simple techniques such as moving the leads V1-2 one interspace lower than standard or combining frontal axis and precordial transition as an algorithmic approach may correctly identify the correct position of the lead. Echocardiography and fluoroscopy are also useful confirmatory techniques that are inevitable in cases of doubt.