Presence of Alternating Bundle Branch Block on Electrocardiogram: What is the Likely Mechanism?

A 52-year-old male, nondiabetic, nonhypertensive, and smoker presented with recurrent syncope six times in the last 2 months with the normal cardiovascular system examination. Baseline electrocardiogram (ECG), echocardiography, and serum chemistry were within the normal limits. The patient was subjected to 24-h Holter analysis which revealed alternating bundle branch block [Figure 1]. The patient was managed with dual chamber pacemaker implantation with no recurrence of syncope in the follow-up. 1st and 2nd leads have T-wave inversions in all three leads, the PR interval appears varying and this could be possible because of: (a) atrioventricular (AV) dissociation, then the QRS is likely to be an escape rather than conducted aberrantly and (b) conducted with varying PR indicate multilevel AV nodal disease with diseased left bundle able to recover in the prolonged PR and conducting with delay in the right bundle. The first half of the ECG shows right bundle branch block (RBBB) pattern when the heart rate is around 35 beats per min, here RBBB is occurring when heart rate is low[1] or left ventricle escape is happening with preferential conduction in left bundle resulting in RBBB. 3rd QRS has left axis deviation, shorter PR, and a QRS morphology different to others. It is a fusion complex (between the escape and conducted wave fronts). This also adds to the likelihood that the earlier two complexes were escape beats. In the 4th to 6th QRS complexes, PP interval has shortened abruptly and conducted wide QRS beats are seen. Prominent S wave in lead 1 suggests atypical left bundle branch block (LBBB). Here, ECG shows sinus rhythm with LBBB with the heart rate of around 75 beats per min. With the acceleration of heart rate, preferential conduction is occurring in the right bundle causing LBBB [Figure 2] or there may be acceleration-dependent aberrancy or Phase III aberrancy. 7th QRS is associated with a similar PR but a narrower complex. In lead 3, S wave is deeper than lead 2 suggesting a left anterior hemi block. The posterior fascicle has recovered conduction. After four beats with left bundle aberrancy, we can observe the sinus beat with no RBBB or LBBB morphology with heart rate of 65 beats per min suggestive of simultaneous conduction across both right and left bundles. The ECG depicts significant conduction system disease with intermittent conduction with LBBB and without LBBB. Furthermore, there is significant variation in PP interval suggestive of 2:1 SA exit block. Normally, right bundle branch aberrancy occurs with the acceleration of heart rate when preferential conduction occurs in the left bundle because the refractory period of RB is longer than LB. Here with slightly higher heart rate, preferential conduction was occurring in the right bundle-producing LBBB morphology which may be suggestive of refractory period of LB is longer than RB. In severely infrahisian conduction system disease, the natural relation between the refractoriness of two bundle branches may be lost. Reversal of refractoriness of bundle branches may also be a feature of significant infrahisian conduction system disease. In the absence of a simultaneous diagnostic electrophysiological study, it is difficult to confirm the reversal of refractoriness as a likely mechanism, although it is a rare possibility.Figure 1: Holter tracing showing alternating bundle branch block with CHB: Complete heart block, RBBB: Right bundle branch block, LBBB: Left bundle branch blockFigure 2: Ladder diagram showing conduction across the left and right bundle branchesDeclaration of patient consent The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.