Understanding Right Bundle Branch Block Morphology

Author: Malithi Navarathna

Peer Editors: Alec Feuerbach, Nicole Anthony

Faculty Reviewer: Mark Silverberg

A 24-year-old female with a past medical history of obesity and a “heart problem” during childhood presents to the ED complaining of palpitations and chest pain for one week.

What is your analysis of this ECG? Which treatment would you administer?

Rate: Approximately 150/min

Rhythm: Irregularly irregular, no P waves

Axis: Borderline left axis deviation

Among several notable abnormalities on this ECG is the absence of p-waves. This finding, an irregularly irregular rhythm, and rate > 120/min are characteristics consistent with atrial fibrillation with rapid ventricular response.

Examination of the QRS morphology reveals findings consistent with a right bundle branch block (RBBB).[1] Though generally a benign electrophysiologic abnormality, this is an important ECG finding to recognize.[2]

The main criteria of RBBB are:

QRS > 120 ms
RSR’ pattern in V1-V3
Slurred S waves in leads I, aVL, V5-V6

The RSR’ pattern in V1-V3 is the classic QRS morphology of RBBB in which an upward deflection is followed by a downward deflection, and then one more upward deflection (see figure 1). To understand why this occurs, remember that electrical depolarization towards a lead will result in a positive deflection; depolarization away from a lead will result in a negative deflection. Also remember that leads V1-V3 are located at the anterior wall of the right ventricle.

The first deflection of the RSR’ pattern in RBBB reflects normal septal depolarization from left to right towards leads V1-V3. This creates a positive R wave (blue arrows in figure 1).

Subsequently, there is negative deflection due to the rapid, unilateral depolarization of the left ventricle through the normally conducting left bundle branch pathway away from V1-V3 (yellow arrows).

Finally, depolarization of the right ventricle is delayed by the “blocked” bundle branch. And this slower depolarization towards V1-V3 creates the terminal R’ wave (green arrows). This slow depolarization of the right ventricle also leads to a slurred, broad, S wave in the lateral leads (clearly seen in this case’s ECG in leads I and aVL).

Figure 1: Blue, yellow, and green arrows represent the stepwise depolarization of myocardium that creates the classic RSR’ morphology seen in leads V1-V3 in RBBB. (left image from StrongMedicine, right from ECGPedia, arrows added)

After in-depth chart review, you learn that this patient had a ventral septal defect repair as a child which may explain her RBBB. Ventral septal defect repair results in RBBB 30% to 80% of the time depending on the surgical approach.[3] You also discover that the patient has a known history of atrial fibrillation, which is more prevalent in patients with bundle branch block.[4] The patient had been previously on the antiarrhythmic dronedarone; however, the patient self-discontinued this medication a year ago.

In the ED, the physicians started a diltiazem drip for rate control. She later spontaneously converted to normal sinus rhythm while in the CCU and was discharged on metoprolol, apixaban, and sacubitril-valsartan.

References

1. Buttner, E., 2022. Right Bundle Branch Block (RBBB). [online] Life in the Fast Lane • LITFL. Available at: <https://litfl.com/right-bundle-branch-block-rbbb-ecg-library/> [Accessed 5 April 2022].

2. Harkness WT, Hicks M. Right Bundle Branch Block. [Updated 2021 Aug 13]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK507872/

3. Karadeniz C, Atalay S, Demir F, et al. Does surgically induced right bundle branch block really effect ventricular function in children after ventricular septal defect closure? [published correction appears in Pediatr Cardiol. 2015 Jun;36(5):1107]. Pediatr Cardiol. 2015;36(3):481-488. doi:10.1007/s00246-014-1037-9

4. Khan MZ, Patel K, Zarak MS, et al. Association between atrial fibrillation and bundle branch block. J Arrhythm. 2021;37(4):949-955. Published 2021 Jun 22. doi:10.1002/joa3.12556