From the Guidelines
The main difference between Right Bundle Branch Block (RBBB) and Left Bundle Branch Block (LBBB) on an EKG lies in their distinctive waveform patterns, with RBBB characterized by an RSR' pattern in leads V1-V3 and a wide S wave in leads I and V6, and LBBB showing a broad, notched R wave in leads I, aVL, V5, and V6, and a QS or rS pattern in V1-V3, as defined in the 2019 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay 1. The key characteristics of RBBB include:
- QRS duration ≥120 ms
- RSR' pattern in leads V1 or V2
- S wave of greater duration than R wave or >40 ms in leads I and V6 in adults
- Normal R peak time in leads V5 and V6 but >50 ms in lead V1 In contrast, LBBB is characterized by:
- QRS duration ≥120 ms in adults
- Broad notched or slurred R wave in leads I, aVL, V5, and V6
- Absent Q waves in leads I, V5, and V6
- R peak time >60 ms in leads V5 and V6 but normal in leads V1, V2, and V3 These differences reflect the altered ventricular activation sequence: in RBBB, the left ventricle activates normally while right ventricular activation is delayed; in LBBB, the right ventricle activates normally while left ventricular activation is delayed, as supported by the 2019 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay 1 and the 2019 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay: executive summary 1. LBBB generally carries more clinical significance as it often indicates underlying structural heart disease, while RBBB can be found in healthy individuals, as noted in the 2009 AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: part III: intraventricular conduction disturbances 1. Both conduction abnormalities can affect interpretation of other EKG findings, particularly when evaluating for myocardial infarction.
From the Research
EKG Patterns
- RBBB (Right Bundle Branch Block) is characterized by a widened QRS complex and changes in the directional vectors of the R and S waves on an ECG, resulting from an interruption in normal electrical activity in the His-Purkinje system 2.
- LBBB (Left Bundle Branch Block) is identified by a widened QRS complex, often with a characteristic notch in the R wave, and is associated with a disruption in the left bundle branch of the conduction system 3.
QRS Duration
- The QRS duration (QRSd) is significantly shortened in patients with LBBB after left bundle branch area pacing (LBBAP), with a mean QRSd reduction from 169.4 ± 22.6 ms to 119.6 ± 9.5 ms 4.
- In patients with RBBB, the QRSd is also shortened after LBBAP, but to a lesser extent, with a mean QRSd reduction from 143.1 ± 16.6 ms to 119.5 ± 11.7 ms 4.
Clinical Significance
- LBBB is a strong predictor of response to cardiac resynchronization therapy (CRT), with patients having a complete LBBB pattern showing a greater improvement in left ventricular function compared to those with incomplete LBBB or non-specific intraventricular conduction delay 5.
- RBBB is generally considered a benign condition, but its presence can affect the efficacy of CRT and other device therapies in patients with heart failure 6.
Device Therapy
- CRT is the therapy of choice for symptomatic patients with heart failure and reduced ejection fraction (HFrEF) and a broad QRS complex with a LBBB morphology, but alternative therapies such as baroreflex activation therapy (BAT) and cardiac contractility modulation (CCM) may be considered for patients with non-LBBB morphology 6.