Memory Tricks for RBBB and LBBB on ECG
The "WiLLiaM MaRRoW" Mnemonic
The most practical mnemonic for distinguishing RBBB from LBBB is "WiLLiaM MaRRoW" - in LBBB you see a "W" pattern in V1 and "M" pattern in V6, while in RBBB you see an "M" pattern in V1 and "W" pattern in V6. 1
How to Apply This:
- LBBB = "WiLLiaM": Look at V1 for a "W" shape (QS or rS pattern) and V6 for an "M" shape (broad notched R wave) 1
- RBBB = "MaRRoW": Look at V1 for an "M" shape (rsr', rsR', or rSR' pattern) and V6 for a "W" shape (prolonged S wave) 1
The "Turn Signal" Mnemonic
Another helpful trick is thinking of bundle blocks as "turn signals":
- LBBB = "Left turn signal": The QRS axis shifts LEFT (posteriorly and superiorly), with the QRS vector pointing leftward 1
- RBBB = "Right turn signal": The terminal QRS forces point RIGHT, creating the characteristic RSR' in V1 1
Key Diagnostic Features to Remember
RBBB Pattern Recognition:
- QRS duration ≥120 ms with the classic "rabbit ears" (rsr', rsR', or rSR') in V1-V2 1
- Prolonged S waves in leads I and V6 (>40 ms or longer than the R wave) 1
- R peak time >50 ms in V1 but normal in V5-V6 1
LBBB Pattern Recognition:
- QRS duration ≥120 ms with broad notched or slurred R waves in leads I, aVL, V5, and V6 1
- Absent Q waves in leads I, V5, and V6 (this is critical - no septal q waves because septal activation goes right-to-left abnormally) 1
- R peak time >60 ms in V5-V6 1
Common Pitfalls to Avoid
- Don't confuse incomplete RBBB with complete RBBB: Incomplete RBBB has QRS <120 ms and is generally benign, while complete RBBB warrants evaluation 1
- LBBB with an initial r wave ≥1 mm in V1: This suggests intact left-to-right septal activation and may indicate the LBBB is not truly "complete" - these patients are at lower risk of complete heart block during procedures 2
- Body habitus affects voltage: In severe obesity, ECG voltage criteria become unreliable due to increased distance between heart and electrodes 3
Clinical Context Matters
- RBBB can be benign: May represent isolated, idiopathic conduction delay in otherwise healthy individuals 1
- LBBB is rarely benign: Almost always indicates underlying structural heart disease and requires cardiology evaluation even if asymptomatic 1
- Athletes with any complete bundle branch block: Mandate full workup including exercise testing, 24-hour ECG, and echocardiography to exclude pathological causes 1