ECG Waveform Patterns in Bundle Branch Blocks
Right Bundle Branch Block (RBBB)
Complete RBBB is diagnosed when the QRS duration is ≥120 ms with characteristic morphologic features including an rsr', rsR', or rSR' pattern in leads V1-V2, prolonged S waves in leads I and V6 (>40 ms or longer than the R wave), and an R peak time >50 ms in V1 but normal in V5-V6. 1
Complete RBBB Criteria:
- QRS duration ≥120 ms 1, 2
- rsr', rsR', rSR', or rarely qR pattern in leads V1 or V2, where the R' or r' deflection is typically wider than the initial R wave 1, 2
- In some patients, a wide and notched R wave may appear in V1 and/or V2 instead of the classic triphasic pattern 1, 2
- S wave duration greater than R wave or >40 ms in leads I and V6 1, 2
- Normal R peak time in leads V5 and V6 but >50 ms in lead V1 1, 2
Incomplete RBBB Criteria:
- QRS duration between 110-119 ms with the same morphologic features as complete RBBB 1, 2
- In children, terminal rightward deflection ≥20 ms but <40 ms 2
Clinical Pearls for RBBB:
- RBBB patterns are stable over time in most patients, making serial ECG comparisons reliable 3
- RBBB reduces S wave amplitude in right precordial leads, which decreases sensitivity for detecting left ventricular hypertrophy 1
- When evaluating for LVH in RBBB, left atrial abnormality and left axis deviation become more valuable diagnostic features 1
Left Bundle Branch Block (LBBB)
Complete LBBB requires QRS duration ≥120 ms with broad notched or slurred R waves in leads I, aVL, V5, and V6, absent Q waves in leads I, V5, and V6, and R peak time >60 ms in V5-V6. 1
Complete LBBB Criteria:
- QRS duration ≥120 ms 1
- Broad notched or slurred R wave in leads I, aVL, V5, and V6 1
- Occasional RS pattern in V5-V6 may occur due to displaced QRS transition 1
- Absent Q waves in leads I, V5, and V6 (though a narrow Q wave may appear in aVL without myocardial pathology) 1
- R peak time >60 ms in leads V5 and V6 but normal in V1, V2, and V3 when small initial R waves are discernible 1
- ST and T waves usually opposite in direction to QRS (discordant ST-T changes) 1
Incomplete LBBB Criteria:
- QRS duration between 110-119 ms 1
- Presence of left ventricular hypertrophy pattern 1
- R peak time >60 ms in leads V4, V5, and V6 1
- Absence of Q waves in leads I, V5, and V6 1
Clinical Pearls for LBBB:
- LBBB causes electrical and mechanical ventricular dyssynchrony that affects regional myocardial function 4
- The abnormal activation pattern interferes with ischemia detection on surface ECG and affects stress testing interpretation 4
- Diagnosing LVH in the presence of LBBB is unreliable unless specific criteria are met: QRS duration >155 ms, left atrial abnormality, and precordial voltage criteria 1
- LBBB may represent the first manifestation of diffuse myocardial disease and is associated with poorer prognosis compared to RBBB 5
Fascicular Blocks
Left Anterior Fascicular Block:
- QRS duration <120 ms (distinguishes it from complete LBBB) 1
- Frontal plane axis between -45° and -90° 1
- qR pattern in lead aVL with R-peak time ≥45 ms 1
- rS pattern in leads II, III, and aVF 1
Left Posterior Fascicular Block:
- QRS duration <120 ms 1
- Frontal plane axis between 90° and 180° in adults 1
- rS pattern in leads I and aVL 1
- qR pattern in leads III and aVF 1
Nonspecific Intraventricular Conduction Delay
QRS duration >110 ms where morphology does not meet criteria for either RBBB or LBBB 1
Important Caveats:
- When LBBB is present with left anterior fascicular block, R wave amplitude in leads I and aVL becomes unreliable for LVH diagnosis, but S wave depth in left precordial leads improves detection 1
- In patients with QRS frequency analysis, low-frequency QRS content (<10 Hz) may predict cardiac resynchronization therapy response better than QRS duration alone, particularly when QRS is <150 ms 6
- Right ventricular pacing typically produces LBBB morphology; if RBBB pattern appears, lead malposition or perforation must be excluded, though rare benign variants exist 7