How to Identify Bundle Branch Blocks on ECG
Bundle branch blocks are identified on ECG by QRS duration ≥120 ms combined with specific morphologic patterns in precordial and limb leads that distinguish right from left bundle branch block.
Right Bundle Branch Block (RBBB) Identification
Essential Diagnostic Criteria
RBBB is diagnosed by the combination of QRS duration >120 ms with characteristic morphology in specific leads 1, 2:
- Lead V1/V2 morphology: Always shows rSR' pattern (M-shaped or "rabbit ears"), rS, or QS complex 1, 2
- Lead V5/V6 morphology: Wide slurred S wave present 3
- Lead I and aVL: Slurred S waves typically present 3
Key Morphologic Features
The right precordial leads (V1-V3) show the most distinctive changes 1:
- Terminal R wave (R') represents delayed right ventricular activation 1
- No Q waves should appear in leads V5/V6 in isolated RBBB 1
Clinical Pitfall Alert
RBBB pattern during right ventricular pacing should produce LBBB morphology—if RBBB pattern appears, suspect lead malposition or myocardial perforation 4. This represents a critical diagnostic error to avoid.
Left Bundle Branch Block (LBBB) Identification
Strict Diagnostic Criteria (Strauss Criteria)
True LBBB requires all of the following 5, 6:
- QRS duration: ≥140 ms in men OR ≥130 ms in women 5, 6
- Lead V1/V2 morphology: QS or rS pattern (never a dominant R wave) 5
- Lead V5/V6 morphology: No Q waves present 5
- Notching/slurring: Present in ≥2 of the following leads: V1, V2, V5, V6, I, aVL 5, 6
Additional Morphologic Features
Dominant R waves appear in leads I and aVL in 97% of true LBBB cases 5. However, Q waves in these leads are uncommon (only 11.6% of cases) 5.
The notching or slurring of QRS complexes is universally present (100%) in at least one of leads I, aVL, V5, or V6 in true LBBB 5.
Distinguishing True LBBB from Nonspecific Conduction Delay
Not all wide QRS complexes with LBBB pattern represent true conduction block 5. True LBBB shows:
- Complete conduction block in the His-Purkinje system 5
- Correctable by conduction system pacing 5
- Meets strict Strauss criteria 5, 6
Nonspecific left ventricular conduction delay may mimic LBBB but lacks these features 5.
Anatomic Localization of Block
LBBB with Left Axis Deviation
When LBBB presents with QRS axis of -60° or more leftward, this suggests the left bundle branch maintains some conduction rather than complete block 7. True complete LBBB typically shows monomorphic morphology 7.
Proximal vs. Distal RBBB
Signal-averaged ECG can distinguish proximal from distal RBBB 8:
- Proximal RBBB: Filtered QRS duration ≥160 ms, >10 fragmented signals, "whole" type pattern 8
- Distal RBBB: Shorter filtered QRS duration, fewer fragmented signals 8
This distinction matters because proximal RBBB combined with left bundle disease may progress to complete AV block 8.
Critical Diagnostic Pitfalls
When Bundle Branch Block Indicates Serious Disease
Bifascicular block (RBBB with left anterior or posterior fascicular block) carries increased risk of progression to complete AV block 1, 2. These patients require comprehensive evaluation 1, 2.
Alternating bundle branch block (RBBB and LBBB on successive ECGs) indicates severe conduction system disease with rapid progression to complete heart block 2.
LBBB in Acute Ischemia
Diagnosing acute MI in the presence of LBBB is challenging due to secondary ST-T wave changes 3. Look for:
- Concordant ST-segment elevation (same direction as QRS) 3
- These changes are associated with higher 30-day mortality 3
Brugada Pattern Mimicry
RBBB with ST-elevation in V1-V3 represents Brugada pattern and requires immediate specialized evaluation due to sudden cardiac death risk 1, 2.
Impact on Other ECG Diagnoses
Left Ventricular Hypertrophy (LVH)
LVH diagnosis should not be attempted in complete LBBB unless specific criteria are met 3:
- QRS duration >155 ms combined with precordial voltage criteria suggests LVH 3
- Otherwise, LBBB obscures LVH diagnosis due to secondary repolarization changes 3
RBBB reduces S-wave amplitude in right precordial leads, decreasing sensitivity of LVH criteria 3. Left atrial abnormality and left axis deviation help support LVH diagnosis in this setting 3.
Fascicular Blocks
ST-segment criteria for acute ischemia remain valid in the presence of fascicular blocks or RBBB 3. Only LBBB significantly affects these criteria 3.