ECG Diagnosis of Bundle Branch Block
Bundle branch block is diagnosed on ECG by identifying QRS prolongation ≥120 ms in adults combined with specific morphologic patterns in the precordial and limb leads that distinguish left from right bundle branch block.
Complete Left Bundle Branch Block (LBBB)
The diagnosis requires all of the following criteria to be met 1, 2:
Essential Diagnostic Criteria
QRS duration ≥120 ms in adults (>100 ms in children 4-16 years; >90 ms in children <4 years) 1, 2
Broad notched or slurred R waves in leads I, aVL, V5, and V6 (occasionally RS pattern in V5-V6 due to displaced transition) 1, 2
Absent q waves in leads I, V5, and V6 (though a narrow q wave may be present in aVL without indicating pathology) 1, 2
R peak time >60 ms in leads V5 and V6, but normal in V1-V3 when small initial r waves are discernible 1, 2
Secondary Features
ST and T waves typically opposite in direction to QRS (appropriate discordance) 1, 2
Positive T waves in leads with upright QRS are normal (positive concordance) 1, 2
Depressed ST segments and/or negative T waves in leads with negative QRS (negative concordance) are abnormal and suggest additional pathology 1, 2
The frontal plane QRS axis may shift right, left, or superior, sometimes in a rate-dependent manner 1, 2
Complete Right Bundle Branch Block (RBBB)
The diagnosis requires the following pattern 1:
Essential Diagnostic Criteria
QRS duration ≥120 ms in adults (>100 ms in children 4-16 years; >90 ms in children <4 years) 1
rsr', rsR', or rSR' pattern in leads V1 or V2, where the R' or r' deflection is usually wider than the initial R wave 1
S wave of greater duration than R wave or >40 ms in leads I and V6 1
Normal R peak time in leads V5 and V6 but >50 ms in lead V1 1
Incomplete Bundle Branch Blocks
Incomplete LBBB
Diagnosed when QRS duration is 110-119 ms in adults (90-100 ms in children 8-16 years; 80-90 ms in children <8 years) with the following features 1, 2:
- Presence of left ventricular hypertrophy pattern 1, 2
- R peak time >60 ms in leads V4, V5, and V6 1, 2
- Absence of q waves in leads I, V5, and V6 1, 2
Incomplete RBBB
Diagnosed when QRS duration is 110-119 ms in adults (90-100 ms in children 4-16 years; 86-90 ms in children <8 years) with morphology matching complete RBBB criteria 1, 3:
- Same rsr', rsR', or rSR' pattern in V1-V2 as complete RBBB 3
- S wave characteristics in leads I and V6 as in complete RBBB 3
- In children, may be diagnosed when terminal rightward deflection is <40 ms but ≥20 ms 3
Critical Diagnostic Pitfalls to Avoid
Do Not Confuse with Normal Variants
An rsr' pattern in V1-V2 with normal QRS duration is a normal variant in children and should not be labeled as incomplete RBBB 3
This pattern may appear when lead V1 is recorded higher than or to the right of normal position, particularly when r' is <20 ms 3
The terms "rsr'" and "normal rsr'" should not be used to describe these patterns due to variable interpretation 3
Nonspecific Intraventricular Conduction Disturbance
QRS prolongation >110 ms without meeting morphologic criteria for RBBB or LBBB should be classified as nonspecific intraventricular conduction disturbance, not bundle branch block 1, 2
This is a common error—QRS duration alone is insufficient for BBB diagnosis 2
Special Populations
Incomplete RBBB criteria do not apply to patients with congenital heart disease in whom left-axis deviation is present in infancy 1, 3
LBBB can mask underlying myocardial pathology due to altered ventricular activation sequence 2
Systematic Approach to ECG Interpretation
Measure QRS duration in the lead where it appears longest (often V3-V4) 1
If QRS ≥120 ms, examine morphology in precordial leads V1-V2 and V5-V6 1
Look for rsr' pattern in V1-V2 → suggests RBBB 1
Look for broad notched R waves in V5-V6 and absent q waves in I, V5-V6 → suggests LBBB 1, 2
Verify all morphologic criteria are met before making the diagnosis—partial fulfillment suggests nonspecific conduction delay 2
Check for rate-dependent changes if multiple ECGs are available, as LBBB appearance may vary 2