What are the key differences between Right Bundle Branch Block (RBBB) and Left Bundle Branch Block (LBBB) on an Electrocardiogram (ECG)?

ECG Characteristics of RBBB vs LBBB

RBBB shows an rsr', rsR', or rSR' pattern in V1/V2 with a wide S wave in lateral leads (I, V6), while LBBB shows broad notched R waves in lateral leads (I, aVL, V5, V6) with absent Q waves and QS or rS complexes in V1/V2. 1

Key Distinguishing Features

Right Bundle Branch Block (RBBB)

Lead V1/V2 Morphology:

• Characteristic rsr', rsR', or rSR' pattern where the R' or r' deflection is usually wider than the initial R wave 2
• The terminal rightward deflection creates the classic "rabbit ears" appearance 2

Lateral Leads (I, V5, V6):

• S wave of greater duration than R wave or >40 ms 2
• Broad, slurred S waves in these leads 2

QRS Duration:

• Complete RBBB: ≥120 ms in adults 1
• Incomplete RBBB: 110-119 ms in adults 2

R Peak Time:

• Normal in V5 and V6 but >50 ms in lead V1 2

Left Bundle Branch Block (LBBB)

Lead V1/V2 Morphology:

• QS or rS pattern (always) 3
• Never shows the rsr' pattern seen in RBBB 1

Lateral Leads (I, aVL, V5, V6):

• Broad notched or slurred R waves 1
• Dominant R waves present in 97.2% of cases 3
• Critical feature: Notched or slurred QRS complexes in at least one of leads I, aVL, V5, or V6 occur in 100% of true LBBB 3

Absence of Q Waves:

• No q waves in leads I, V5, and V6 (though narrow q may appear in aVL) 1
• Q/Q waves present in only 11.6% of lateral leads 3

QRS Duration:

• Complete LBBB: ≥120 ms in adults 1
• Incomplete LBBB: 110-119 ms in adults 1

R Peak Time:

• 60 ms in leads V5 and V6 1

• Normal in V1, V2, V3 when small initial r waves are present 1

ST-T Wave Changes

LBBB:

• ST and T waves usually opposite in direction to QRS (appropriate discordance) 1
• Positive T wave with upright QRS may be normal (positive concordance) 1
• Depressed ST segment and/or negative T wave with negative QRS (negative concordance) is abnormal 1

RBBB:

• Generally preserves normal ST-T wave orientation in lateral leads 4

Critical Clinical Pitfalls

Normal Variants Mimicking RBBB:

• An rsr' pattern in V1/V2 with normal QRS duration is a normal variant in children and should not be labeled incomplete RBBB 2
• This pattern may appear when V1 is recorded higher or more rightward than normal position 2

Atypical Patterns:

• Broad QRS not meeting morphology criteria for RBBB or LBBB is classified as non-specific intraventricular conduction delay (NSIVCD), not bundle branch block 5
• Only 90.6% of true LBBB cases meet strict Strauss criteria, but 100% show notching/slurring in lateral leads 3

Clinical Significance:

• RBBB in acute MI indicates complete coronary occlusion in 51.7% of cases and carries 14.3% in-hospital mortality 6
• New RBBB in acute MI has 18.8% in-hospital mortality, the highest of all ECG presentations 6
• 26% of acute left main coronary artery occlusions present with RBBB (mostly with left anterior hemiblock) 6