ECG Changes in Bundle Branch Block and Mechanisms
Left Bundle Branch Block (LBBB)
In LBBB, the hallmark ECG changes include QRS duration ≥120 ms, absence of septal Q waves, and notched/slurred QRS complexes in lateral leads, caused by delayed left ventricular activation that forces depolarization to spread from right to left through slower myocardial pathways rather than the normal His-Purkinje system. 1, 2
Diagnostic ECG Criteria for LBBB
- QRS duration ≥120 ms is the minimum threshold, though stricter criteria suggest ≥130 ms for women and ≥140 ms for men improve specificity 2, 3
- Absence of initial septal Q waves in leads I, aVL, V5, and V6 (loss of normal left-to-right septal activation) 2
- Notched or slurred QRS complexes (not gradual prolongation) in at least two of leads I, aVL, V1, V2, V5, or V6 2, 3
- Monophasic R waves that are broad and plateau-topped in lateral leads (I, aVL, V5, V6) 1
Mechanism of LBBB
The left ventricle activates late because the normal rapid conduction through the left bundle is blocked, forcing depolarization to spread from the right ventricle across the interventricular septum through slow cell-to-cell conduction in working myocardium 4. This produces:
- Delayed and prolonged left ventricular activation (the fundamental pathophysiology) 4
- Reversed septal activation (right-to-left instead of left-to-right), eliminating normal septal Q waves 2
- Prolonged QRS duration due to slower myocardial conduction versus His-Purkinje conduction 4
Secondary Repolarization Changes in LBBB
- ST segments and T waves are directed opposite to the mean QRS vector (discordant ST-T changes) 1
- These are secondary repolarization abnormalities that result from altered depolarization sequence, not primary myocardial disease 1
- The magnitude of ST-T changes correlates with the magnitude of QRS changes 1
Critical Pitfall: LBBB makes diagnosis of acute MI difficult. Use Sgarbossa criteria: concordant ST elevation ≥1 mm, concordant ST depression ≥1 mm in V1-V3, or discordant ST elevation ≥5 mm have high specificity but low sensitivity for acute MI 1
Right Bundle Branch Block (RBBB)
RBBB produces QRS duration ≥120 ms with a characteristic RSR' pattern in V1-V2 and wide slurred S waves in lateral leads, caused by delayed right ventricular activation after the left ventricle has already depolarized. 1, 5
Diagnostic ECG Criteria for RBBB
- QRS duration ≥120 ms 1, 5
- RSR' pattern (M-shaped QRS) in right precordial leads V1-V2 1
- Wide, slurred S waves in lateral leads (I, aVL, V5, V6) 1
- Normal septal Q waves preserved in lateral leads (unlike LBBB) 1
Mechanism of RBBB
The right ventricle depolarizes late because conduction through the right bundle is blocked, forcing activation to spread from the left ventricle across the septum to reach the right ventricle through slow myocardial conduction 1. This produces:
- Terminal rightward and anterior forces (the late R' in V1) representing delayed RV activation 1
- Preserved initial septal activation (left-to-right), maintaining normal Q waves 1
Secondary Repolarization Changes in RBBB
- ST segments and T waves are directed opposite to the slow terminal QRS component (the R' wave) 1
- ST-T changes in V1-V2 are typically discordant (opposite direction from terminal R') 1
- Unlike LBBB, RBBB does not interfere with ST-segment criteria for acute MI in most leads 1
Fascicular Blocks (Hemiblocks)
Left Anterior Fascicular Block (LAFB)
LAFB causes left axis deviation between -45° to -90°, qR pattern in aVL, and QRS duration <120 ms, resulting from blocked conduction in the anterior fascicle that forces activation to spread from inferior to superior left ventricle. 6, 7
Diagnostic Criteria
- Left axis deviation -45° to -90° (mandatory criterion) 6
- qR pattern in lead aVL with small initial q wave and tall R wave 6, 7
- R-peak time in aVL ≥45 ms 6
- QRS duration <120 ms (distinguishes from complete LBBB) 6
- Small q waves in leads I and aVL with tall R waves 6
- Deeper S waves in V5-V6 1, 7
Mechanism
The anterior fascicle is blocked, forcing activation to travel first through the posterior fascicle to activate the inferior left ventricle, then spread superiorly and laterally through myocardium, creating a superior and leftward QRS axis 1, 7
Critical Pitfall: R-wave amplitude in leads I and aVL cannot be used to diagnose LVH when LAFB is present; use S-wave depth in V5-V6 instead 1, 6, 7
Left Posterior Fascicular Block (LPFB)
- Right axis deviation (QRS axis >+90°) 1
- rS pattern in leads I and aVL 1
- qR pattern in inferior leads 1
- Very rare as an isolated finding; usually occurs with RBBB 1
Management Implications
Indications for Permanent Pacing
Permanent pacing is mandated for alternating bundle branch block (alternating LBBB and RBBB morphologies) due to high risk of sudden complete heart block. 1
- Class I (Recommended): Syncope with BBB and HV interval ≥70 ms at electrophysiology study 1
- Class I (Recommended): Alternating bundle branch block 1
- Class III (Harm): Asymptomatic isolated BBB with 1:1 AV conduction requires no pacing 1
Cardiac Resynchronization Therapy (CRT)
- LBBB with QRS ≥150 ms and heart failure (LVEF 36-50%, NYHA Class II or greater) may benefit from CRT 1
- Stricter LBBB criteria (QRS ≥130-140 ms with mid-QRS notching) improve CRT patient selection 3, 8
Evaluation Algorithm
When BBB is detected 1:
- Assess for symptoms: syncope, presyncope, heart failure
- Evaluate for underlying disease: exercise testing, 24-hour ECG monitoring, echocardiography 1
- Consider electrophysiology study if syncope present to measure HV interval 1
- Screen family members if bifascicular block in young patient (consider genetic conduction disease) 1
Common Pitfall: Exercise-induced LBBB (but not RBBB) is associated with increased cardiac events and mortality, requiring thorough evaluation 1