Can a bundle branch block affect the QRS complex?

Bundle Branch Block Effects on the QRS Complex

Yes, bundle branch blocks fundamentally alter the entire QRS complex, including its terminal portion (the "bottom" or end of the QRS), by changing both the sequence and duration of ventricular depolarization. 1

How Bundle Branch Blocks Affect the QRS Complex

Complete Morphological Changes

Bundle branch blocks cause secondary repolarization abnormalities that affect the entire QRS waveform, not just isolated segments 1:

• Right bundle branch block (RBBB) produces characteristic terminal changes with QRS duration ≥120 ms, featuring an rsr', rsR', or rSR' pattern in leads V1-V2, where the terminal R' deflection is typically wider than the initial R wave 2, 3
• The terminal portion shows prolonged S waves (>40 ms or longer than the R wave) in leads I and V6, with R peak time >50 ms in V1 2, 3
• Left bundle branch block (LBBB) creates broad, notched or slurred R waves in leads I, aVL, V5, and V6, with R peak time >60 ms in V5-V6 3

Terminal QRS Forces Are Specifically Altered

The "bottom" or terminal portion of the QRS is where bundle branch blocks manifest most prominently 1:

• In RBBB, the ST-segment and T-wave vectors are directed opposite to the slow terminal component of the QRS complex 1
• In LBBB, the ST-segment and T-wave vectors are generally directed opposite to the mean QRS vector 1
• These terminal forces represent the delayed activation of the ventricle whose bundle branch is blocked 1

Clinical Implications for ECG Interpretation

Wide QRS Tachycardia Differentiation

When evaluating wide QRS complexes, the terminal portion is critical for diagnosis 1:

• QRS width >0.14 seconds with RBBB pattern or >0.16 seconds with LBBB pattern favors ventricular tachycardia over supraventricular tachycardia with aberrancy 1
• Bundle branch block can occur with any supraventricular arrhythmia, making the terminal QRS morphology essential for accurate diagnosis 1

Acute Ischemia Detection

Bundle branch blocks complicate ischemia diagnosis differently 1:

• ST-segment criteria for acute ischemia are NOT affected by fascicular blocks or RBBB because the initial QRS forces remain relatively preserved 1
• LBBB significantly affects ST-segment interpretation due to pronounced secondary ST- and T-wave changes that involve the entire repolarization sequence 1
• In LBBB, concordant ST-segment elevation ≥0.1 mV in leads with positive QRS or ST depression ≥0.1 mV in V1-V3 suggests acute MI with high specificity but low sensitivity 1

Important Caveats

• RBBB can paradoxically produce Q waves that simulate myocardial infarction when the initial QRS forces are altered, though this is uncommon 4
• "Masquerading" RBBB can occur when severe left anterior fascicular block with left ventricular enlargement reorients terminal forces leftward and upward, making the characteristic terminal S wave in lead I disappear and mimicking LBBB 5
• Intermittent bundle branch blocks that appear and disappear with heart rate changes (phase 3 or phase 4 blocks) indicate true bundle branch pathology rather than simple rate-related aberrancy 6

Practical Algorithm for Recognition

When evaluating whether a bundle branch block is affecting the QRS:

1. Measure QRS duration: ≥120 ms indicates complete block 2, 3
2. Examine terminal forces in V1: rsr', rsR', or rSR' pattern = RBBB; broad notched R wave = LBBB 2, 3
3. Check lateral leads (I, V6): Prolonged S waves (>40 ms) = RBBB; broad notched R waves = LBBB 2, 3
4. Assess R peak time: >50 ms in V1 with normal V5-V6 = RBBB; >60 ms in V5-V6 = LBBB 2, 3

The terminal portion of the QRS is where bundle branch blocks create their most distinctive and diagnostically important changes, making this region essential for accurate ECG interpretation 1, 2, 3.