• Wide QRS Complex
• Causes of LBBB
• Heart Vector and LBBB
• LBBB and Leads (V1, V6)
• Septal Q Wave
• Ventricular Septal Infarction and LBBB
• ECG and LBBB
• QRS Variants in LBBB
• ECG and Incomplete LBBB

Wide QRS Complex

Wide QRS Complex (≥0.12s)

• Left bundle branch block (LBBB) has 2 variants:
• Block at the trunk level
• Block at the fascicular level (note, this is not a bifascicular block)
• In LBBB, the impulse is blocked in the left bundle branch
• Supraventricular impulse activates the ventricles only through the right bundle branch
• The impulse coming from the atria activates the ventricles only through the right bundle branch
• First, the right ventricle is activated
• And then the impulse passes through the myocardium to the left ventricle and depolarizes it
• The impulse spreads more slowly through the myocardium than through the bundle branches
• Activation of the left ventricle is therefore delayed
• Thus, the QRS complex will be widened ≥ 0.12s (≥ 3 small squares)

Heart Vector and LBBB

Ventricular Activation in Left Bundle Branch Block (LBBB)

• Both ventricles are activated only through the right bundle branch, with the impulse activating the ventricles sequentially
• During the activation of different parts of the ventricles, heart vectors are generated
• 1. First, the ventricular septum is activated
• The septum is activated from the right bundle branch (which is located on the right side of the septum)
• The initial vector (V_I) of the septum points to the left (1)
• No septal Q wave appears in lateral leads
• 2. Then, the right ventricle is activated
• The impulse exits from the right bundle branch and activates a small area of the right ventricle
• A small right ventricular vector is generated, which is not visible because it is overlaid by the large vector of the left ventricle
• 3. Then, the massive left ventricle is activated
• The impulse slowly travels to the left ventricle through the myocardium (not through the blocked left bundle branch)
• A wide QRS complex ≥ 0.12s is generated
• The main heart vector (V_H) is generated, which points to the left (2)
• Imagine the vectors in a 3D space
• If the vector points toward the ECG lead, a positive deflection is generated
• If the vector points away from the ECG lead, a negative deflection is generated

Septal Q Wave

• Forms in lateral leads (V5-V6, I, aVL)
• Occurs with proper activation of the ventricular septum (from left to right):
• With intact bundle branches
• Also with right bundle branch block
• Not with LBBB (the septum is activated from right to left)
• In LBBB, the septum is activated from right to left
• Thus, no Q wave forms in lateral leads
• It may only form in lead aVL

ECG and LBBB

• Widened QRS ≥ 0.12s (≥ 3 squares)
• Dominant S wave (V1)
• Dominant R waves (aVL, I, V5, V6)
• Absence of Q wave laterally (I, V5, V6) in lateral leads
• In aVL, a q wave may be present
• Prolonged R peak > 60ms
• The duration from the start of the QRS complex to the R peak is more than 60ms
• Discordance
• ST segment and T waves are oriented in the opposite direction to the QRS complex.
• Reduced R wave progression
• Left axis deviation
  
  
• If LBBB is newly developed, consider:
• anterior wall infarction (ST elevations in V1-V3)
• The Sgarbossa criteria are used to diagnose infarction in the presence of LBBB

QRS Variants in LBBB

Lead V1 (V2)

• QRS can have the following configurations:
• qrS "W"
• rS
• Q (QS)

qrS "W" Configuration (V1)

rS Configuration (V1)

Q (QS) Configuration (V1)

Lateral Leads (V5-V6, I, aVL)

• QRS can have the following configurations:
• RsR "M"
• R (with notch)
• R (monophasic)
• RS

RsR "M" Configuration (aVL)

R Configuration (with notch) (V6)

R Configuration (monophasic R) (V6)

RS Configuration (V5)

Incomplete LBBB (V6)

• QRS width 0.1s
• Dominant S (V1)
• Discordance
• Positive QRS and Negative T waves

Complete LBBB (V6)

• QRS width 0.2s
• Dominant S (V1)
• Discordance
• Positive QRS and Negative T waves

Incomplete Left Bundle Branch Block (iLBBB)

• Narrow QRS < 0.12s
• The width of the QRS is the main parameter distinguishing iLBBB from complete LBBB
• Deep S (V1)
• Dominant R (V6)
• Discordance
• QRS complexes are oriented opposite to T waves (negative QRS and positive T waves)
• Best assessed in (V1-V3)
• If LBBB is newly developed, consider:
• anterior wall infarction (ST elevation V1-V3)
• Diagnosis of infarction in LBBB can be assisted by Sgarbossa criteria
• Laterally (aVL, I, V5, V6) descendant ST depressions are a sign of left ventricular overload

Left Bundle Branch Block (LBBB)

• Wide QRS ≥ 0.12s
• Deep S (V1)
• Dominant R (V6)
• Discordance
• QRS complexes are oriented opposite to T waves (negative QRS and positive T waves)
• Best assessed in (V1-V3)
• If LBBB is newly developed, consider:
• anterior wall infarction (ST elevation V1-V3)
• Diagnosis of infarction in LBBB can be assisted by Sgarbossa criteria

Left Ventricular Hypertrophy

• This EKG resembles LBBB, but:
• There are huge S and R waves in the precordial leads
• It indicates left ventricular hypertrophy because the voltage criteria are met:
• Positive Sokolow index: S (V2) + R (V6) > 35mm
• Positive McPhie index: S (V2) + R (V6) > 40mm
• Left axis deviation
• In the lateral leads (aVL, I, V5, V6) there are descending ST depressions
• These arise from left ventricular overload
• Rarely, hypertrophy and LBBB can occur simultaneously