Left Bundle Brunch Block is a conduction abnormality seen on the Electrocardiogram (ECG). In this condition, activation of left ventricule is delayed, which causes the left ventricule to contract later than right Ventricule of the heart.

Notched R wave and rS wave. no Septal q wave in V6.

Right Ventricular activation begins First than left Ventricular activation start. Spread of Depolarization is right to left results small right Septal r wave with Deep S wave formed in Lead V1 due to right Septal activation occurs normally but left septal activation doesn’t occurs with LBBB as a result no Septal q wave in V6.

Diagnosis of left Bundle Branch Block: Requires all of the following

  1. QRS duration equal or more than 0.12 second
  2. Broad notch R wave in lateral precordial leads ( V5 and V6) and usually limb leads l and aVL.
  3. Small or absent initial r waves in right precordial leads (V1 and V2) followed by deep S wave.
  4. absent septal q wave in lateral leads. and
  5. Prolonged intrinsicoid deflection ( more than 0.06 second ) in V5 and V6.

Incomplete left Bundle Branch Block has typical morphology of the LBBB But the QRS duration is only 0.10 to 0.12 second.

QRS duration is between Normal with incomplete left Bundle Branch Block.

Clinical Significance of LBBB:

  • Usually appears in patients with underlying heart disease, although as many as 12% of patiens with LBBB have no demonstrable heart disease.
  • Even among persons without overt Heart disease, LBBB is associated with a higher than normal risk of cardiovascular and all cause of mortality.
  • It is also associated with higher than expected risks of high grade AV block and sudden cardiac death.
  • Among patients with coronary artery disease, presence of Left Bundle Branch correlates with more extensive disease, more severe LV dysfunction and reduced survival rates.
  • Left axis deviation is associated with more severe conduction system disease that includes the fascicle as well as main left Bundle,
  • whereas right axis deviation suggests dilated cardiomyopathy with biventricular enlargement .
  • Left Bundle Branch Block may simulate other ECG changes example: Myocardial infarction and left Ventricular hypertrophy.