Critical Care Trauma Centre


Question of the Week: March 3, 2000

What are the criteria for left bundle branch block? 


  • broad QRS > .11 seconds
  • initial negative deflection in V1; V1 is mainly negative
  • broad R wave in leads I, aVL and V6, without a Q or S wave
  • there may be secondary T wave changes

A large lesion is required to block the entire left bundle and is generally associated with very significant heart muscle damage.

Normal Conduction:
Normally, the septum depolarizes before the ventricles, with septal activation being initiated from the left bundle towards the right.

Normally, the right and left ventricles depolarize immediately following septal depolarization. Although both ventricles normally depolarize together, the left ventricle sends a stronger electrical signal.  The sum of the forces from the left ventricle are greater than from the right, therefore, the right ventricular depolarization is usually "hidden" under the stronger left ventricular depolarization wave.  The QRS displays the dominant left ventricular force.

Conduction in Left Bundle Branch Block
In left bundle branch block, the initial left to right depolarization of the septum is blocked.  Consequently, the initial depolarization of the septum occurs from the right bundle towards the left.  Following septal depolarization, the ventricles depolarize.  Because the left bundle branch is blocked, the impulse travels only down the right bundle.  The right ventricle depolarizes normally, while the left ventricle depolarizes afterward, through the "back door" route. The prolonged time for depolarization of both ventricles widens the QRS >.11 seconds.  The asymmetrical (left after right) ventricular depolarization gives the QRS an abnormal pattern in the extreme right (V1) or extreme left (V6) leads.

Right Ventricular Lead
The V1 lead is closest to the right ventricle. It will identify depolarization of the right ventricle  (current flowing towards the lead) as a positive deflection and left ventricular depolarization (current flowing away from the lead) as a negative deflection.

Normal V1
In a normal V1 pattern, the QRS begins with a small initial "r" wave, indicating depolarization of the septum towards the lead.  This is followed by simultaneous transmission of the impulse through the right and left bundle branches, producing simultaneous left and right ventricular depolarization.  Because the left ventricle sends a stronger impulse, the net sum of the ventricular forces is away from V1, producing a downward deflection (or S).  The right ventricular depolarization wave (upward deflection) is buried in the S wave. Thus, the normal QRS pattern in V1 is rS.

V1 in Left Bundle Branch Block (LBBB)
In LBBB, the septum depolarizes abnormally, from the right bundle to the left.  This produces an initial negative deflection in V1 (away from the lead).  Because the left bundle is blocked, the impulse then travels towards the right bundle branch only, producing an upward depolarization as the right ventricle depolarizes towards V1.  Following depolarization of the right ventricle, the left ventricle depolarizes (away from V1), producing a negative deflection or S wave.  Thus, LBBB produces a V1 QRS pattern with an initial negative deflection, with a QRS that is predominantly negative. Because depolarization is abnormal, abnormal repolarization may also be seen by T wave changes.

Normal I, aVL and V6
Leads I, aVL and V6 are in the best position to observe "leftward" depolarization.   A small initial "q" wave may be present normally, reflecting the initial depolarization of the septum from left to right (or away from I, aVL and V6).  The right and left ventricles then depolarize simultaneously.  Because both ventricles depolarize within the same period of time, the QRS is narrow (< .12 seconds).  Because the left ventricular force is strongest, leads I, aVL and V6 display a tall "R" wave.   The right ventricular depolarization wave (away from I, aVL and V6) is buried in the R wave. The normal pattern is a narrow "R" or narrow qR pattern.

Leads I, aVL and V6 in Left Bundle Branch Block (LBBB)
With LBBB, the initial septal depolarization is from right to left.  This may be seen as an initial R wave in the left sided leads I, aVL and V6.  The right ventricle then depolarizes away from I, aVL and V6, producing a brief downward deflection.  As the left ventricle depolarizes after the right ventricle, a second R wave or upward deflection demonstrates impulse transmission towards the left sided leads.  This results in a wide QRS with an RR' pattern.

Alternatively,  the right ventricular depolarization may not be detected in I, aVL and V6.  This would result in a broad R wave versus RR'.

Normal V1 Appearance V1 Appearance in LBBB
  • rS pattern, narrow QRS 
  • the initial small "r" is produced as the septum depolarizes towards V1 
  • the left and right ventricles depolarize simultaneously, with the stronger left signal causing a net downward deflection (S wave) 
  • wide QRS > .11 seconds
  • initial negative deflection
  • predominantly negative QRS
  • T wave changes 

Normal I, aVL and V6 Appearance Leads I, aVL and V6 in RBBB
  • narrow QRS, qR or R pattern
  • a small initial "q" may be present, representing septal depolarization from left to right (away from I, aVL and V6)
  • as the ventricles depolarize together, the stronger net force of the left ventricle is displayed as an R
  • broad QRS > .11 seconds
  • broad R wave or RR'


Conover, M. (1990). Pocket Guide to Electrocardiography. Mosby: Toronto.

Conover, M. (1984). Understanding Electocardiography (4th Edition). Mosby: Toronto.

Brenda Morgan
Clinical Educator, CCTC
March 3, 2000
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