Left bundle branch block (LBBB)


Left bundle branch block (LBBB) is a pattern seen on the surface ECG that results from interruption of normal electrical activity of the left ventricle. 

We can divide LBBB by QRS duration to:

  • Incomplete LBBB - QRS complex widening to 100 – 120ms and loss of the septal q wave in leads I and V5-6. There is a slowing or delay of conduction through the left bundle, or diffusely through the Purkinje system and the ventricular myocardium

  • Complete LBBB – QRS duration > 120 ms. Complete block of stimulus in the left bundle or within the left ventricular Purkinje network. Activation is initiated by conduction through the septum from the right-sided Purkinje system in a right to left direction (opposite to the normal activation – left to right)
  • This delay in left ventricular activation causes interventricular dyssynchrony, with the right ventricle contracting before the left ventricle, which has a deleterious effect on left ventricular function

ECG features

  • QRS duration > 120ms, slurring in the mid-portion of the QRS
  • QS or rS pattern in V1 – usually deep and abnormal waves
  • Single broad R wave in leads I, aVL and V5,V6 
  • aVR – a QS pattern with a positive T wave
  • ST and T wave abnormalities - T wave inversions and ST segment depression, in the opposite direction from the QRS complex
  • Absent Q wave in lead V6 – septum depolarizes from right to left
  • Leftward QRS axis

Figure 1 QRS morphology in leads I, aVL, V5 and V6

  • monomorphic broad R waves
  • Prolonged time to peak > 60 ms in lead V6 (delayed intrinsicoid deflection)

Figure 2 QRS morphology in leads V1-V2

  • deep S waves with a small r wave and positive T wave
  • The electrocardiographic changes in LBBB can cause diagnostic problems in a variety of clinical conditions such as myocardial ischemia or ventricular hypertrophy.

ECG 1 Left bundle branch block (LBBB)

  • sinus rhythm with 1st degree AV block and Left bundle branch block
  • QRS duration 160ms
  • PR interval 240ms
  • broad monophasic R waves in leads I, aVL and V6
  • deep broad S waves in leads V1-V4
  • Axis deviation to the left – 32°

ECG 2  Left bundle branch block (LBBB)

  • sinus rhythm with left bundle branch block, rate 88 bpm
  • QRS duration 122 ms
  • QS waves in leads V1, V2
  • monophasic broad R waves in leads I and aVL
  • rSR QRS morphology in lead V6
  • Q waves present in the inferior leads II, III, aVF suggest prior inferior wall infarction


  • The prevalence of LBBB increases with age, occurring in less than 1 percent of the general population
  • Conduction in the left bundle branch can be compromised by both structural and functional factors:

    1) Structural heart disease: 
    - usually results from slowly progressive degenerative myocardial fibrosis (hypertension, coronary artery diseases, cardiomyopathies)
    - may result following an acute myocardial insult such as myocardial ischemia or myocardial infarction (approx. 7% of acute MI)
    - may also develop following certain cardiac surgeries or procedures

    2) Functional LBBB: 
    - Ashman’s phenomenon – functional RBBB as a result of a long preceding R-R interval following by a short cycle


  • The presence of LBBB is almost always an indication of underlying pathology and the patient should be assessed accordingly 
  • For asymptomatic patients with an isolated LBBB and no other evidence of cardiac disease, no specific therapy is required
  • in patients with LBBB and heart failure, cardiac resynchronization therapy may be indicated

Picture 1 Cardiac resynchronization therapy recommendation as per ESC guidelines


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