Last reviewed 12/2020

If there is left bundle branch block then depolarisation has to occur from right to left instead of the normal left to right.

Left bundle branch block (LBBB), unlike right bundle branch block, is always an indication of heart disease, usually on the left side.

The most common causes of LBBB are:

  • coronary artery disease
  • hypertensive heart disease
  • cardiomyopathy

LBBB and coronary heart disease:

  • the Framingham study revealed that during an18 year period, 89% of those individuals who developed LBBB also developed some apparent cardiovascular abnormalities. Also within a decade of the development of LBBB, 50% of these individuals had died secondary to a cardiovascular cause (2)
  • many studies that have shown that patients who have acute myocardial infarction and left bundle branch block (BBB) on admission electrocardiogram have been found to have unfavorable short- and long-term outcomes (1,2,3,4)
    • a study has shown that the presence of LBBB on admission electrocardiogram is an independent predictor of in-hospital mortality in patients who have AMI and undergo primary angioplasty (5)
  • LBBB is seen in acute myocardial infarction in 2-4% of cases
  • in a series of patients studied in the cardiac catheterization laboratory, patients with LBBB and concomitant left axis deviation have been found to be significantly more likely to have organic heart disease, with a sensitivity of 42% and a specificity of 92% (3)


  • assessment of left ventricular hypertrophy (LVH) in presence of LBBB
    • usual voltage criteria no longer apply -however an S wave in V2 plus an R wave in V6 of > 45 mm has been found to be highly specific and slightly less sensitive for LVH in the presence of LBBB

Click here for an example ECG of LBBB


  1. Harrigan RA et al. Electrocardiographic manifestations: bundle branch blocks and fascicular blocks. The Journal of Emergency Medicine 2003;25(1):67-77
  2. Schneider JF, Thomas HE, Kreger BE, McNamara PM, Kannel WB. Newly acquired left bundle-branch block: the Framingham study. Ann Intern Med 1979;90:303-10.
  3. Parharidis G, Nouskas J, Efthimiadis G, et al. Complete left bundle branch block with left QRS axis deviation: defining its clinical importance. Acta Cardiol 1997;52:295-303.
  4. Woo KS Norris RM. Bundle branch block after myocardial infarction - a long term follow-up. Aust N Z J Med 1979;9 : 411-416
  5. Guerrero M et al. Comparison of the Prognostic Effect of Left Versus Right Versus No Bundle Branch Block on Presenting Electrocardiogram in Acute Myocardial Infarction Patients Treated With Primary Angioplasty in the Primary Angioplasty in Myocardial Infarction Trials. American Journal of Cardiology 2005; 96(4): 482-488