Bundle Branch Block

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This condition is an abnormal ventricular excitation as the result of blockage to the right bundle branch, the left bundle branch, or left anterior fascicular block.

Right bundle branch block is characterised by the failure or delay of conduction through the right bundle branch. The left ventricle is unaffected. Some causes of this condition include pulmonary embolism, heart worm, electrolyte disturbances, congenital diseases etc.

In left bundle branch block, there is failure of conduction through the left bundle branch and the right ventricle is unaffected. Causes of this include congenital abnormalities and myocardial diseases.

There can also be a left anterior fasicular block, which is characterised by the failure of conduction through the anterior fascicle of the left bundle branch. Causes of this include electrolyte abnormalities and cardiomyopathies e.g. in the cat.


The disease is described in both cats and dogs.

Clinical Signs

Many cases of bundle branch block are asymptomatic and are only detected on ECG. However, more severe cases will cause ventricular muscle damage and therefore signs of heart disease will occur. Some animals may present due to syncope, weakness or exercise intolerance.


An Electrocardiograph (ECG) is the diagnostic method of choice.

In a right bundle branch block, one may see a wide QRS complex and a wide S wave.
In left anterior fasicular block there will be a normal QRS wave, a tall R waves and deep S waves.


Unless the condition is congenital, treatment is by treating the underlying cause.


Ettinger, S.J. and Feldman, E. C. (2000) Textbook of Veterinary Internal Medicine Diseases of the Dog and Cat Volume 2 (Fifth Edition), W.B. Saunders Company

Ettinger, S.J, Feldman, E.C. (2005) Textbook of Veterinary Internal Medicine (6th edition, volume 2), W.B. Saunders Company

Fossum, T. W. et. al. (2007) Small Animal Surgery (Third Edition), Mosby Elsevier

Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition), Merial

Nelson, R.W. and Couto, C.G. (2009) Small Animal Internal Medicine (Fourth Edition), Mosby Elsevier.

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