Heart Block or Conduction Delay (Left Bundle) in Cats

Left Bundle Branch Block (LBBB) in Cats

Left Bundle Branch Block (LBBB) is a defect in the heart’s electrical conduction system. It occurs when the left ventricle (one of the cat's four heart chambers) is not directly activated by the electric impulses through the left posterior and anterior fascicles of the left bundle branch, causing the deflections in the electrocardiographic tracing (QRS) to become wide and bizarre. LBBB may be complete or partial in nature.

Symptoms and Types

Often, no specific symptoms are seen that can be attributed to LBBB, only those that are related to the underlying disease causing the defect.


  • Cardiomyopathy
  • Cancerous tumors
  • Direct or indirect cardiac trauma (e.g., hit by car and cardiac needle puncture)
  • Narrowing just below the aortic valve, which supplies the body with oxygenated blood (subvalvular aortic stenosis)
  • Replacement of heart muscle with scar tissue (fibrosis)
  • Ischemic cardiomyopathy (i.e., hardening or thickening of the coronary arteries, death of heart muscle due to lack of oxygen)


You will need to give a thorough history of your cat’s health, including the onset and nature of the symptoms, to the veterinarian. He or she will then perform a complete physical examination, as well a biochemistry profile, urinalysis, and complete blood count (CBC) -- the results of which are typically non-specific.

Left bundle branch block is often only found accidentally, perhaps while performing an echocardiogram. In the case of this defect, he or she may identify structural defects in the heart without left-side enlargement. Thoracic and abdominal radiography may also show masses and other abnormalities, while Holter monitoring may reveal intermittent LBBB.

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