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Heart Block or Conduction Delay (Left Bundle) in Dogs

Left Bundle Branch Block (LBBB) in Dogs

Left Bundle Branch Block (LBBB) in dogs refers to an anomaly in the heart’s electrical conduction system where the left ventricle, one of the four heart chambers, fails to receive direct activation from electric impulses through the left posterior and anterior fascicles of the left bundle branch. Consequently, this leads to widened and irregular deflections in the electrocardiographic tracing (QRS). LBBB can manifest as either complete or partial in its presentation.

Symptoms and Types

In many cases, there are no distinct symptoms directly associated with LBBB; rather, any observable symptoms are typically linked to the underlying disease responsible for 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)


When visiting the veterinarian, it’s important to provide a comprehensive history of your dog’s health, detailing the onset and characteristics of any symptoms. The veterinarian will conduct a thorough physical examination along with various tests including a biochemistry profile, urinalysis, and complete blood count (CBC), which may yield nonspecific results.

Left bundle branch block is often discovered incidentally, possibly during an echocardiogram. In cases of this defect, structural abnormalities in the heart may be identified without left-side enlargement. Additionally, thoracic and abdominal radiography may reveal masses or other irregularities, while Holter monitoring could uncover intermittent LBBB.


The focus of treatment is on addressing the underlying cause.

Living and Management

LBBB itself isn’t typically life-threatening, and resolving the underlying cause often leads to complete resolution of the issue. However, if left unaddressed, LBBB can progress to more serious heart rhythm disturbances or even complete heart block.

Regular follow-up exams may be necessary to monitor the disease’s status and your dog’s response to treatment. Dietary adjustments aren’t usually needed unless specifically recommended to manage the underlying condition.

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