Heart Block or Conduction Delay (Left Anterior) in Dogs


PetMD Editorial

Updated Aug. 16, 2022

Left Anterior Fascicular Block in Dogs

Left Anterior Fascicular Block (LAFB) is a heart problem that originates due to an abnormally functioning conduction system, which is responsible for generating electrical impulses (waves) that propagate throughout the musculature of the heart, stimulating the heart muscles to contract and pump blood. If the conduction system is affected, not only will contraction of the heart muscles be affected, but the timing and frequency of heartbeats too. Fortunately, this condition is uncommon in dogs.

Symptoms and Types

There are no specific symptoms related to this condition itself, rather, related to the underlying cause of the LAFB.


  • Heart surgery
  • Electrolyte abnormalities
  • Heart problems (e.g., ischemic cardiomyopathy, ventricular septal defect, aortic valvular disease, etc.)


You will need to give a thorough history of your dog’s health, including the onset and nature of the symptoms. 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 may reveal electrolyte imbalances.

Electrocardiography, however, remains the most important tool for diagnosis. Your veterinarian will record your dog’s electrocardiogram (ECG) and compare it with a normal ECG to see if any abnormalities are present. Further evaluation of the heart is usually done with echocardiography. This helps in the diagnosis of underlying heart disease or problem, and the extent of heart involvement.

Your veterinarian will also take X-rays of both the thoracic and abdominal regions to see if there are any abnormal masses, tumor, foreign body, and/or abnormal heart position.


The form of treatment recommended for your dog grossly depends on the diagnosis and may vary patient to patient. Therefore, correctly diagnosing the underlying cause of the LAFB is paramount.

Living and Management

Prognosis and follow-up exam schedules varies greatly depending on the underlying disease. However, in cases of severe or advanced heart problems or cancer, prognosis is not good. Consult with your dog's veterinarian in all cases.

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