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Heart Block (First-Degree) in Dogs

Atrioventricular Block, First Degree in Dogs

First-degree atrioventricular block in dogs occurs when the electrical conduction from the atria to the ventricles experiences a delay or prolongation. Normally, the heart’s contraction is initiated by an electrical impulse originating from the sinoatrial node, which stimulates the atria. This impulse then travels to the atrioventricular node before reaching the ventricles. On an electrocardiogram (EKG), first-degree AV block is indicated by a prolonged PR interval, which is the time between the P wave (the main electrical impulse) and the QRS complex (the heart beat).

This condition can be observed in young, healthy dogs due to a high vagal tone, which refers to impulses from the vagus nerve that inhibit heart beat. Additionally, it is commonly found in elderly Cocker Spaniels and Dachshunds with degenerative conduction system disease.

Symptoms and Types

Symptoms and types of first-degree atrioventricular block in dogs typically remain unnoticed. However, if the condition is triggered by an overdose of digoxin, a medication used for heart issues, symptoms such as loss of appetite, vomiting, and diarrhea may manifest.


Causes of first-degree atrioventricular block may vary, though it can occur even in otherwise healthy dogs. Dachshunds and Cocker Spaniels are particularly susceptible to this condition. Certain prescription medications like digoxin, bethanechol, physostigmine, and pilocarpine may also increase the likelihood of first-degree AV block in animals. Other common causes include:

  • Calcium deficiency
  • Degenerative diseases affecting the electrical conduction system
  • Hypertrophic cardiomyopathy
  • Inflammation of the heart
  • Infiltrative diseases such as tumors or amyloidosis
  • Additionally, intravenous administration of atropine, which is used to control spasms, may briefly prolong the PR interval, contributing to the condition.


For diagnosis, your veterinarian will conduct a comprehensive physical examination and gather a thorough background history of your dog’s health, including the onset of symptoms and any relevant details pointing to the underlying cause. Standard tests such as a chemical blood profile and complete blood count will be performed to identify imbalances or infections.

An echocardiogram (EKG) will be conducted to rule out specific types of heart diseases, while X-ray or ultrasound imaging may be used to visualize the internal structures of the heart, confirming or ruling out the presence of masses. It’s important to note that disorders like gastrointestinal issues, high eye pressure, and upper airway diseases can lead to this condition, though they are not directly related to heart function.

EKG recordings will help examine the electrical currents within the heart muscles, potentially revealing abnormalities in cardiac electrical conduction, which is fundamental to the heart’s ability to contract and beat.


Treatment for atrioventricular block will vary depending on the underlying disease causing it.

Living and Management

Living and management of your dog’s condition will involve following dietary guidelines prescribed by your veterinarian to address the underlying cause of the disease. Regular follow-up appointments with your veterinarian are essential to address any changes promptly. During these appointments, EKGs will be performed to monitor the progress of the heart’s ability to conduct properly.

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