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Atrioventricular Block, First Degree in Dogs

A normal heart contraction is caused by an electrical impulse originating from the sinoatrial node, stimulating the atria, traveling to the atrioventricular node and finally to the ventricles. First-degree atrioventricular block is a condition in which the electrical conduction from the atria to the ventricles is delayed, or prolonged. On an electrocardiogram (EKG) this shows as a prolonged PR interval -- the time between the main electrical impulse, called the P wave, and the QRS complex, which is recognized as the heart beat.

First-degree AV block may be found in young, healthy dogs due to a high vagal tone (impulses from the vagus nerve that produce an inhibition in the heart beat), and it is also often noted in elderly Cocker Spaniels and Dachshunds with degenerative conduction system disease.

Symptoms and Types

Most dogs with this condition will not show signs of the condition. However, if induced by digoxin (heart medication) overdose, there may be a loss of appetite, vomiting, and diarrhea.

Causes

Although it may occur in otherwise healthy dogs, Dachshunds and Cocker Spaniels are known to be more prone to first-degree AV block. Additionally, prescription medication such as digoxin, bethanechol, physostigmine, and pilocarpine, may predispose animals to first-degree AV block. Some other common causes for the condition include:

• Calcium deficiency
• Degenerative disease of the electrical conduction system
• Hypertrophic cardiomyopathy
• Inflammation of the heart
• Infiltrative diseases (tumors, amyloidosis)
• Atropine (used to control spasms) administered intravenously may also briefly prolong the PR interval

Diagnosis

In addition to the full physical exam, your veterinarian will take a complete background history of your dog's health, when the symptoms began and any other symptoms that may point your doctor to the underlying cause. Standard tests include a chemical blood profile and a complete blood count to check for imbalances or infections.

An echocardiogram (EKG) will be conducted to rule out certain types of heart diseases, and X-ray or ultrasound imaging can be used for internal imaging of the heart, confirming the presence of masses, or ruling them out. Gastrointestinal disorders, high pressure in the eye and upper airway disease are some of the diseases that can lead this disorder, all of which are unrelated (directly) to the heart.

An EKG recording can be used to examine the electrical currents in the heart muscles, and may reveal the exact abnormalities that are taking place in the cardiac electrical conduction (which underlies the heart’s ability to contract/beat).