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Atrioventricular Block, Complete (Third Degree) in Dogs

The heart's sinoartial node (SA) is very much like a control center, responsible for controlling the heart rate. This electrical conduction system generates electrical impulses (waves), which propagate through the atrioventricular (AV) node and into the ventricles, stimulating the heart's muscles to contract and push blood through the interior arteries and out into the body.

Complete, or third-degree, atrioventricular block is a condition in which all impulses generated by the SA node are blocked at the AV node, leading to independent and non-coordinated beating of atria and ventricles.

Cocker spaniels, Pugs, and Doberman breeds are predisposed to heart defects leading to complete heart block. Third-degree atrioventricular block also occurs in older dogs more frequently.

Symptoms and Types

• Weakness
• Coughing
• Difficulty breathing
• Inability to perform routine exercise
• Slow heart rate (bradycardia)
• Fainting

Causes

• Congenital (present at birth) heart defects
• Idiopathic fibrosis (scarring of heart tissue due to unknown cause)
• Inflammation of heart (myocarditis)
• Inflammation of lining of heart (endocarditis)
• Infiltration of heart muscle by some abnormal substance or cancer (amyloidosis or neoplasia)
• Drug toxicity (i.e., digitalis)
• Electrolyte imbalances
• Heart attack (myocardial infarction)
• Lyme disease
• Chagas' Disease

Diagnosis

You will need to give a thorough history of your dog’s health, including the onset and nature of the symptoms, to your veterinarian. He or she will then perform a complete physical examination, as well as a biochemistry profile, urinalysis, and complete blood count (CBC). Dogs suffering from infections of the heart will demonstrate high white blood cell count on blood testing, while biochemistry profile may reveal electrolyte imbalances.

Your veterinarian will record the electrocardiograph, or ECG, which is extremely beneficial to make an initial diagnosis. Echocardiography and Doppler ultrasound are performed in animals with abnormal ECG finding, and those with symptoms associated with heart issues.