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Atrioventricular Block, Second Degree–Mobitz Type II in Dogs

A dog's heart is divided into four chambers. The two top chambers are the atria (singular: atrium), and the bottom chambers are the ventricles. The heart has an electrical conduction system that is responsible for controlling the heart rate. This electrical conduction system generates electrical impulses (waves), which propagate throughout the musculature of the heart, stimulating the heart's muscles to contract and push blood through the interior arteries and out into the body. There are two nodes (masses of tissue) present in the heart that play an important role in this conduction system. The sinus node, or sinoatrial (SA) node, is a clustered collection of similar cells located in the right atrium, its purpose being to generate electrical impulses and to serve as the heart's pacemaker. The other node is called the atrioventricular (AV) node. Like the SA node, it is a clustered collection of similar cells situated in the right atrium, close to the ventricle. The AV node receives impulses from the SA node, and after a small delay, directs the impulses to the ventricles. This delay allows for the atrium to eject blood into the ventricle before the ventricular muscles contract. The AV node can also take the place of the SA node as the heart's pacemaker, should the SA node be affected adversely by a pathological condition of the heart.

Second degree AV block in dogs is a disease in which the above mentioned electrical conduction system goes off course, as some impulses are not passed down from the atria to the ventricles, thus impairing contraction and pumping functions of the heart muscles. AV block is rare in healthy dogs, but older dogs are at higher risk. American cocker Spaniels, pugs, and dachshund breeds are known to be predisposed to second degree AV block.

Symptoms and Types

Some dogs may remain asymptomatic while others may show the following symptoms:

• Weakness
• Lethargy
• Sudden collapse
• Syncope

In case of digoxin intoxication (a drug used to treat many cardiac diseases and which is sometimes over-dosed), an animal may show the following symptoms:

• Vomiting
• Poor appetite
• Diarrhea
• Symptoms related to the underlying disease

Causes

• Inheritable in pugs
• Involvement of non-cardiac diseases
• Age related degeneration within the cardiac conduction system in older dogs
• Drug side effects (e.g., digoxin, a drug used to treat many cardiac diseases)
• Cardiac neoplasia
• Infections involving heart (e.g., bacterial, viral, parasitic)
• Cardiomyopathy (heart muscle disease)
• Trauma

Diagnosis

You will need to give a thorough history of your dog's health, onset of symptoms, and possible incidents that might have preceded this condition. A previous illness or treatment history is important in these cases. After performing a complete physical examination, your veterinarian will measure your dog's arterial blood pressure to check for hypertension (high blood pressure) related to cardiac disease. Laboratory tests include the standard complete blood count, biochemistry profile, and urinalysis. These tests are of importance in the diagnosis of this problem as there are some biochemical changes that can predispose your dog to AV block. For example, if digoxin toxicity is suspected, the level of digoxin will be measured in your dog's serum. More specific tests may be conducted to evaluate the presence of infectious disease or parasitism. Blood culture/sensitivity tests will show evidence of a type of organism that is involved in infection and its sensitivity to various antibiotics.

Other diagnostic tools that are important for the evaluation of the heart's structural and functional parameters include electrocardiography (ECG) and echocardiography for measuring the heart's electrical impulses.