Heart Block (Mobitz Type I) in Dogs

Atrioventricular Block, Second Degree–Mobitz Type I in Dogs

The sinoatrial node (SA Node, or SAN), also called the sinus node, is the initiator of electrical impulses within the heart, triggering the heart to beat, or contract, by firing off electrical surges. The atria, the two upper chambers of the heart that receive  and send out blood, are prompted into action by the SA node's electrical impulse, which then activates the atrioventricular node (AV node). The AV node conducts the normal electrical impulses from the atria to the ventricles, coordinating the mechanical activity so that the atria have forced the blood down into the ventricles before the ventricles contract to send the blood out into the body through the pulmonary artery and aortic artery.

Second-degree atrioventricular block occurs when the electrical conduction within the AV node is delayed. 

Most dogs with this condition show no signs, appearing to be in perfect health. The condition is also rarely noted in geriatric cocker spaniels and dachshunds due to fibrosis.  Low calcium levels and certain drugs (e.g., digoxin, bethanechol, physostigmine, pilocarpine) may predispose some animals to second-degree AV block–Mobitz Type 1.  Second-degree AV block–Mobitz Type 1 can also be brought about by diseases unrelated to the heart.

Symptoms and Types

• Most affected dogs show no symptoms
• If induced by digoxin (a heart medication) overdose, dog may have vomiting and lack of appetite
• Fainting
• Weakness

Causes

• May occur in normal, healthy animals
• Certain drugs can affect the AV node
• Diseases that are not directly related to the heart
• Cardiac neoplasia – masses of the heart

Diagnosis

You will need to provide a thorough history of your dog's health leading up to the onset of symptoms. Your veterinarian will perform a full physical exam, with a chemical blood profile and a complete blood count. A thorough history from you will allow your veterinarian to rule out masses, gastrointestinal disorders, high pressure in the eye and upper airway disease. X-rays may help detect some of these disorders as well. An atropine response test, which increases the firing action of the sinoatrial node and the conduction of the AV node will indicate if the disease is originating from the heart. 

An electrocardiogram (ECG, or EKG) recording can be used to examine the electrical currents in the heart muscles, and may reveal any abnormalities in cardiac electrical conduction (which underlies the heart’s ability to contract/beat).

Treatment

Treatment will vary depending on the underlying disease causing the second-degree–Mobitz Type 1 atrioventricular block.  Most of the time however, the affected dog will be otherwise healthy and no treatment will be needed. 

Living and Management

Your veterinarian will guide you through a health plan for your dog that emphasizes the necessary diet and activity guidelines that will effectively treat the underlying cause of disease, if one is present.