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Heart Block (Mobitz Type I) in Dogs

Atrioventricular Block, Second Degree–Mobitz Type I in Dogs

Second-degree atrioventricular block, specifically Mobitz Type I, affects the electrical conduction within the atrioventricular (AV) node in dogs. The sinoatrial node (SA Node), responsible for initiating heartbeats, triggers the atria to contract through electrical impulses. Subsequently, the atrioventricular node (AV node) transmits these impulses from the atria to the ventricles, ensuring coordinated heart function.

In this condition, there is a delay in the electrical conduction within the AV node. While many dogs with this block show no symptoms and seem healthy, it is occasionally observed in elderly cocker spaniels and dachshunds due to fibrosis. Certain factors like low calcium levels or the administration of specific drugs such as digoxin, bethanechol, physostigmine, or pilocarpine can predispose animals to this type of AV block. Additionally, unrelated diseases can also lead to Mobitz Type I second-degree AV block.

Overall, this condition may manifest subtly in affected dogs and is associated with various predisposing factors and health conditions.

Symptoms and Types

  • The majority of affected dogs exhibit no noticeable symptoms.
  • If triggered by an overdose of digoxin (a heart medication), dogs may experience vomiting and a decrease in appetite.
  • Symptoms such as fainting and weakness can occur in some cases.


Causes of second-degree atrioventricular block in dogs include:

  • Occurrence in otherwise healthy animals is possible.
  • Certain medications can influence the function of the atrioventricular (AV) node.
  • Diseases unrelated to cardiac issues can also contribute to this condition.
  • Cardiac neoplasia, referring to the presence of masses in the heart, is another potential cause.


To diagnose second-degree atrioventricular block in your dog, your veterinarian will begin by gathering a comprehensive history of your pet’s health leading up to the appearance of symptoms. A thorough physical examination will be conducted, along with a chemical blood profile and complete blood count. Your input regarding your dog’s history will aid in ruling out other potential causes such as masses, gastrointestinal disorders, elevated eye pressure, and upper airway disease. X-rays may also be utilized to detect certain conditions.

An atropine response test may be performed to assess the function of the sinoatrial node and the conduction of the atrioventricular node, helping to determine if the issue originates from the heart.

Additionally, an electrocardiogram (ECG or EKG) will be administered to analyze the electrical activity of the heart muscles, potentially revealing any abnormalities in cardiac electrical conduction, which is fundamental to the heart’s ability to contract and beat.


Treatment for second-degree atrioventricular block, specifically Mobitz Type 1, will depend on the underlying cause. In many cases, if the affected dog is otherwise healthy, no treatment will be necessary.

Living and Management

Your veterinarian will provide you with a health plan tailored to your dog, focusing on essential dietary and activity recommendations to effectively address any underlying causes of the disease, if applicable.

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