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Heart Block (Mobitz Type II) in Dogs

Atrioventricular Block, Second Degree–Mobitz Type II in Dogs

In dogs, a condition known as Atrioventricular Block, Second Degree – Mobitz Type II affects the heart’s electrical conduction system, which is vital for controlling the heart rate. The heart consists of four chambers: two atria (singular: atrium) on top and two ventricles on the bottom. This system generates electrical impulses that spread throughout the heart’s muscles, triggering contractions and facilitating blood circulation. Two important nodes in this system are the sinus node (or sinoatrial node) located in the right atrium, responsible for generating impulses and acting as the heart’s pacemaker, and the atrioventricular (AV) node, also situated in the right atrium near the ventricle. The AV node receives impulses from the sinus node and after a brief delay, transmits them to the ventricles, allowing for coordinated contraction and efficient blood pumping. In case of any malfunction affecting the sinus node, the AV node can take over as the pacemaker.

Second degree AV block in dogs occurs when some impulses fail to pass from the atria to the ventricles, disrupting the heart’s contraction and pumping functions. Although rare in healthy dogs, older dogs are more susceptible to this condition. Certain breeds such as American Cocker Spaniels, Pugs, and Dachshunds are predisposed to second degree AV block.

Symptoms and Types

While some dogs may display no symptoms, others might exhibit the following signs:

  • Weakness
  • Lethargy
  • Sudden collapse
  • Syncope (fainting)

In instances of digoxin intoxication, a medication used to manage various cardiac conditions and occasionally administered in excessive doses, the animal may present the following symptoms:

  • Vomiting
  • Decreased appetite
  • Diarrhea
  • Symptoms associated with the underlying disease


The causes of second degree AV block in dogs include:

  • Inheritance in pugs
  • Impact of non-cardiac diseases
  • Age-related degeneration within the cardiac conduction system in older dogs
  • Side effects of medications like digoxin, commonly used to treat various cardiac conditions
  • Cardiac neoplasia
  • Heart infections such as bacterial, viral, or parasitic infections
  • Cardiomyopathy (a disease affecting the heart muscle)
  • Trauma


To diagnose second degree AV block in your dog, your veterinarian will gather a comprehensive history of your dog’s health, including the onset of symptoms and any previous illnesses or treatments. A thorough physical examination will be conducted, including measurement of arterial blood pressure to assess for hypertension related to cardiac disease.

Laboratory tests such as a complete blood count, biochemistry profile, and urinalysis will be performed to identify any biochemical changes that may predispose your dog to AV block. For instance, if digoxin toxicity is suspected, the level of digoxin in your dog’s serum will be measured. Additional tests may be conducted to detect the presence of infectious diseases or parasitism, including blood culture/sensitivity tests to identify the type of organism involved in infection and its sensitivity to antibiotics.

Diagnostic tools such as electrocardiography (ECG) and echocardiography will be utilized to evaluate the heart’s structural and functional parameters, including measuring the heart’s electrical impulses.


Second degree AV block in dogs is typically not aggressively treated. If the heart rate remains sufficient to pump an adequate amount of blood for normal bodily functions, no specific treatment may be necessary. However, if an underlying disease is identified as the cause of the AV block, your veterinarian will address it accordingly.

Living and Management

Special nursing care is typically not necessary for dogs with second degree AV block. If symptoms persist, strict cage rest may be advised by your veterinarian. Additionally, if an underlying disease is identified, your veterinarian may recommend a specialized diet to complement treatment.

It’s important to note that if an underlying cause is contributing to the AV block, addressing it is essential for resolving the issue. Your veterinarian can discuss various treatment options available.

In persistent cases where medication alone is insufficient for long-term management, a permanent pacemaker may be required. This small device is implanted under the skin in your dog’s thoracic (chest) cavity to regulate abnormal heart rhythms. Regular visits to your veterinarian for evaluations of your dog’s cardiac health status and progress are crucial, as untreated AV block can lead to serious complications if not monitored closely.

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