First-Degree Atrioventricular Block in Dogs

Atrioventricular block is classified into three degrees of severity: first-degree, second-degree (which is broken down into Mobitz Type I and Mobitz Type II), and third-degree. First and second-degree consist of partial interruption of impulse transmissions between the atria and ventricles, while a third-degree atrioventricular block is a complete interruption of these transmissions. Symptoms are uncommon for First-Degree and Second-Degree Mobitz Type I, while Second-Degree Mobitz Type II and Third-Degree blocks may exhibit symptoms, including light-headedness, rapid heart rate, fatigue, and heart failure. Treatment is typically unnecessary for First-Degree and Second-Degree Mobitz Type I. Treatment for Mobitz Type II and Third-Degree requires the insertion of a pacemaker.

Most dogs with First-Degree atrioventricular block will not show symptoms.

Dogs with Second-Degree Mobitz type II and Third-Degree atrioventricular blockage may show the following symptoms:

• Light-headedness
• Presyncope (light-headedness, muscular weakness, blurred vision and feeling faint)
• Syncope (loss of consciousness due to drop in blood pressure)
• Rapid heart rate
• Fatigue
• Lethargy
• Slower heart rate
• Heart failure

Types

While this article deals primarily with first-degree atrioventricular block, there are three different types of atrioventricular block:

• First-degree - First-degree atrioventricular block rarely exhibits any symptoms and is typically found in younger dogs with a high vagal tone (refers to the activity of the vagus nerve on the brainstem). Treatment isn’t necessary unless underlying conditions warrant it.
• Second-degree - Second-degree atrioventricular block can be broken into two categories, Mobitz type I and Mobitz type II. Mobitz type I also occurs in younger dogs who are more active. Treatment is not necessary unless the block causes other problems, such as bradycardia. Mobitz type II is caused by disease, though there may be no symptoms exhibited. There is a risk of developing complete atrioventricular block.
• Third-degree - In third-degree atrioventricular there are no impulse transmissions between the atria and ventricles. Mild symptoms will likely appear. Treatment is required.

• Idiopathic fibrosis (a disease in which lung function declines progressively)
• Sclerosis (atypical hardening of body tissue) of the conduction system (cardiac muscles in the walls of the heart)
• Ischemic heart disease (hardening of the arteries)
• Drugs, specifically beta-blockers, calcium channel blockers, digoxin, and amiodarone, which are used to treat heart-related problems
• Increased activity of the vagus nerve, located in the brainstem
• Valvulopathy (stiff/inflamed heart valve)
• Other heart or genetic disorders

Your veterinarian will ask you questions regarding your pet’s health history to rule out any other potential heart diseases or problems. In addition, your veterinarian will use the following tests:

• Physical exam
• Complete blood count
• Chemistry profile
• Electrocardiogram
• X-Rays of the heart
• Echocardiogram

Typically there is no treatment necessary for the first-degree atrioventricular block. If the atrioventricular block is the result of drugs that are being administered, usage should be discontinued.

Depending on what the underlying cause of the condition is, treatment specific to that treatment may be necessary.

In Second-Degree Mobitz Type II and Third-Degree atrioventricular block, pacemaker insertion may be necessary.

Recovery and management will vary depending on the cause of the condition and any treatments administered. Your veterinarian will want to monitor the condition regularly so that they are able to adequately address any changes in the condition when they occur. If the atrioventricular block appears to be caused by drugs, your veterinarian will likely discontinue use and check back periodically to see if the condition has improved, stayed the same, or worsened.