What is First-Degree Atrioventricular Block?
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.Much like it sounds, an atrioventricular block is either a partial or complete interruption of impulse transmissions between the atria (the upper cavities of the heart) and the ventricles (the two large chambers of the heart). Atrioventricular block is classified into three categories—first-degree, second-degree, and third-degree. First and second-degree atrioventricular blocks are partial and third-degree atrioventricular block is complete.
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Symptoms of First-Degree Atrioventricular Block in Dogs
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:
- Presyncope (light-headedness, muscular weakness, blurred vision and feeling faint)
- Syncope (loss of consciousness due to drop in blood pressure)
- Rapid heart rate
- Slower heart rate
- Heart failure
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.
Causes of First-Degree Atrioventricular Block in Dogs
- 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
Diagnosis of First-Degree Atrioventricular Block in Dogs
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
- X-Rays of the heart
Treatment of First-Degree Atrioventricular Block in Dogs
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 of First-Degree Atrioventricular Block in Dogs
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.
First-Degree Atrioventricular Block Questions and Advice from Veterinary Professionals
Took my 11 year old dachshund in for teeth cleaning. After receiving pre-op HR dropped to 40's Did not continue with medication ---gave atropine. First time has had this reaction to pre-op. Will f/u with cardiologist but what do I need to know.
In reading your information looks like a First Degree Atrioventricular Block but will it progress?
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