Atrioventricular Block, Complete (Third Degree) in Dogs
The heart's sinoartial node (SA) is very much like a control center, responsible for controlling the heart rate. This electrical conduction system generates electrical impulses (waves), which propagate through the atrioventricular (AV) node and into the ventricles, stimulating the heart's muscles to contract and push blood through the interior arteries and out into the body.
Complete, or third-degree, atrioventricular block is a condition in which all impulses generated by the SA node are blocked at the AV node, leading to independent and non-coordinated beating of atria and ventricles.
Cocker spaniels, Pugs, and Doberman breeds are predisposed to heart defects leading to complete heart block. Third-degree atrioventricular block also occurs in older dogs more frequently.
Symptoms and Types
- Difficulty breathing
- Inability to perform routine exercise
- Slow heart rate (bradycardia)
- Congenital (present at birth) heart defects
- Idiopathic fibrosis (scarring of heart tissue due to unknown cause)
- Inflammation of heart (myocarditis)
- Inflammation of lining of heart (endocarditis)
- Infiltration of heart muscle by some abnormal substance or cancer (amyloidosis or neoplasia)
- Drug toxicity (i.e., digitalis)
- Electrolyte imbalances
- Heart attack (myocardial infarction)
- Lyme disease
- Chagas' Disease
You will need to give a thorough history of your dog’s health, including the onset and nature of the symptoms, to your veterinarian. He or she will then perform a complete physical examination, as well as a biochemistry profile, urinalysis, and complete blood count (CBC). Dogs suffering from infections of the heart will demonstrate high white blood cell count on blood testing, while biochemistry profile may reveal electrolyte imbalances.
Your veterinarian will record the electrocardiograph, or ECG, which is extremely beneficial to make an initial diagnosis. Echocardiography and Doppler ultrasound are performed in animals with abnormal ECG finding, and those with symptoms associated with heart issues.
The ultimate goal of therapy is to clear the blockage of electrical impulses at the AV node. To achieve this, a special device called a pacemaker is used to resolve the electrical impulse conduction problems and normalize the heart's beating. (Chest x-rays are taken to confirm the proper placement of pacemaker.) Both temporary and permanent pacemakers are available, and your veterinarian will recommend which will work best for your dog. The blockage can be rectified surgically too, but this is often riskier for the dog.
Living and Management
If your dog has had a pacemaker implanted, he will require extra care as well as cage rest. Typically, permanent pacemakers are placed in a pocket created surgically under the skin. To prevent the pacemaker from moving, a bandage is applied over the surgical wound for three to five days. Because pacemakers are battery operated, a malfunction can occur at any time; the pacemaker may also become infected, dislodged, or run out of battery. In such cases, the dog's heart may again go into a complete atrioventricular block. Therefore, it is vital that you restrict the dog's movements and monitor him for untoward symptoms.
Depending on the severity of the underlying disease, the dog's diet may require modifying. In addition, you will need to visit your veterinarian at regular intervals for ECG and chest radiography, which are used to assess proper pacemaker function. Unfortunately, long term prognosis of dogs with complete atrioventricular block is very poor.
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