Atrial Fibrillation in Dogs

By Malcolm Weir, DVM, MSc, MPH; Robin Downing, DVM, DAAPM, DACVSMR, CVPP, CRPP

The heart is a hollow organ with four separate chambers that are involved in pumping blood around the body. The bottom chambers are the right and left ventricles; the right ventricle pumps the blood to the lungs, and the left ventricle pumps blood to the rest of the body.

The top chambers are the right and left atria. The right atrium accepts blood from the general circulation, and the left atrium accepts blood from the lungs.

When the heart relaxes, the valves between the atria and the ventricles open, allowing the blood to move from the atria to the ventricles. When the heart is beating normally, the contractions of the atria and the ventricles are coordinated to move the blood smoothly around the body, as well as into the lungs where carbon dioxide is exhaled, and oxygen is picked up by the red blood cells.

Prior to having anesthesia, my senior dog had an electrocardiogram (ECG). The ECG showed an abnormal heart rhythm that my veterinarian called atrial fibrillation. What is atrial fibrillation?

Atrial fibrillation describes very rapid contractions or twitching of the heart muscle, specifically in the atria. The ventricles will then contract more rapidly than normal, but the rhythm may be either regular or irregular. Most of the time, atrial fibrillation in dogs occurs secondary to heart disease. Sometimes, in large breed dogs (Irish Wolfhound, Newfoundland, German Shepherd), atrial fibrillation occurs as a primary heart problem.

What causes atrial fibrillation?

Sometimes, no specific cause is determined for atrial fibrillation. In that case it is called idiopathic. Atrial fibrillation can be the result of chronic heart valve disease, or disease of the heart muscle itself (cardiomyopathy). Atrial fibrillation can also be caused by heart disease that is present at birth, but this is rare.

My dog had no signs there was anything wrong with his heart. What signs are typical in a dog with atrial fibrillation?

Most dogs who develop atrial fibrillation have underlying heart disease, so the signs that are observed are related to the underlying disease. If the dog is experiencing congestive heart failure, the heart cannot pump blood effectively enough to provide adequate oxygen to the tissues and to keep the body’s fluid balance as it should be.

"The dog may be exercise intolerant, becoming exhausted after very little exertion."

The dog may be exercise intolerant, becoming exhausted after very little exertion. The dog may cough or have difficulty breathing. Atrial fibrillation causes an erratic heart rhythm that may sound like an extra heart sound is present when heard through a stethoscope.

Your veterinarian may describe what is called a pulse deficit. This describes the situation in which there are fewer pulses felt in an artery (like the femoral artery in the rear leg) than are heard through a stethoscope.

Is atrial fibrillation treatable?

For a dog with atrial fibrillation, medications that slow the rate of conduction of electrical signals between the atria and the ventricles are used. These include beta-adrenergic blockers such as atenolol (brand name: Tenormin®), or calcium-channel blockers such as diltiazem (brand names: Cardizem®, Dilacor XR®, Tiazac®, Diltia XT®, Taztia XT®, Dilt-XR®). Digoxin (brand name: Lanoxin®, Lanoxicaps®, Toloxin®, Digitek®, Digox®) is another medication that is sometimes used for treatment of this condition.

Dogs with atrial fibrillation also often have underlying heart disease, meaning the focus of medical management also needs to center on treatment of this condition. For congestive heart failure, medication is given to relieve the body of excess fluid, to control hypertension (if present), and to help the heart beat more effectively.

For those dogs with primary atrial fibrillation (i.e., no other heart problems are present), cardioversion (using electrical or drug methods) can be attempted. This is a very specialized treatment and generally requires referral to a heart specialist to be performed.

"Your veterinarian may recommend a special diet for your dog with atrial fibrillation."

Your veterinarian may recommend a special diet for your dog with atrial fibrillation. If your dog is experiencing congestive heart failure, then a mild to moderate restriction of sodium in the diet may be in order.

Is there any monitoring or follow-up for dogs with atrial fibrillation?

Heart rate is monitored closely, and there will be follow-up ECGs to measure the success of treatment. In addition, radiographs (X-rays) of the chest and heart ultrasounds to assess heart health are recommended to monitor progress of the disease.

If you dog is being managed with digoxin therapy, this drug has a narrow margin of safety and your dog will need to have blood levels of the medication monitored very closely to make sure no additional problems develop.

Over time, heart function will deteriorate, ultimately leading to congestive heart failure that cannot be managed. Eventually, euthanasia will be appropriate. That said, many dogs can live quite a long time with medical management, especially if they do not have underlying structural heart disease or heart failure to begin with.

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