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Atrial Fibrillation in Dogs
Veterinary & Aquatic Services Department, Drs. Foster & Smith
Heart and Respiratory
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Click here for a pdf version of this article.  See related products at Pet Supplies
To appreciate what atrial fibrillation or A-fib is and how it affects the dog's heart, it is important first to understand the normal anatomy of the heart and how it works.

First, let's follow blood as it travels through the heart.

illustration of heart
LA: Left atrium
LV: Left ventricle
RA: Right atrium
RV: Right ventricle
The heart is a four chambered pump located in the dog's chest. The heart's chambers are called the left and right atria (atrium) and the left and right ventricles. We'll use the right atrium as a starting point. The right atrium receives the oxygen-depleted blood coming from the body and passes it on to the right ventricle. The right ventricle then pumps blood to the lungs so that it can be replenished with oxygen. The oxygenated blood from the lungs is passed on to the left atrium. The left atrium passes it on to the left ventricle. The left ventricle then pumps the oxygenated blood out to the body. At the same time this oxygenated blood is being pumped out of the heart by the left ventricle, the oxygen-depleted blood is entering the right atrium and the cycle continues. The flow of blood between these chambers is controlled by a series of valves. These valves are designed to prevent the back flow of blood and keep it flowing in the proper direction.

To pump blood efficiently, the heart muscles must work in a regular rhythmic pattern. The muscles of the heart contract in response to an electrical impulse that originates from an area in the right atrium called the sinoatrial (or SA) node. The impulse travels through the right and left atria to another area in the center of the heart called the atrial-ventricular (or AV) node. The electrical impulse then travels down a set of fibers called the right and left bundle branches to the respective ventricular muscles causing them to contract. These impulses continue uninterrupted as long as life is present.

So, what is atrial fibrillation in dogs?
Atrial fibrillation is a malfunction of the heart's electrical system. Instead of the electrical impulse originating from the SA node, the impulse originates from many different areas of the right atrium in an unorganized manner. This causes the atrial tissue to fibrillate (quiver). With the atrium in this state, only a small percentage of the electrical impulse reaches the bundle branches and the ventricles. As a result, the ventricular contractions are irregular and weakened, the ventricular chambers are not filled properly, and the heart's ability to effectively deliver blood is diminished. During atrial fibrillation, the dog's blood flow is slowed down which may lead to blood clot formation. If a clot is then pumped out of the heart, it can become lodged somewhere downstream and lead to further problems: pulmonary embolism (blood clot in the lung), stroke, or a myocardial infarction (heart attack - rare in dogs).

Which dogs get atrial fibrillation?
The size of the dog's heart and in particular the size of the right atrium is directly related to the risk of development of atrial fibrillation. Atrial fibrillation is more common in larger breed dogs simply because their hearts are larger. A-fib can occur in smaller dogs whose hearts have been affected by diseases of the heart including degenerative heart valve disease, hypertrophic cardiomyopathy and dilated cardiomyopathy. These heart diseases typically increase the size of the atrium.

How is atrial fibrillation in dogs diagnosed?
Oftentimes, atrial fibrillation is discovered when a veterinarian listens to the dog's heart during an annual exam and detects an irregular rhythm. These dogs may not have any outward symptoms. Some have described this rhythm as very irregular or as a "tennis shoe in a dryer." This is called "lone atrial fibrillation." Some dog owners may notice that the dog's exercise tolerance is reduced which may prompt a visit to their veterinarian. In these cases, A-fib may be accompanied with another form of heart disease as listed above.

The only way to confirm a diagnosis of atrial fibrillation is with an electrocardiogram or ECG. This involves connecting several electrical wires to the dog in strategic locations. The electrical impulses of the heart are then recorded and examined. Atrial fibrillation has a very unique rhythmic pattern on an ECG.

How is atrial fibrillation in dogs treated?
The treatment of atrial fibrillation involves re-establishing a normal heart rhythm. In people, this may involve an electric shock to the heart that "re-sets" the heart's beat. This process is called "electrocardioversion." In addition, certain medications may be used to control the heart's beat. In dogs with lone atrial fibrillation, electrocardioversion may or may not be used. It is more common that antiarrhythmic medications will be used to control the dog's heart beat. If A-fib is caused by an underlying heart disease, then the underlying disease will need to be addressed as well. In either case, the goal is a normal heart rhythm that will deliver blood in a consistent and efficient manner.

A-fib is a heart condition that is more common in larger breed dogs and dogs with a heart condition that leads to an enlargement of the atrium. This condition affects the heart's ability to efficiently move blood throughout the body and compromises the health of the animal. A-fib can occur as an isolated incident or as a result of a progressive disease condition such as congestive heart failure. The effects of A-fib can typically be controlled with heart medications that strengthen and control the frequency of the heart's beat.

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