Atrial Fibrillation in Dogs and Cats

What is atrial fibrillation?

Atrial fibrillation refers to very rapid contractions of the atria that cause the ventricles to also contract more rapidly than normal. To have a better picture of the abnormality, it is necessary to understand how the heart works.

Heart Anatomy and Function

The heart is a muscular organ that is separated into four chambers that are involved in pumping blood for the body. The upper chambers of the heart are the right and left atrium and the lower chambers are the right and left ventricles.

• Right atrium - accepts poorly-oxygenated blood from the general circulation
• Left atrium - accepts oxygenated blood from the lungs
• Right ventricle - pumps poorly-oxygenated blood to the lungs
• Left ventricle - pumps oxygen-rich blood to the rest of the body

When the heart is in a state of relaxation, the valves between the atria and ventricles open to allow blood to move from the atria to the ventricles. In a heart that is beating normally, there is precise coordination in the contractions of the atria and ventricles to ensure that blood is moved smoothly throughout the body.

What happens during heart contraction?

An electrical impulse stimulates the contraction of the heart muscles. It originates in a special tissue located in the right atrium called the sino-atrial (SA) node. The electrical impulse travels to the atrial muscle and stimulates it to contract. At the same time, it simultaneously travels down special electrical networks (composed of the Purkinje fibers, AV node, and bundle branches) to the muscles of the ventricles causing them to contract.

It’s these electrical impulses that coordinate the rhythmic contraction of the heart. Since the electrical impulses get to stimulate the atria first, they’re the first to contract. When the atria contract, they push blood into the ventricles. As the electrical impulses reach the ventricles and cause contraction, blood is pushed either to the lungs or circulated throughout the body.

An electrocardiogram (ECG or EKG) can record all these contractions of the heart muscles including the electrical activity.

When does atrial fibrillation occur?

Atrial fibrillation occurs when, instead of electrical impulses being started and coordinated by the SA node, various areas within the atria send out rapid electrical impulses. These disorganized electrical impulses spread over the atrial tissues causing them to fibrillate instead of contracting in a coordinated manner.

Only a few of these electrical impulses can get into the special electrical networks to reach the AV node and ventricles to make them contract. The irregular pattern by which the abnormal electrical impulses from the atria are conducted through the AV node is what causes the irregular beating of the ventricles.

How Atrial Fibrillation Affects Dogs and Cats

In dogs and cats, atrial fibrillation is generally associated with underlying problems that cause heart disease. The most common of these conditions include dilated cardiomyopathy (DCM), mitral valve regurgitation, and the consequent development of congestive heart failure (CHF).

For atrial fibrillation to occur in dogs and cats, the atrial chambers must be bigger than normal. This abnormal atrial enlargement is usually present in dilated cardiomyopathy, degenerative disease affecting the mitral valve, and hypertrophic cardiomyopathy.

In large and giant breeds dogs, such as German Shepherds, Great Danes, and Irish Wolfhounds, AF can occur even without structural heart defects. This is referred to as primary or lone atrial fibrillation. In some cases, the lone AF can be an early red flag indicating the presence of dilated cardiomyopathy. In other cases, there is no underlying heart disease that is associated with the arrhythmia.

In cats, atrial fibrillation is always linked with severe heart disease, thus the symptoms that can be seen are often related to the underlying problem. If your cat has congestive heart failure, the heart is unable to pump blood effectively, thus the oxygenation of the tissues and body fluid balance is compromised. Prominent symptoms include poor exercise tolerance, coughing, difficulty breathing, and becoming exhausted even with very little effort exerted.

How Atrial Fibrillation is Diagnosed in Pets

Veterinarians generally use specific diagnostic criteria in the identification of AF in dogs and cats.

Historical information and predispositions

These include the patient’s age, breed, pet owner’s observations, and medical history. Atrial fibrillation in large and giant breed dogs is sometimes associated with gastrointestinal conditions. A dog that has been inserted with intravenous catheters may also have AF as a result of atrial muscle irritation. A history of cardiac disease can also be a cause of secondary AF.

Physical findings

• Auscultation involves listening to your pet’s heart using a stethoscope to check for the presence of arrhythmia or a heart murmur.
• Pulse is assessed to check for deficits or a variation in the pulse quality. Tachycardia (increase in heart rate) is always present with secondary AF.
• Signs manifested by your pet - Are the clinical symptoms associated with AF exhibited by your dog? Does your dog have a cough? Is it a gagging cough?

Diagnostic Procedures

• Electrocardiography (ECG) is considered the gold standard when it comes to diagnosing atrial fibrillation.
• Chest x-rays can help identify abnormalities or changes in the heart and lungs.
• Echocardiography can help identify the type of underlying heart disease in cases of secondary atrial fibrillation.
• Intracardiac electrography records the activity within the atria.
• Holter Monitoring (ambulatory ECG) may be recommended to help differentiate whether it’s lone (primary) or secondary AF. A Holter device is worn by a dog for 24-48 hours to monitor the activity of the heart.

Is atrial fibrillation treatable in pets?

Atrial fibrillation in dogs and cats can be treated with medications that slow down the rate by which electrical signals are conducted between the atria and ventricles. Treatment will also include addressing any underlying heart disease. If your pet has congestive heart failure, medication is given to eliminate excess body fluid, help keep the heart beating normally, and control hypertension, if present.

Cardioversion may be attempted in dogs with primary atrial fibrillation. The treatment is a very specialized procedure that should only be performed by a heart specialist.

A special diet may also be recommended by your vet. Pets with congestive heart failure may be placed on a diet with mild or moderate sodium restriction.

Managing a Pet with Atrial Fibrillation

A dog or cat that has been diagnosed with atrial fibrillation will need regular monitoring. There will be follow-up ECGs to assess if treatment is effective. The heart rate will also be monitored closely.

Unfortunately, with time, the function of the heart often deteriorates. This can lead to severe congestive heart failure that can’t be managed with medications. When this happens, euthanasia is generally recommended. That said, many dogs and cats with atrial fibrillation can live longer and have a good quality of life with appropriate medical management.

Read more:

Heart Murmurs in Cats

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Heartworm Infection in Cats

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