Atrial Fibrillation: Why is My Dog’s Heart Beating so Fast?

Senior dog laying down

Atrial fibrillation is an abnormal rhythm that affects the two upper chambers (atria) of the heart. During episodes of abnormal atrial rhythm, the upper chambers beat out of synch with the lower chamber (ventricles).

Cause of atrial fibrillation in dogs
Atrial fibrillation is caused by an electrical stimulus to the atrium. Atrial fibrillation may be associated with underlying heart muscle disease or it may occur in an otherwise seemingly normal heart1. It has been associated with increased size of the atrium, as occurs normally in giant breeds and as a result of atrial/ventricular valve disease.

How likely is atrial fibrillation in my dog?
The condition is more common in large breeds of dogs; for example Great Danes, Doberman Pinschers, Boxers and Irish Wolf Hounds1; but it can occur in small dogs as well2.

Clinical signs of atrial fibrillation
Atrial Fibrillation generally causes the heart to beat too fast. As a result the ventricles do not have time to normally fill and empty—ultimately resulting in heart failure.
Affected dogs may be:

  • Listless
  • Weak
  • Unable to exercise

Frequently, there is an accumulation of fluid in the lungs or abdomen. There may be an associated shortness of breath, cough or fainting3. The heart rate may exceed 200 beats per minute.

How is atrial fibrillation diagnosed and confirmed?
The physical findings associated with poor heart function and the classical abnormal rate and rhythm of the heart are the main findings to prompt a diagnosis. An EKG will show rapid irregular heartbeats. Radiographs will often show the enlargement of the heart, abnormal distension of blood vessels and fluid retention in and around the lungs.

Prognosis of atrial fibrillation
The prognosis is varied but tends to be much worse when associated with underlying dilated cardiomyopathy. Atrial fibrillation is usually associated with underlying heart disease. These underlying conditions make the resultant arrhythmia difficult to manage.

Treatment of atrial fibrillation
Abnormal, electrical rhythms may respond to electrical conversion in humans, but this is not generally successful in dogs for long. The usual treatment involves slowing the heart rate with medications1. These drugs treat the underlying disease and focus on slowing the heart rate

Ongoing care involves monitoring the heart rate, rhythm and radiographs to determine if other treatments are indicated.

Because most animals with atrial fibrilation have a coexisting cardiac condition that must also be treated, the management of dogs with atrial fibrilation is very challenging for pet guardians and veterinarians alike.

Questions to ask your veterinarian

  • My dog looks to have a distended abdomen full of fluid? Do dogs that retain fluid have heart disease?
  • Are some breeds of dogs more likely to develop Atrial Fibrillation?


  1. "Arrhythmias (Abnormal Rhythms) in Dogs." Cornell University Hospital for Animals. College of Veterinary Medicine - Cornell University, Web. 02 Mar. 2015.
  2. Foster, Dr., and Smith, Dr. "Atrial Fibrillation in Dogs." Pet Education. Web. 02 Mar. 2015.
  3. Hoskins, Johnny D. "Atrial Fibrillation Tough to Manage at times Due to Many Other Concurring Problems." DVM360 MAGAZINE, 1 Apr. 2003. Web. 02 Mar. 2015.

If you have any questions or concerns, you should always visit or call your veterinarian -- they are your best resource to ensure the health and well-being of your pets.