Atrial Fibrillation is an irregular heart rhythm (arrhythmia) and is usually a very fast and irregular rhythm (tachyarrhythmia). In the dog it is always associated with heart disease though in people and horses it can be found with no underlying heart disease.
Cause of Atrial Fibrillation in the Dog
The normal heartbeat started by its own “pacemaker” called the SA Node is disrupted by an erratic firing of electrical stimuli coming from the atrium. The exact reason for this abnormality is unknown, but it always is found in association with enlargement of the atria which is, in turn, secondary to heart disease. These other electrical stimuli usually are very rapid and irregular which, in turn, stimulate the ventricle to contract in a similar fashion. The heart rate is often very fast exceeding 200 beats per minute. This rapid and irregular rhythm leaves little time for normal filling of the ventricle; therefore, the contractions often have little blood to eject. The net result is a worsening of cardiac output and often causes decompensation if the dog is not already in heart failure.
Underlying Heart Disease in Atrial Fibrilation in the Dog
Several diseases of the heart may cause enough enlargement of the atrium to precipitate atrial fibrillation. DCM (Dilated Cardiomyopathy) is the most common cause of increased atrial size in dogs. DCM typically occurs in large breed dogs with naturally occurring large atria. Combined with poor contraction of the ventricle, the atria become further enlarged. As stated above, the larger the atria, the more chance of developing atrial fibrillation. Massive atrial enlargement can occur in small breeds of dogs secondary to mitral valve disease which often predisposes to atrial fibrillation. Congenital abnormalities like which cause enlargement of the atrium can also result in atrial fibrillation (Triatriatum Dexter where there are essentially 3 atria due to an abnormal septum from birth).
Clinical signs and symptoms of dogs with atrial fibrillation
Clinical signs of atrial fibrillation depend on the severity of the underlying heart disease. Some dogs will show minimal changes when the arrhythmia begins. Others may become lethargic, have increased exercise intolerance or begin to have fainting episodes (syncope). Still other dogs that are on the edge of heart failure will decompensate and develop signs of heart failure. These signs include coughing and varying degrees of labored respiration.
Diagnosis of atrial fibrillation in dogs
After a full history including signs mentioned above, a thorough stethoscope evaluation (auscultation) of the heart finding a characteristic rhythm of a rapid and irregularly, irregular heart beat. An ECG evaluation will provide the definitive diagnosis. Chest x rays will help determine the severity of the disease and whether the dog is in congestive heart failure. An echocardiogram will be performed to further assess the severity of heart disease and appropriate therapy.