Both atrial fibrillation (AF) and the related rhythm, atrial flutter, originate in the top chambers of the heart called the atria. Both rhythms can lead to a decrease in heart function related to chaotic rhythm and excessively rapid heart rates. These electrical disturbances can rarely be "cured" back to normal because the majority of dogs with AF have progressive underlying heart disease. When AF occurs as the only heart problem (lone atrial fibrillation) or suddenly develops in a hospitalized patient, there is a greater likelihood that the rhythm can be made "normal" again. However, over 95 percent of cases maintain AF for life.

A number of other heart rhythm disturbances can also lead to a rapid heart rate and deterioration in heart function. Ventricular tachycardia is a good example; this is a rapid heart rate that originates in the lower chambers of the heart called the ventricles. These disturbances are readily distinguished by an electrocardiogram (EKG) examination.

A key to management of AF is the medical control of co-existing congestive heart failure (CHF) when present.