The most common heart arrythmia in horses is atrial fibrillation. Young adult standardbreds and thoroughbreds appear to be affected more than other breeds, and younger horses may have this heart condition and not show any symptoms until they begin training. This may be the reason the condition is found in the two breeds mentioned earlier as these breeds are generally in race training early in life and affected horses do not perform up to expectations. In contrast to younger horses, older horses with atrial fibrillation usually have actual heart abnormalities, such as congestive heart failure or valvular insufficiency. Along with exercise intolerance, other clinical signs are an increased respiratory rate, difficulty breathing, muscle disease, colic, variable intensity of heart sounds, and a bloody nasal discharge. A murmur may or may not be present.
Low potassium levels have been shown to be associated with atrial fibrillation and can occur in horses who are given steroids or furosemide (a diuretic) before a race. An electrocardiogram, or ECG, is required for diagnosis of atrial fibrillation. A cardiac ultrasound is also helpful in the diagnosis to observe the heart and determine the severity of the disease. Horses with short-term atrial fibrillation that are not in congestive heart failure, do not have severe murmurs, and have heart rates less than 60 beats per minute, can be converted to a normal rhythm with the drug quinidine. However, horses with long-term fibrillation, congestive heart failure, severe murmurs, and heart rates greater than 60 have a much poorer prognosis. If your horse is not performing up to your expectations, consult your veterinarian.