Atrial Fibrillation in a Horse

History. The owner of a 3-year-old Standardbred gelding is concerned because the horse is no longer able to complete its training program. Up until a week ago, the horse had been running well during its daily bouts of training.

Clinical Examination. Inspection of the horse reveals a normal-appearing Standardbred in excellent condition. It is standing in its box stall, eating, and looks alert when you enter the stall. Clinical examination reveals normal-colored mucous membranes, no abnormality of lung sounds, and no abnormalities in the gastrointestinal, urinary, or nervous system. When you take the pulse, you note that it is irregular in both amplitude and rate. Several pulses fol­low one another rapidly, and then there are prolonged pauses, with no consistent pattern to the irregularity. Aus­cultation of the heart reveals a similar irregularity in the heart sounds.

You take a blood sample for measurement of the hemato­crit, which is normal. You also obtain an electrocardiogram (ECG), which reveals a continuous irregular pattern of multiple P waves with occasional and irregularly occurring QRS complexes.

Comment. The history, heart rhythm, and ECG findings in this horse are typical of αtriαl fibrillation. The multiple P waves observed on the ECG are a result of circuitous depolarization of the atria. In atrial fibrillation the atria contract and relax in an uncoordinated manner. The atrioventricular node is acti­vated at intervals that vary considerably from cycle to cycle; thus there is no constant interval between ventricular contrac­tions. The variable time between ventricular contractions allows for variable degrees of ventricular filling and therefore results in uneven stroke volume; consequently, the pulse varies in amplitude as well as frequency.

The irregular ventricular rhythm may be sufficient to main­tain cardiac output in the resting animal, but during exercise the cardiac output cannot be maintained. As a result, oxygen delivery to the muscles is inadequate to sustain exercise. This is an example of a failure of oxygen delivery resulting from inadequate blood flow.

Treatment. Treatment for atrial fibrillation in the horse is the administration of quinidine sulfate, which has a negative inotropic effect on the myocardium and slows atrioventricular conduction time. This allows the reestablishment of normal atrial and ventricular rhythm. The horse’s heart rate will likely return to normal with treatment if there is no underlying cardiac disease. The horse is rested for at least a week, at which time training is reinstituted. Several months later the owner reports that the horse is still doing well.

This horse is a young adult, with no evidence of cardiac disease (e.g., murmur, signs of heart failure). Treating a horse with underlying cardiac disease has associated risks, and the horse may not return to normal heart rhythm. Therefore, in most cases, an echocardiogram is recommended before treat­ment to determine that the horse has no underlying cardiac disease. If present, any disease should be addressed first. In some horses the underlying disease is more significant, and treatment of fibrillation will not be attempted.