A 17-year old Thoroughbred gelding presented to the Cornell Equine Hospital for evaluation of a cardiac arrhythmia identified incidentally two weeks prior to presentation during an examination of mandibular lymphadenopathy. On presentation, the patient had an irregularly irregular cardiac arrhythmia and a grade 2/6 left diastolic heart murmur with the point of maximal intensity over the aortic valve. There was mild pulsation of the jugular veins bilaterally in conjunction with a capillary refill time and peripheral pulses that were within normal limits. Electrocardiography revealed atrial fibrillation. Echocardiography revealed mild mitral and tricuspid valve regurgitation, trace aortic valve regurgitation, and mild left atrial enlargement. Medical conversion of the arrhythmia was elected.

This seminar will discuss the clinical signs, diagnosis, and treatment, of atrial fibrillation in the horse. Common side effects of quinidine treatment will also be discussed.