Abstract
A three-year-old male neutered Bullmastiff cross was assessed for lethargy. The dog had recently been diagnosed with atrial fibrillation and hypothyroidism and had been started on thyroid supplementation and cardiac medications. ECG on presentation confirmed atrial fibrillation. Echocardiography showed normal chamber dimensions, making heart failure unlikely. With the lack of structural heart disease, a diagnosis of lone atrial fibrillation was made. After initial medical management of the atrial fibrillation with rate control, the client elected electrical cardioversion. The dog was pretreated with amiodarone and transthoracic electrical cardioversion was performed. Atrial fibrillation was successfully converted to normal sinus rhythm, and the dog was discharged on a maintenance dose of amiodarone. The dog remained in a normal sinus rhythm at the 1-, 2-, 4-, 8- and 14-month revisits. This case report describes management options for atrial fibrillation and the potential association with hypothyroidism.