Treatment of supraventricular tachycardia in a horse

OBJECTIVE: To describe the treatment of persistent supraventricular tachycardia (SVT) in a young horse in endurance training. CASE SUMMARY: A 6-year-old Arab gelding in endurance training presented for a dysrhythmia and decreased performance. SVT was diagnosed and conversion to a normal sinus rhythm was achieved following administration of a constant rate infusion of amiodarone. However, reversion to SVT occurred shortly after initiation of ridden exercise. A second attempt to convert the dysrhythmia with amiodarone failed, but normal sinus rhythm was achieved with transvenous electrical cardioversion (TVEC). Postmortem examination of the heart revealed extensive fibrous replacement of most of the left atrial myocardium; these changes likely provided the structural substrate for the dysrhythmia. The underlying cause of the fibrosis was not identified. NEW OR UNIQUE INFORMATION PROVIDED: SVT is a form of supraventricular tachyarrhythmia rarely diagnosed in the horse. A recent report has described sudden death of a horse following attempted conversion of SVT with oral flecainide acetate. In the present report, we describe short-term conversion of SVT in a horse using intravenous amiodarone with no significant adverse effects. When the dysrhythmia recurred, the animal was donated for teaching purposes and conversion was achieved with TVEC. Normal sinus rhythm persisted for 2 weeks until the horse was euthanized for postmortem evaluation of the heart. Intravenous amiodarone or TVEC could be considered as treatments for supraventricular tachyarrhyhmias other than atrial fibrillation in the horse.