At present, there are a few effective anti-arrhythmic drugs available for use in veterinary medicine. Amiodarone is a class III antiarrhythmic drug with potent antiarrhythmic and low proarrhythmic effects. However, amiodarone has possessed beta-adrenergic and calcium channel blocking activities which may have adverse effects on cardiac contractility, relaxation and thyroid function. Objectives of this study were to evaluate effects of amiodarone in terms of controlling arrhythmia, cardiac contractility and relaxation, and thyroid toxicity. Eight dogs with organic heart diseases were recruited and confirmed arrhythmia types by Lead II and 24-h ambulatory ECGs. Echocardiograms were performed before and after treated with amiodarone at a 10 mg/kg loading dose twice a day for 7 days, followed by a 5 mg/kg maintenance dose, once a day. After treated with amiodarone for 15 days, the result showed that the total arrhythmic count was significantly decreased (p < 0.05). Pre-ejection period and isovolumic contraction time were significant prolongations (p < 0.05). Tei index tended to increase (p = 0.07). After 60 days, plasma tri-iodothyronine (T₃) and tetra-iodo-thyronine (T₄) were not significant alterations (p = 0.44 and 0.37, respectively).

In conclusion, amiodarone is a potent antiarrhythmic drug which has minor negative effects on inotropy and lusitropy. Thyroid toxicity of amiodarone is still controversial, but does not have impact on clinical signs. Therefore, the dose of amiodarone recommended in this study is safe for treated arrhythmic dogs with organic heart diseases.