A 6‐year‐old, 43 kg, male Shiloh shepherd dog was anaesthetised for computed tomography to investigate the source of respiratory distress, lethargy and decreased appetite. Arterial hypotension during anaesthesia was treated with an infusion of phenylephrine and a bolus of ephedrine, 0.05 mg/kg intravenously (IV). Ephedrine resulted in increased blood pressure and reflexive bradycardia; atrial fibrillation developed immediately after. Approximately 10 minutes later, a bolus of lidocaine 2 mg/kg IV was administered, and normal sinus rhythm was restored within 1 minute. Lone atrial fibrillation may be triggered by agents or disease processes that increase vagal activity. While lidocaine has historically been used only to treat unstable ventricular dysrhythmias, it is effective for terminating this particular type of atrial fibrillation.