A 25-year-old Belgian Warmblood mare with pre-existing atrial fibrillation was presented for surgical treatment of sinusitis. Echocardiography evidenced signs of compromised cardiovascular function, including a decreased contractility of the left ventricle and mitral valve insufficiency. The horse was treated with 2.2 µg/kg digoxin IV, followed by 11 µg/kg orally twice a day for one week before surgery. Premedication consisted of 0.02 mg/kg acepromazine IM and 80 µg/kg romifidine IV. Anesthesia was induced with 0.06 mg/kg midazolam and 2.2 mg/kg ketamine IV and maintained with isoflurane in oxygen. During the anesthesia, a continuous rate infusion with lidocaine (bolus 1.5 mg/kg, then 1.7 mg/kg/h) was administered and a temporary ventricular pacing unit, placed preoperatively, was available to treat possible bradycardia. Dobutamine was used to increase blood pressure but was accompanied by an obvious increase in heart rate. No major problems were encountered during the course of the anesthesia and recovery.