A 19-year-old 555-kg (1,221-lb) warmblood gelding was evaluated because of poor performance and an irregular heart rhythm. The gelding was used as a jumper and had exercise intolerance of 1 week's duration. On evaluation, cardiac auscultation revealed a high heart rate (60 beats/min) with an irregularly irregular rhythm. No heart murmurs were detected via auscultation. The quality of the mandibular arterial pulses was considered normal. Irregular jugular venous pulsation was evident in the distal third of the jugular groove. Results of a CBC and serum biochemical analysis were unremarkable. A base-apex ECG examination revealed that the heart rate ranged from 40 to 60 beats/min; the rhythm was irregularly irregular with an absence of P waves (Figure 1). These findings were consistent with atrial fibrillation (AF). Echocardiographic variables, including cardiac chamber dimensions, valve morphology, and trans-valvular flow velocities, were within reference limits. On the basis of clinical examination and diagnostic test findings, a diagnosis of lone AF was made.
To convert the AF, the horse was admitted to the hospital for a transvenous electrical cardioversion (TVEC) procedure. Serum electrolyte concentrations were within reference intervals at this time. A baseline serum cardiac troponin I (cTnI) concentration was determined to be normal (0.053 ng/mL; reference interval, < 0.29 ng/mL). A continuous rate infusion of amiodarone hydrochloride (5 mg/kg/h [2.27 mg/lb/h]) was administered IV before the TVEC procedure. While introducers were placed in the right jugular vein for the procedure (after 60 minutes of constant rate infusion with amiodarone), it was noted that the rhythm changed from AF to atrial flutter (AFL), followed by conversion to normal sinus rhythm 10 minutes later. The continuous rate infusion of amiodarone was continued thereafter for 8 hours at a rate of 1 mg/kg/h (0.45 mg/lb/h). The plasma cTnI concentration measured 18 hours after medical conversion of AF to normal sinus rhythm was 0.087 ng/mL, and no abnormalities were observed during a recheck ECG and echocardiographic examinations.
One month after the conversion, a recheck cardiologic examination was performed. The ECG tracing revealed a regular rhythm and a heart rate of 30 to 40 beats/min. The plasma cTnI concentration was 0.027 ng/mL. Ambulatory Holter monitoring was conducted for a 24-hour period and revealed no evidence of cardiac arrhythmia, although the heart rate varied in accordance with the level of exercise performed. Therefore, it was determined that the horse could return to performance. After 7 months with no antiarrhythmic treatment, the horse had remained healthy and in normal sinus rhythm.
1. Bonagura JD, Reef VB, Schwarzwald CC. Cardiovascular diseases. In: Reed MS, Bayly WM, Sellon DC, eds. Equine internal medicine. 3rd ed. St Louis: Saunders Elsevier, 2010; 459–463.
3. Fenton FH, Cherry EM, Kornreich BG. Termination of equine atrial fibrillation by quinidine: an optical mapping study. J Vet Cardiol 2008; 10: 87–103.
4. Reef VB, Bonagura J, Buhl R, et al. Recommendations for management of equine athletes with cardiovascular abnormalities. J Vet Intern Med 2014; 28: 749–761.
5. Reef VB, Levitan CW, Spencer PA. Factors affecting prognosis and conversion in equine atrial fibrillation. J Vet Intern Med 1988; 2: 1–6.
6. Decloedt A, Schwarzwald CC, De Clercq D, et al. Risk factors for recurrence of atrial fibrillation in horses after cardioversion to sinus rhythm. J Vet Intern Med 2015; 29: 946–953.
7. Jesty SA, Reef VB. Cardiovascular system. In: Orsini JA, Divers TJ, eds. Equine emergencies: treatment and procedures. 3rd ed. St Louis: Saunders Elsevier, 2008; 67–77.
8. McGurrin MKJ, Physick-Sheard PW, Kenney DG, et al. Transvenous electrical cardioversion in equine atrial fibrillation: technique and successful treatment of 3 horses. J Vet Intern Med 2003; 17: 715–718.
9. McGurrin MK, Physick-Sheard PW, Kenney DG. How to perform transvenous electrical cardioversion in horses with atrial fibrillation. J Vet Cardiol 2005; 7: 109–119.
12. Khan IA, Mehta NJ, Gowda RM. Amiodarone for pharmacological cardioversion of recent-onset atrial fibrillation. Int J Cardiol 2003; 89: 239–248.
13. Vardas PE, Kochiadakis GE, Igoumenidis NE, et al. Amiodarone as a first-choice drug for restoring sinus rhythm in patients with atrial fibrillation: a randomized, controlled study. Chest 2000; 117: 1538–1545.
14. Capucci A, Villani G, Aschieri D, et al. Oral amiodarone increases the efficacy of direct-current cardioversion in restoration of sinus rhythm in patients with chronic atrial fibrillation. Eur Heart J 2000; 21: 66–73.
15. De Clercq D, van Loon G, Baert K, et al. Intravenous amiodarone treatment in horses with chronic atrial fibrillation. Vet J 2006; 172: 129–134.
16. Trachsel D, Tschudi P, Portier CJ, et al. Pharmacokinetics and pharmacodynamic effects of amiodarone in plasma of ponies after single intravenous administration. Toxicol Appl Pharmacol 2004; 195: 113–125.
17. De Clercq D, Loon G, Baert K, et al. Effects of an adapted intravenous amiodarone treatment protocol in horses with atrial fibrillation. Equine Vet J 2007; 39: 344–349.