Atrial fibrillation in the horse : review of 27 cases

Atrial fibrillation (AF) is a common arrhythmia in horses and it is the most common cardiovascular cause of poor performance in the equine athlete. AF seems to be related with a “reentry” mechanism with or without underlying cardiac diseases and there do not appear to be any gender predisposition. Aim of the present report is to review the epidemiological, clinical and therapeutic aspects of AF in 27 horses. The horses had an history of poorperformance (6 horses), cardiovascular disorders (19 horses), exercise induced pulmonary hemorrhage (1 horse) and collapse (1 horse). The symptoms were present for less than 1 month in 17 cases and for longer than 1 month in 10 horses. The horses were 16 trotters and 11 warmblood; 10 males, 10 females and 7 geldings with a mean body weight of 497,11±61,3 kg and a mean age of 8±6 years. All horses underwent a thorough physical examination, laboratory evaluation, electrocardiography (ECG), 24 h Holter recording (HR) and echocardiography (ECC). According with the results obtained 17 horses were treated for AF (AFt), 9 horses were not treated and spontaneous resolution of AF was recorded in 1 horse. 15 horses received 22 mg/kg quinidine sulfate (QS) by nasogastric tube every 2 hours until cardioversion (CV); 1 horse, since QS was ineffective, received amiodarone cloridrate (Am) intravenously (De Clercq et al 2006) until CV, and 1 further AFt case received Am first and then QS. During treatments the horses were monitored by ECG. All horses had normal body temperature (37,8±0,3 °C); on pulse palpation 12 had reduced intensity and amplitude and all had irregular rhythm. In all cases, heart auscultation revealed an irregularly irregular rhythm and murmurs were detected in 10 horses. ECG and HR confirmed the arrhythmia with mean rate of 44±11 bpm. ECC showed increased ventricular and/or atrial diameters in 4 horses, reduced left ventricular fractional shortening in 9 horses, valvular incompetence in 19 horses. In the 15 QS treated horses CV was achieved after different number of treatments: 2 in 1 case, 3 in 4 cases, 4 in 2 cases, 5 in 1 case and 6 in 6 cases. 2 out of this 15 horses received a double QS treatment at 1 week interval and in these CV was obtained after a total of 6+5 and 6+6 treatments. In the QS+Am case CV was obtained after 26 hours of Am administration. In the Am+QS case Am was discontinued due to signs of toxicity and a subsequent QS administration was ineffective. The treatment was effective in 16 out of 17 horses. In our review QS showed its efficacy for AF in 94% of cases without side effects. The Am+QS treated horse did not convert to sinus rhythm probably due to chronic AF. Probably, the high rate of success obtained can be related with the accurate ECC selection of the cases. However, since QS is becoming difficult to obtain, other cardioverting techniques or drugs like Am, which is currently used in horses for chronic cases of AF, need to be further investigated in the equine patient.