Paroxysmal Atrial Fibrillation in a Racehorse

Abstract

A 4-year-old thoroughbred stallion with a history of loss of racing form was studied over a period of approximately 7 months. At the initial examination he showed positive T waves in 1 chest lead and wandering of the pacemaker. This was followed by an episode of paroxysmal atrial fibrillation which disappeared spontaneously and was replaced by significant T wave changes in all the chest leads. During a period when these changes persisted, there was progressive lengthening of the P wave and the P-R interval. Because the last ECG showed evidence of intra-atrial block, first degree A-V block and T wave changes, the owner was advised that a fluctuating myocarditis was affecting the horse's performance and it was retired to stud. This appears to be the first report of paroxysmal atrial fibrillation in Australia, and it demonstrates the value of regular follow-up examinations in horses with initial electrocardiographs showing only minor departures from normal. The case also suggests that when sharp reversals of racing form occur, the possibility of paroxysmal atrial fibrillation being the cause should be kept in mind.

Cardiopathological Changes in Racehorses with Abrupt Deceleration of Speed during a Race

“…A few reports seem to have been published to deal with VPCs clinico-pathologically [8,11,12,24,34], but no details for cardiopathology were given, at least, in the horse. In addi tion, paroxysmal AF often occurs suddenly during a race in the racehorse [2,3,16,28]. Although the persistent type of AF occurs K. KIRYU, T. NAKAMURA, N. MACHIDA ET AL.…”

Paroxysmal Atrial Fibrillation in Horses: Pathophysiology, Diagnostics and Clinical Aspects

“…AF is a supraventricular arrhythmia characterized by an irregularly irregular heart rhythm [7,8,15], which may occur either secondary to an underlying disease (e.g., myocardial disease, mitral regurgitation or pulmonary disease) [16,17] or without any concomitant disease. The terms "idiopathic AF" or "lone AF" have previously been used to emphasize the absence of underlying disease, yet this should be avoided as even subclinical pathophysiological changes in the atria are considered to be "underlying" causes [18].…”

“…Immediate clinical examination of horses with AF will reveal the following: irregular heart rhythm, delayed recovery of the HR and respiratory rate, and variation in heart sound intensity [7,15,60,62]. In rare cases, horses can show distress [8].…”

“…In racehorses, AF is commonly characterized by an instant loss of speed that causes the horse to finish markedly behind the winner [3,7,8,15]. This is caused by the sudden lack of peripheral oxygenation because AF impairs ventricular filling and affects the cardiac output [8].…”

Fundamentals of arrhythmogenic mechanisms and treatment strategies for equine atrial fibrillation

“…In contrast, the goal of treating horses with AF is the return and maintenance of normal sinus rhythm – otherwise known as cardioversion, to restore athletic performance 2 . Because of the possibility of a spontaneous return to normal sinus rhythm (paroxysmal AF), cardioversion is typically delayed for the first 48 hours after AF detection 2,8,11,12,147–149 …”