7.8.6 Paroxysmal atrial fibrillation
In some horses, AF occurs for a short period of time before sinus rhythm returns without treatment. This usually occurs during exercise. When sinus rhythm is rewithin 24 hours this is termed paroxysmal atrial fibrillation. ParoxAF can result in a significant reduction in performance during exercise. In racehorses, the usual pattern is for a sudden tailing-off from the rest of the field. In some cases, epistaxis and ataxia are seen. It can be difficult to establish the diagnosis in these cases because the paroxysm has often resolved by the time that a veterinary examination is performed. Horses with this history need a thorough examination for all potential causes of poor athletic performance (see section 8.2). Radiotelemetry is a very useful aid to detecting paroxysmal AF itself, or APCs which may predispose to AF. However, it not always possible to reproduce the clinical signs seen on the track and a presumptive diagnosis may have to be made. In horses in which repeated paroxysmal AF is suspected, trainers should be encouraged to seek a veterinary examination directly after the end of the race if the horse unexpectedly tails-off.
It is unclear how many horses have repeated bouts of paroxysmal AF. Many animals which have an episode of paroxysmal AF do not have repeated episodes even without treatment or rest. In most cases in which paroxysmal AF is susno specific treatment can be given because of the restrictions on the use of drugs in animals in competition. Even if treatment of horses during compewas permitted, it would be difficult to ensure that paroxysmal AF did not occur. Therefore, in animals in which it occurs repeatedly, possible predisposing factors should be assessed. Valvular heart disease is a significant factor and, if present, carries a poor prognosis because there is no treatment. The most common predisposing factors are atrial myocardial disease (usually a myoand electrolyte imbalance. These should be evaluated in the same way as for persistent AF. Other conditions which reduce performance and may put extra pressure on the horse at exercise may also be important. For example, if respiratory disease has been present but is managed appropriately, paroxysmal AF may not recur. Upper airway obstruction may be a complicating factor in some cases and if corrected may make paroxysmal AF less likely to recur.