Racehorses performing at the highest levels of exertion are at risk of developing atrial fibrillation, with a sudden loss of speed the most common sign.
Episodes of atrial fibrillation are characterized by rapid and chaotic electrical activity in the two upper chambers of the heart, known as the atria, causing reduced function.
It is the most common cardiac arrhythmia affecting performance in horses. The sporadic form of the condition, first described in the 1950s, is called paroxysmal atrial fibrillation.
The condition may go undetected, as episodes can occur intermittently at different frequencies and vary in duration from minutes to hours. However, it may still result in poor performance, especially in racehorses.
“An unexpected and abrupt decrease in performance in horses without any previous clinical signs may be related to the sudden onset of cardiac arrhythmias,” Sofie Troest Kjeldsen, Sarah Dalgas Nissen, Rikke Buhl and Charlotte Hopster-Iversen wrote in their just-published review.
The University of Copenhagen researchers, writing in the journal Animals, set out to investigate the current knowledge and possible risk factors that may predispose horses to paroxysmal atrial fibrillation.
The estimated incidence in racehorses with poor performance is less than 5% when using short-term monitoring, but 33% when using long-term monitoring devices, they noted.
“In recent decades, numerous studies have investigated the mechanisms of atrial fibrillation in horses along with different treatment options and the effect on performance,” they said. “However, less is known about paroxysmal atrial fibrillation, likely due to its sporadic nature and lack of clinical signs in non-racing horses.”
It can easily be overlooked, they said, if the horse is not examined during the short window of time when the arrhythmia is present. The condition is therefore likely to be underdiagnosed.
Atrial fibrillation can cause electrical, structural and functional remodeling of the heart’s upper chambers. This forms the basis of a vicious cycle as the atrial fibrillation-induced remodeling acts as a substrate that allows the condition to become self-sustained.
Looking at risk factors, the authors noted that most cases of paroxysmal atrial fibrillation have been reported in Thoroughbreds and Standardbreds, but it has also been noted in other breeds. The condition is more common in larger horses compared to ponies.
A study from Canada found that Standardbreds are more commonly affected by atrial fibrillation than other breeds. It was later discovered that it is a moderately heritable arrhythmia in the breed, and frequently used stallions might have helped increase the incidence.
A recent study in Thoroughbreds from Hong Kong and Australia found the heritability of atrial fibrillation to be low.
Most cases of sporadic atrial fibrillation have been reported in male horses, but this likely reflects their greater numbers on the racecourse, and no significant difference in frequency between sexes has been found.
Similar to humans, the risk of atrial fibrillation in horses increases with age.
Larger animals, including horses, cattle and giant dog breeds, are thought to be more prone to developing sporadic atrial fibrillation in the absence of other diseases when compared to smaller animals. Larger animals have a larger atrial mass, which is likely a factor, they said.
The authors noted that there are several known risk factors for human atrial fibrillation, including age, high blood pressure, valve disease, sleep apnea and obstructive lung diseases, whereas only age or valvular disease seem to be of relevance in horses.
There is also the role of the autonomic nervous system, which interacts with cardiac electrophysiology. “Altered autonomic tone affects the electrophysiological properties of the atria and plays a role in the onset of atrial fibrillation,” they said.
Turning to clinical signs, the review team said an immediate clinical examination of horses with atrial fibrillation will reveal an irregular heart rhythm, delayed recovery of the heart rate and respiratory rate, and variation in heart sound intensity. In rare cases, horses can show distress.
Most horses with atrial fibrillation have a normal ventricular rate at rest, while some have an increased resting heart rate.
In racehorses, atrial fibrillation is commonly characterized by an instant loss of speed that causes the horse to finish markedly behind the winner. This, they said, is caused by the sudden lack of peripheral oxygenation because the condition impairs ventricular filling and affects cardiac output.
“If the onset of atrial fibrillation begins at the finish line or during deceleration, it will not affect the horse’s racing ability,” they said. In fact, in one study, 3 out of 13 horses won their race despite being diagnosed with post-race atrial fibrillation.
“In pleasure horses, clinical signs are thought to be more subtle or they may even be asymptomatic due to less stress on the cardiovascular system.
“The sporadic nature of paroxysmal atrial fibrillation may explain why some horses present with intermittent episodes of reduced performance.”
Holter ECG monitoring is currently considered the gold standard for diagnosing arrhythmias. In order to diagnose sporadic arrhythmia, long-term cardiac monitoring is essential.
The authors noted that computational analysis of ECGs of horses with an apparently normal sinus rhythm has recently been tested to identify those more likely to develop sporadic atrial fibrillation.
It is an area that shows considerable promise, they said. “In future diagnostics, when artificial intelligence becomes more and more powerful, computational ECG analysis will be an important dimension and may play a role in future neural networks for diagnosing paroxysmal atrial fibrillation.”
The authors, who cited 119 previous studies in their review, said there are few treatment options for horses with the condition, but an effort to return the heart to normal rhythm using electricity or drugs – known as cardioversion – is currently recommended if the episode lasts longer than 48 hours.
Cardioversion can be achieved either with quinidine sulphate or by transvenous electrical cardioversion.
Cardioversion of horses with atrial fibrillation presenting with sudden reduced performance but without underlying diseases can return them to their previous level of performance, they noted.
The researchers said large-scale longitudinal studies that include different breeds and equine disciplines of both apparently healthy horses and horses with reduced performance are needed to explore the true prevalence of paroxysmal atrial fibrillation and its effect on performance.
Long-term continuous cardiac monitoring is essential for diagnosing the condition, they said, but suitable devices with high sensitivity during both rest and exercise are currently lacking.
Better knowledge of the pathophysiology and mechanisms involved in the early remodeling process is also needed to fully understand how the condition progresses into persistent atrial fibrillation.
“Insights into the pathophysiology of the remodeling process can also help to identify possible targets for future treatments,” they said.
Future diagnostic techniques, including computational analysis of ECGs, could have considerable value in screening and early diagnosis of the condition in horses.
The challenge in horses is detecting the problem early and recognizing horses at risk. Early diagnosis of the sporadic form is crucial to head off progression to persistent atrial fibrillation.
Kjeldsen, S.T.; Nissen, S.D.; Buhl, R.; Hopster-Iversen, C. Paroxysmal Atrial Fibrillation in Horses: Pathophysiology, Diagnostics and Clinical Aspects. Animals 2022, 12, 698. https://doi.org/10.3390/ani12060698