Arrhythmias common in poorly performing Standardbred horses, findings show

The positioning of the electrodes on the Standbardbreds used in the study.
The positioning of the electrodes on the Standbardbreds used in the study. Image: Alberti et al.

Premature complexes – abnormal heartbeats that occur too early, disrupting the organ’s rhythm – are frequent in poorly performing Standardbred racehorses, the findings of fresh research suggest.

In sport horses, cardiovascular problems represent the third cause of poor performance after musculoskeletal and respiratory disorders.

Issues such as arrhythmias can cause poor performance. However, only the role of atrial fibrillation and severe bradyarrhythmias on athletic performance has been well established in horses.

The significance of other arrhythmias, such as premature beats originating in the lower chambers, is still not well defined, and their exact influence on athletic performance needs to be clarified.

Elena Alberti and her fellow researchers at the University of Milan in Italy, in a study reported in the journal Animals, set out to explore the prevalence of cardiac arrhythmias during maximal treadmill exercise in poorly performing Standardbreds.

Electrocardiographic Holter recordings were taken before, during and after treadmill exercise tests on 158 poorly performing Standardbreds.

Analysis of the readings showed that 50 horses did not have any type of arrhythmia.

In all, 108 horses had at least one type of arrhythmia, such as sinus arrhythmia (8.2%), sinoatrial block (3.2%), second-degree atrioventricular block (33.5%), supraventricular premature complexes (7.6%), and ventricular premature complexes (48.1%).

Further work showed only weak evidence that the occurrence of these premature complexes decreases as the minimum heart rate increases. However, there was a tendency for these arrhythmias to increase with increasing age and maximum lactate concentrations.

“Our results suggest that premature complexes are frequent in poorly performing Standardbred racehorses, but further studies are necessary to clarify their role and clinical significance,” they said.

Discussing their findings, the researchers said the total prevalence of premature complexes in the study seemed to be quite similar to those reported in a study conducted on poorly performing Thoroughbred racehorses. Also, in both studies, the main incidence of the premature complexes was recorded during the high‐speed phases and immediately after the maximal exercise.

They noted that a significant prevalence of premature complexes has also been reported in normal and poorly performing Standardbred racehorses, as well as poorly performing Thoroughbreds.

“Given the high prevalence of these arrhythmias in both normal and poorly performing racehorses, the observation of isolated premature complexes alone cannot explain why a horse shows a decline in athletic performance.”

Thus, the role of premature complexes remains to be defined, they said.

Although arrhythmias were quite common in the study population, the number that would be considered severe was low.

“The results obtained in the present study did not clarify the role and clinical significance of premature complexes in poorly performing horses,” they said, adding that they could not rule out a possible influence of other causes of poor performance, such as respiratory issues, in their results.

“Nevertheless, we would like to suggest that the presence of premature complexes in racehorses, recorded during and after strenuous exercise, should be further investigated.”

The study team comprised Alberti, Luca Stucchi, Chiara Maria Lo Feudo, Giovanni Stancari, Bianca Conturba, Francesco Ferrucci and Enrica Zucca.

Alberti, E.; Stucchi, L.; Lo Feudo, C.M.; Stancari, G.; Conturba, B.; Ferrucci, F.; Zucca, E. Evaluation of Cardiac Arrhythmias before, during, and after Treadmill Exercise Testing in Poorly Performing Standardbred Racehorses. Animals 2021, 11, 2413.

The study, published under a Creative Commons License, can be read here

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