Exercise-associated rhythm disturbances in poorly performing Thoroughbreds: risk factors and association with racing performance.
Authors: Marr Celia M, Franklin Samantha, Garrod Georgie, Wylie Claire, Smith Lewis, Dukes-McEwan Jo, Bright Jan, Allen Kate
Journal: Equine veterinary journal
Summary
# Exercise-Associated Cardiac Rhythm Disturbances in Poorly Performing Racehorses Cardiac rhythm disturbances during exercise are frequently encountered in poorly performing Thoroughbreds, yet clinical interpretation remains problematic due to limited evidence distinguishing clinically significant arrhythmias from benign findings. Marr and colleagues reviewed medical records from 245 racehorses undergoing simultaneous exercise endoscopy and electrocardiography, identifying rhythm disturbances in 158 horses (64.5%): isolated premature depolarisations in 110 horses (44.9%) and complex tachydysrhythmias in 48 horses (19.6%), with most complex rhythms occurring during recovery phases rather than gallop. Treadmill testing, exercise-associated upper respiratory tract obstruction, and National Hunt racing were associated with increased rhythm disturbance detection; however, only 15 horses were clinically considered rhythm disturbances contributory to poor performance, and subsequent racing records showed no significant associations between arrhythmia presence or type and future racing success (82% of all groups raced again). These findings highlight that whilst cardiac arrhythmias are prevalent in underperforming racehorses, many detected rhythm disturbances—particularly isolated premature depolarisations and some complex rhythms during recovery—appear benign and should not automatically be attributed as performance-limiting factors; clinicians should carefully consider the exercise phase during which disturbances occur and integrate findings with upper airway assessment when evaluating poor performance cases.
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Practical Takeaways
- •Exercise-associated cardiac rhythm disturbances are common in poorly performing racehorses (64.5% prevalence) but most do not prevent return to racing, suggesting clinicians should use caution when attributing poor performance solely to detected arrhythmias
- •Exercise-associated upper respiratory tract obstruction increases arrhythmia risk during testing; investigating and managing respiratory issues may help reduce exercise-induced rhythm disturbances
- •Most complex rhythms occur during recovery rather than gallop phase; ensure adequate monitoring post-exercise during clinical evaluations and use consistent testing conditions when comparing horses
Key Findings
- •Of 245 racehorses, 35.5% had no ectopic rhythms, 44.9% had isolated premature depolarisations, and 19.6% had complex tachydysrhythmias during exercise testing
- •Rhythm disturbances were detected during warm-up (8.2%), gallop (24.9%), and recovery (50.6%), with most complex events occurring during recovery phase
- •Treadmill exercise tests, exercise-associated upper respiratory tract obstructions, and National Hunt racing type were associated with increased risk of rhythm disturbances
- •82% of horses with detected rhythm disturbances returned to racing, with no significant association between rhythm disturbance presence or type and subsequent racing performance