Electrocardiographic characteristics of trained and untrained standardbred racehorses.
Authors: Nissen Sarah D, Weis Rikke, Krag-Andersen Elisabeth K, Hesselkilde Eva M, Isaksen Jonas L, Carstensen Helena, Kanters Jørgen K, Linz Dominik, Sanders Prashanthan, Hopster-Iversen Charlotte, Jespersen Thomas, Pehrson Steen, Buhl Rikke
Journal: Journal of veterinary internal medicine
Summary
# Electrocardiographic characteristics of trained and untrained standardbred racehorses Long-term athletic conditioning induces structural remodelling of the equine heart, yet its influence on electrical conduction pathways remains poorly characterised. This cross-sectional study examined resting electrocardiograms from 201 trained and 52 untrained Standardbred racehorses, supplemented by invasive electrophysiological studies in 13 animals, to determine whether training alters cardiac conduction properties. Trained horses demonstrated significantly lower resting heart rates (30.8 versus 32.9 bpm) and paradoxically exhibited a 2.59-fold higher odds of developing second-degree atrioventricular block (2AVB)—findings that challenge conventional assumptions about the benignity of training-induced bradycardia. Invasive studies revealed variable atrioventricular nodal conduction intervals within individual horses during 2AVB episodes, suggesting a more nuanced mechanism of block than simple nodal delay. These observations warrant heightened clinical vigilance for arrhythmias in trained populations, particularly given the potential welfare and performance implications of conduction disturbances, and highlight the need for further investigation into whether training-induced 2AVB represents pathological remodelling or an incidental electrophysiological variant in athletic horses.
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Practical Takeaways
- •Second-degree atrioventricular block is a normal training adaptation in racehorses and should not automatically warrant clinical concern when found on routine ECG in trained athletes
- •The lower resting heart rate in trained horses reflects appropriate cardiac conditioning but practitioners should monitor for arrhythmias as part of pre-competition screening
- •Variable atrioventricular conduction patterns in trained horses suggest the need for careful interpretation of ECG findings and consideration of electrophysiological studies when arrhythmias are detected
Key Findings
- •Trained Standardbred horses had significantly lower resting heart rate (30.8 ± 2.6 bpm vs 32.9 ± 4.2 bpm; P = 0.001) compared to untrained horses
- •Trained horses displayed second-degree atrioventricular block 2.59 times more frequently than untrained horses (P = 0.04)
- •No significant difference in sinus node recovery time was detected between trained and untrained horses (n = 13)
- •His signal recordings revealed variable H-V intervals within individual horses during atrioventricular conduction, providing novel insight into equine AV physiology