Key result
Exercise-induced increases in heart rate in Thoroughbred horses reduced critical values of QT and TQ intervals by 38.9% and 86.2% respectively, ensuring stable electrophysiological activation.
Observational (n=7)
No
In vivo restitution analysis in exercising horses demonstrates electrophysiological adaptations that prevent arrhythmias over a wide range of heart rates, suggesting the horse as a novel animal model for studying sudden cardiac death in human athletes.
No immediate implications for human care; leaves open translational relevance of rate-dependent QT stability in equine models.
Exercising horses uniquely accommodate 7-8-fold increases in heart rate (HR). The present experiments for the first time analysed the related adaptations in action potential (AP) restitution properties recorded by in vivo telemetric electrocardiography from Thoroughbred horses. The horses were subjected to a period of acceleration from walk to canter. The QRS durations, and QT and TQ intervals yielded AP conduction velocities, AP durations (APDs) and diastolic intervals respectively. From these, indices of active, λ = QT/(QRS duration), and resting, λ0 = TQ/(QRS duration), AP wavelengths were calculated. Critical values of QT and TQ intervals, and of λ and λ0 at which plots of these respective pairs of functions showed unity slope, were obtained. These were reduced by 38.9±2.7% and 86.2±1.8%, and 34.1±3.3% and 85.9±1.2%, relative to their resting values respectively. The changes in λ were attributable to falls in QT interval rather than QRS duration. These findings both suggested large differences between the corresponding critical (129.1±10.8 or 117.4±5.6 bpm respectively) and baseline HRs (32.9±2.1 (n = 7) bpm). These restitution analyses thus separately identified concordant parameters whose adaptations ensure the wide range of HRs over which electrophysiological activation takes place in an absence of heart block or arrhythmias in equine hearts. Since the horse is amenable to this in vivo electrophysiological analysis and displays a unique wide range of heart rates, it could be a novel cardiac electrophysiology animal model for the study of sudden cardiac death in human athletes.
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Li et al. (2018) conducted an observational in Healthy Thoroughbred horses (n=7). Exercise-induced heart rate increase vs. Resting state was evaluated on Reduction in critical values of QT and TQ intervals relative to resting values. Exercise-induced increases in heart rate in Thoroughbred horses reduced critical values of QT and TQ intervals by 38.9% and 86.2% respectively, ensuring stable electrophysiological activation.
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