QT interval and cardiac restitution dynamics in racehorses: Observations during maximal effort.

Overview

• This study investigates how the QT interval and the cardiac restitution ratio (QT/TQ interval) change in Thoroughbred racehorses during maximal exercise effort, to better understand the risk factors associated with sudden cardiac death in athletic animals and potentially humans.

Background and Motivation

• Sudden cardiac death is a major cause of death in athletes, including humans and horses.
• Racehorses, due to their high-intensity exercise and susceptibility, provide a valuable model for studying the mechanisms underlying sudden cardiac death.
• The cardiac restitution ratio (QT interval divided by TQ interval) is a known electrophysiological marker used to assess arrhythmia risk in humans but has been less studied in athletes or racehorses.

Research Aim

• To characterize changes in the QT interval and cardiac restitution ratio in Thoroughbred racehorses during different phases of maximal exertion.
• To understand how these cardiac electrophysiological parameters vary before, during, and after maximal effort.

Methodology

• Data Collection:
  • Electrocardiographic (ECG) data from 30 Thoroughbred racehorses were collected, focusing on 2709 pairs of cardiac cycles.
  • Cardiac cycles were segmented into five distinct exercise periods: pre-race, acceleration out of the gate, race (maximal effort), immediate post-race, and recovery.
• Data Analysis:
  • Automated cardiac restitution analysis was performed by analyzing QT and TQ intervals within these different periods.
  • Multivariable linear regression was used to analyze the relationships between QT interval, cardiac restitution ratio, RR interval (time between heartbeats), exercise period, and their interactions.

Key Findings

• The relationship between QT interval and RR interval (QT/RR relationship) varied significantly depending on the exercise period.
• During maximal effort (racing), the QT/RR relationship was distinct compared to pre-race and recovery periods, implying exercise-specific cardiac electrophysiological changes.
• Exercise period, RR interval, and their interaction were all significantly associated with changes in both QT interval and cardiac restitution ratio, indicating a complex interplay between heart rate and exercise intensity on cardiac function.
• Post-race QT intervals showed relatively little change in response to RR interval changes, suggesting altered cardiac responsiveness immediately after intense exercise.
• Pre-race QT intervals were generally longer, and the cardiac restitution ratio was higher compared to the recovery phase at the same RR intervals, signalling changes in myocardial electrophysiology between rest and recovery.

Interpretations and Implications

• The QT/RR relationship is multifactorial and influenced by different intracellular and extracellular mechanisms across exercise periods.
• Possible mechanisms include changes in:
  • Intracellular ionic processes affecting cardiac repolarization.
  • Extracellular factors such as autonomic nervous system activity modulating cardiac rhythm during different exercise states.
• These dynamic changes in cardiac electrical properties during maximal exertion might have biologically significant implications for myocardial electrophysiology and the pathophysiology of arrhythmias in athletes.
• Findings support the utility of racehorses as an animal model to study cardiac function and sudden cardiac death risk during intense exercise.

Conclusion

• This study provides novel insights into how cardiac electrical intervals adapt during different phases of exercise in Thoroughbred racehorses.
• By characterizing QT interval behavior and cardiac restitution ratio changes across exercise periods, it better informs the understanding of arrhythmia risk associated with maximal exertion.
• Such knowledge may help improve prevention strategies for sudden cardiac death in both equine athletes and potentially in human athletes through translational research.