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

Sudden cardiac death is a leading cause of athletic death in both humans and horses, making racehorses a potentially valuable model for investigating sudden cardiac death. Cardiac restitution ratio (QT/TQ interval) is used to assess arrhythmia risk in humans, but investigations in athletes are scarce. The objective was to characterize the QT interval and cardiac restitution ratio in Thoroughbred racehorses during maximal effort. Automated restitution analysis was performed using 2,709 pairs of cardiac cycles from 30 Thoroughbred horses during races. Cardiac cycles were obtained during: prerace; acceleration out of the gate; race; immediate postrace; and recovery phases. Multivariable linear regression analyses were performed with both QT interval and cardiac restitution ratio as outcomes of interest. The QT/RR relationship varied by exercise period with cycles during maximal effort being distinct from those of prerace and recovery periods. Exercise period, RR interval, and their interaction were significantly associated with both QT interval and cardiac restitution ratio during maximal effort. QT intervals during the postrace period were relatively unresponsive to changes in RR interval compared with other exercise periods. QT intervals were longer, and cardiac restitution ratio were higher prerace than recovery at the same RR intervals. The QT/RR relationship is multifactorial, and variables associated with QT interval during maximal effort differ from other exercise periods. These changes suggest differences in intracellular mechanisms and/or extracellular controls (e.g., autonomic activity) between exercise periods that have biologically significant effects on myocardial electrophysiology and pathophysiology. Racehorses may be a useful model for studying cardiac function during exercise.<b>NEW & NOTEWORTHY</b> ECG recordings obtained from racehorses during competition provide insight into the cardiac physiology of athletes and potentially pathophysiologic mechanisms that underlie the risk of exercise-associated arrhythmias. The QT/RR relationship is uniquely altered during maximal effort compared with other exercise periods, and the response of the QT interval to changes in heart rate (RR interval) is decreased immediately postrace.