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 QT interval and cardiac restitution ratio in Thoroughbred racehorses during maximal effort. Automated restitution analysis was performed using 2709 pairs of cardiac cycles from 30 Thoroughbred horses during races. Cardiac cycles were obtained during: pre-race; acceleration out of the gate; race; immediate post-race; 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 pre-race 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 post-race period were relatively unresponsive to changes in RR interval compared with other exercise periods. QT intervals were longer and cardiac restitution ratio higher pre-race 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 mechanism 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.