Effect of gait on, and repeatability of heart rate and heart rate variability measurements in exercising Warmblood dressage horses

Heart rate variability (HRV) is a non-invasive measurement of the balance between the sympathetic and parasympathetic innervation to the myocardium. Used as an indicator of health, welfare and fitness level, its measurement during exercise has been suggested to be helpful in sport horses. The aims of this study were to assess the within- and between-day repeatability of time-domain HRV-measurements during exercise, and to evaluate the effect of gait (as a factor of increasing exercise intensity) on HRV. Heart rate (HR), root mean square of successive differences (RMSSD) between normal heartbeats and the standard deviation of normal (R-R) intervals (SDNN) were assessed during a standardised exercise test (SET) on five consecutive days in a sample of 21 healthy Warmblood dressage horses. All variables showed good between-day repeatability (Spearman’s rank correlation coefficient), while only RMSSD showed reliable within-day repeatability (Wilcoxon signed-rank test). Increase of exercise intensity was significantly associated with increased HR and decreased RMSSD and SDNN, with a negative correlation between HR and the two HRV-measurements. HR and HRV values varied significantly with intensity of exercise (rest, walk, trot, canter and whole SET), though HRV at rest was not different to that at walk, and HRV at trot was not different to that at canter. In conclusion, the results of this study support the use and show the repeatability of time-domain HRV-measurements during exercise in Warmblood dressage horses. Further investigation for use of HRV in other equestrian disciplines, levels and health state is warranted.