Effect of gait on time-domain heart rate variability parameters in exercising Warmblood horses

Heart rate variability (HRV) is influenced by the autonomic nervous system and is used to monitor welfare, health and fitness in resting horses. Exercising HRV has been used to detect arrhythmias and to monitor the effect of exercise. The aim of this study was to observe the effect of gait on exercising HRV. Heart rate (HR) and two time-domain HRV-parameters – the root mean square of successive differences (RMSSD) and the standard deviation of R-R intervals (SDRR) – were assessed in 16 healthy Warmblood horses during rest (6 minutes) and a standardised 18-minute warming-up, including walk (6 minutes), trot (2×3 minutes) and canter (2×2 minutes). HRV-parameters were calculated using a commercial software on the interbeat intervals obtained from a HR monitor (Hylofit). One-way analysis of variance (ANOVA) showed significant changes in HR throughout the whole protocol. RMSSD differed significantly between rest and all gaits independently, but not between gaits (rest 51.71±25.26, walk 16.41±8.61, trot 6.09±2.25, canter 4.64±1.80; ms; mean ± SD). SDRR differed significantly between rest and all gaits and between walk and other gaits (rest 47.84±22.27, walk 25.50±8.37, trot 12.47±7.10, canter 8.38±3.10; ms). Pearson’s test showed significant correlation between HR and RMSSD (r=-0.7617; P<0.00001) or SDRR (r=-0.7691; P<0.00001). These preliminary findings in a small cohort of horses suggest gait and HR should be considered when monitoring HRV-parameters during standardised exercise tests in horses.