A consort-guided randomized, blinded, controlled clinical trial on the effects of 6 weeks training on heart rate variability in thoroughbred horses

Background: Fitness assessment of horses remains challenging. Heart rate variability (HRV) can be used to monitor human athlete's training, but its value is unknown in horses. Hypothesis: The linear domain HRV variables are affected by fitness. Animals: Twelve healthy untrained thoroughbreds were randomly split into a training group (6 weeks of incremental racetrack training) and a control group (no training). Methods: Linear domain HRV variables were analyzed (high frequency [HF], low frequency [LF], their normalized units [Hf-nu, Lf(nu)], root mean square of successive differences between beats [RMSSD], Poincar & eacute; plot features [SD1 and SD2]) while resting overnight before (baseline) and after 2, 4 and 6 weeks of training. VO2max and echocardiographic indexes were measured at baseline and after 6 weeks. Changes in HRV variables over time (ANOVA), and correlation with VO2max (Pearson's chi-squared test) were tested (P < .05 significance). Results: VO2max, LF/HF ratio, and LFnu increased while HFnu decreased in the training group (before and after training mean [SD] values: VO2max 134 [12.8]-146 [16.5] mL/[kg min]; P < .001; LF/HF ratio 0.51 [0.2]-0.91 [0.3] [P = .02]; LFnu 37.5 [10.1]-46.8 [7.8] [P = .02]; HFnu 76.2 [7.9]-53.2 [7.7] [P < .001]). Training did not affect RMSSD, SD1, SD2, resting HR, or echocardiographic indexes. Strong correlations were found between VO2max and HRV variables (VO2max and LFnu [r = -0.59, P = .04]; Delta VO2max and the corresponding Delta LF [r = -0.88, P = .02]). Conclusions and clinical importance: Six weeks of training affected some frequency domain HRV variables. Further studies are necessary to validate the use of HRV for monitoring horses' responses to training.