Quantification of a Circadian Rhythm in Heart Rate Variability in horses

Abstract

Background: HRV is a recognized parameter for autonomic nervous system functioning 1 and it’s relationship with physiological, mental and disease states has been described. In humans, circadian rhythms in HRV have been described. Horses do show circadian behavior and many variables exist that show a circadian pattern. The possible existence of a circadian rhythm in HRV in horses should be taken into account to diminish the potential for incorrect conclusions and to add precision to conclusions by qualifying measurements by the time of day when they were made. Objectives: to find and describe circadian rhythms in HRV parameters and determine their significance. Methods: 12 healthy Dutch Royal Warmblood horses were housed under stationary conditions with a daily routine in an accustomed environment. An ECG was recorded for two consecutive 24hrs periods. RR intervals were calculated and all RR intervals were analyzed. Data was chronobiologically analyzed and statistical analysis was performed to find significant rhythms. Results: Clear rhythms for mean RR, SD RR, mean HR, SD HR, RMSSD, pNN50, NN50, LF(Hz), VLF(ms2), LF(ms2), HF(ms2), TP, LF/HF, ApEn, SampEn, SD1 and SD2 were found. Statistical analysis with significance level set at p<0,05 showed rhythms for mean RR, mean HR, pNN50, VLFpower, LFpower and TP to be significant. Conclusions & clinical relevance: Measurements taken on HRV at different times of day lead to different results and therefore may result in misleading conclusions. Clinical relevance can thus be found in the fact that future research can add precision to conclusions by qualifying measurements by the time of day when they were made.