Abstract:
Heart rate variability (HRV) is an important non-invasive method to quantify stress by
measuring sympathetic and parasympathetic activity of the autonomic nervous system. Few studies
exist on the repeatability and reliability of HRV in equids. The objectives of this study were to (a)
compare the effect of different correction factors (CF) available in HRV analysis software on HRV
indicator values and (b) to determine the repeatability and reliability of HRV indicators in an
unrestricted (pasture) and a restricted movement (equine examination stocks) environment. Data were
recorded on five consecutive days from six adult pony mares. Short term tachograms were compared
with regards to software CF by graphical and statistical (Friedman’s and Wilcoxon signed rank test)
comparison. The results showed that the specific CF influences the HRV indicator values. The Strong
CF was able to balance the elimination of artefacts without removing the variability of RR-intervals
and was subsequently used to determine repeatability and reliability. HRV indicators showed good
repeatability over the 5 days using Friedman’s test (pasture: p=0.162-0.898; examination stocks:
p=0.29-0.865), indicating that there were no significant differences between HRV indicator values.
The reliability, represented by intraclass correlation coefficient (ICC) and coefficient of variation
(CV), was poor to good for pasture data (ICC=0.44-0.79; CV=10-68.10) and examination stocks data
(ICC=0.22-0.83; CV=16.07-62.29). Measurements obtained from the examination stocks were less
reliable than those from pasture. Using normalized low-frequency and normalized high-frequency
components improved reliability. Free-movement environment based HRV recordings could ensure
better reliability, but may require the use of a stronger CF.
