Cardiac adaptations in early equine pregnancy: heart rate elevation without heart rate variability alteration in Thai...

Pregnancy induces significant anatomical and physiological changes, many of which are regulated by the autonomic nervous system (ANS). Heart rate variability (HRV) is a well-established non-invasive tool for assessing ANS activity. While changes in heart rate (HR) and HRV during the third-trimester of equine pregnancy are documented, there is limited understanding of cardiac autonomic adaptations during the early stages of gestation. This study aimed to compare HR and time-domain HRV parameters between healthy non-pregnant mares and those in the first and second trimesters of pregnancy.

A total of 45 Thai native crossbred mares were enrolled and divided into three groups: Non-pregnant (n = 5), first-trimester pregnant (0-114 days; n = 18), and second-trimester pregnant (115-226 days; n = 22). All mares were clinically healthy and free from cardiac abnormalities. Electrocardiographic data were collected using a Holter electrocardiogram system over a 15 min period at rest, and HRV was analyzed using time-domain measures: Standard deviation of all NN intervals (SDNN), SDNN index, root mean square of successive differences, standard deviation of 5-min mean NN intervals, percentage of successive NN intervals >50 ms, and vasovagal tonus index (VVTI). Data were analyzed using Kruskal-Wallis and Mann-Whitney U-tests.

HR was significantly higher in first-trimester pregnant mares compared to non-pregnant mares (p < 0.05), and even higher in the second-trimester compared to the first (p < 0.05). However, there were no significant differences among the groups in any of the HRV parameters or VVTI.

The findings indicate that cardiovascular adaptation during early pregnancy in mares is characterized by a progressive increase in HR, likely reflecting increased cardiac output to support fetal development. However, the lack of significant changes in time-domain HRV parameters and VVTI suggests that ANS balance is maintained during the first and second trimesters. These results provide valuable reference values for equine reproductive monitoring and contribute to a better understanding of physiological changes in early gestation.