Background and Aim: Pregnancy affects maternal hemodynamics. The changes in autonomic nervous system activity for hemodynamics adaptation in pregnant horses are still unclear. Thus, this study aimed to examine the effect of pregnancy on electrocardiographic, vasovagal tonus index, and echocardiographic variables in horses. Materials and Methods: A total of 23 Thai native crossbred mares without any cardiac abnormalities were included in this study. The animals were assigned into two groups, a non-pregnant mare group (n =12) and a pregnant mare group (n = 11). Electrocardiogram recordings (paper speed = 25 mm/s and calibration = 10 mm/mV) were performed to obtain six limb leads (leads I, II, III, aVR, aVL, and aVF). The vasovagal tonus index (VVTI) was calculated to assess variability in heart rate over short periods using just 20 consecutive beats. Cardiac structure and function were evaluated by echocardiography. Results: Heart rate, P wave duration, PR interval, QRS duration, QT interval, and T wave duration were significantly different between non-pregnant and pregnant horses (p < 0.05). Pregnant horses had significantly lower VVTI than non-pregnant (p < 0.05). There were no significant differences in cardiac structures including % interventricular septum (IVS), % left ventricular posterior wall (LVPW), IVS in diastole, left ventricular internal diameter at end-diastole, LVPW thickness at end-diastole, IVS in systole, left ventricular internal diameter at end-systole, LVPW thickness at end-systole, and left atrium/aortic roots ratio between the two groups. However, the pregnant horses had a significantly higher cardiac output and % ejection fraction than non-pregnant horses (p < 0.05). Conclusion: This study provided the first evidence that hemodynamic adaptations during pregnancy modified cardiac conduction, vasovagal tonus index, and echocardiographic variables in horses. Keywords: echocardiography, electrocardiogram, horses, pregnancy, vasovagal tonus index.

Background and Aim: Vasovagal tonus index (VVTI) serves as a straightforward assessment tool for autonomic function during both physiological and pathological conditions, including pregnancy, in horses. Obtaining VVTI through a modified base apex lead system could be a practical and comfortable solution. In this study, we assessed VVTI in horses with respect to training status and electrocardiographic measurements utilizing a novel modified base apex lead technique. Materials and Methods: A total of 12 Thai native crossbred horses and 12 Arabian horses, all free of cardiac abnormalities, were enrolled in the study. Animals underwent electrocardiogram (ECG) and VVTI using both the base-apex lead method and its modified version. 25 mm/s and 10 mm/mV ECG recordings provided standard bipolar limb leads. The amplitudes and durations of P waves, QRS complexes, PR interval, QT interval, and T duration were assessed. The T wave’s shape was examined. Each recording’s R-R interval was utilized to assess heart rate. Twenty consecutive beats were used to compute the variability of heart rate (VVTI). Results: The P wave amplitude was the only significant difference (p < 0.05) between the base apex lead method and the modified base apex lead method, with no variations in heart rate, P duration, PR interval, T duration, and QRS duration and amplitude. Both methods showed mainly biphasic T wave patterns. The VVTI values of all horses did not differ significantly between the base apex and modified base apex methods. There was no significant difference in VVTI between Thai crossbred horses and Arabian horses in either method. Conclusion: This study provided the first evidence that VVTI can be evaluated using the modified base apex lead system and may be useful for cardiovascular function monitoring in horses. Keywords: electrocardiogram, evaluation, horses, vasovagal tonus index.

Background and Aim: 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. Materials and Methods: 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. Results: 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. Conclusion: 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. Keywords: heart rate, heart rate variability, pregnancy, mares, vasovagal tonus index, autonomic regulation.