CONCLUSION

This study demonstrated that horses maintained under tropical urban conditions exhibit attenuated circadian autonomic modulation, characterized by only modest nocturnal reductions in HR and the absence of a pronounced nocturnal increase in parasympathetic-related HRV indices. Age-related alterations in autonomic regulation were evident, with older horses showing significantly reduced vagal activity, lower HRV indices, and higher LF/HF ratios, indicating diminished autonomic flexibility and a shift toward sympathetic predominance. In addition, elevated PM2.5 and AQI levels were consistently associated with impaired autonomic balance, as reflected by reduced RMSSD, pNN100, HF power, and total power, along with an increased LF/HF ratio. These findings provide the first evidence linking ambient air pollution with alterations in resting HRV in horses under real-world tropical conditions.

A major strength of this study was the integrative approach combining continuous 24-hour HRV monitoring, age stratification, and synchronized real-time environmental measurements under field conditions. Unlike previous studies conducted primarily under temperate climates or focused on individual determinants of HRV, the present study simultaneously evaluated circadian rhythm, aging, and environmental influences, thereby providing ecologically relevant insights into equine autonomic regulation in tropical urban environments.

Overall, the findings indicate that environmental conditions and aging exert a greater influence on cardiac autonomic function than time of day alone in tropical horses. The observed blunting of nocturnal parasympathetic activity and the association between air pollution and autonomic dysfunction suggest that horses may exhibit a unique physiological response to chronic environmental stressors. These results emphasize the importance of age-specific management and environmental monitoring to safeguard equine welfare. Furthermore, the study provides a foundation for future investigations to elucidate the mechanisms underlying pollution-induced autonomic alterations and to develop mitigation strategies to promote the health and well-being of horses living in increasingly urbanized environments.