Circadian rhythm of heart rate variability in tropical horses: age-associated autonomic alterations and environmental air pollution effects in an urban field setting

Background and Aim: Heart rate variability (HRV) reflects autonomic nervous system activity and is influenced by age, environmental conditions, and circadian rhythms. Information regarding these interactions in horses maintained under tropical conditions remains limited. This study aimed to characterize 24-h heart rate (HR) and HRV patterns in horses raised under tropical conditions and to investigate the effects of aging and environmental factors, including air pollution, on autonomic regulation. Materials and Methods: Fifteen clinically healthy horses aged 4-20 years were allocated to three age groups: 4-7 years, 8-14 years, and 15-20 years (n = 5 per group). Continuous 24-h recordings of HR and HRV were obtained using a Polar H10 sensor. Time-domain and frequency-domain HRV indices were analyzed using Kubios Scientific software. Simultaneously, environmental parameters, including temperature, humidity, feels-like temperature, light intensity, air quality index (AQI), and particulate matter ≤2.5 μm (PM2.5), were continuously monitored under field conditions in Bangkok, Thailand. Circadian variations, age-associated differences, and correlations between HRV and environmental variables were evaluated. Results: Only HR and normal-to-normal intervals differed significantly between daytime and nighttime, whereas other HRV indices exhibited limited circadian variation, indicating attenuated nocturnal parasympathetic predominance. Older horses (15-20 years) demonstrated significantly lower parasympathetic-related indices, including root mean square of successive differences (59.44 ± 7.25 ms) and high-frequency (HF) power (805.60 ± 213.37 ms²), compared with younger groups (p < 0.05). Total power was reduced, whereas the low-frequency (LF)/HF ratio increased with age, suggesting diminished autonomic flexibility and sympathetic predominance. Air pollution variables showed significant associations with autonomic imbalance. Both AQI and PM2.5 were negatively correlated with HRV indices, including root mean square of successive differences, percentage of normal-to-normal intervals differing by >100 ms, and HF power, while showing positive correlations with the LF/HF ratio (p < 0.001). Conclusion: Tropical horses exhibited blunted circadian autonomic modulation and age-related reductions in parasympathetic activity. Furthermore, elevated PM2.5 and AQI levels were associated with impaired autonomic balance, providing the first evidence linking ambient air pollution with resting HRV alterations in horses. These findings emphasize the importance of age-specific management and environmental monitoring to promote equine welfare in tropical urban environments.