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

Overview

• This study investigated how heart rate variability (HRV), an indicator of autonomic nervous system function, varies over 24 hours in horses living in tropical urban environments.
• It examined how aging and environmental factors, including air pollution, affect autonomic regulation in these horses.

Study Objectives and Rationale

• HRV reflects autonomic nervous system activity, specifically the balance between sympathetic (fight or flight) and parasympathetic (rest and digest) control.
• While HRV circadian rhythms and effects of aging are studied in humans and some animals, limited data exist on horses raised in tropical climates, where environmental stressors like heat and pollution may impact autonomic function.
• The researchers aimed to characterize the 24-hour patterns of heart rate and HRV in tropical horses under real-life conditions and assess how age and environmental factors—especially air pollution—affect these measures.

Methods

• Subjects: 15 clinically healthy horses divided into 3 age groups: 4-7 years, 8-14 years, and 15-20 years (5 horses per group).
• Data collection: Continuous 24-hour heart rate and HRV data were recorded using Polar H10 heart rate sensors, which are wearable and validated for such monitoring.
• Variables analyzed: Time-domain HRV indices (e.g., root mean square of successive differences, RMSSD) and frequency-domain indices (e.g., high-frequency power, HF; low-frequency power, LF; and LF/HF ratio) using Kubios Scientific software.
• Environmental monitoring: Parameters including temperature, humidity, feels-like temperature, light intensity, Air Quality Index (AQI), and particulate matter ≤2.5 μm (PM2.5) were continuously recorded in Bangkok, Thailand.
• Analysis focus: Circadian (day vs. night) variations in HR and HRV, age-related differences, and correlations between HRV and environmental factors.

Key Findings

• Circadian variations: Heart rate and normal-to-normal (NN) intervals showed significant differences between day and night, but other HRV indices showed limited circadian variation.
• This suggests a blunted nocturnal parasympathetic predominance in tropical horses, meaning their parasympathetic activity normally higher at night was less marked.
• Age-related effects: Older horses (15-20 years) had significantly lower parasympathetic-related HRV indicators, including RMSSD and HF power, compared to younger groups. This indicates reduced parasympathetic nervous system activity with age.
• Environmental influences: Increased PM2.5 and AQI levels correlated negatively with markers of parasympathetic activity (RMSSD and HF power) and positively with the LF/HF ratio, which suggests a shift toward sympathetic dominance or autonomic imbalance when air pollution levels are higher.

Interpretation and Implications

• Tropical horses display attenuated circadian rhythms in their autonomic nervous system activity, potentially due to consistent environmental stressors or adaptation to tropical conditions.
• Age diminishes parasympathetic tone in horses, paralleling findings seen in other species and indicating the need for age-specific health management strategies.
• The novel finding that ambient air pollution, specifically fine particulate matter (PM2.5) and overall AQI, impairs autonomic balance in horses suggests that urban environmental conditions negatively affect equine cardiovascular health.
• These results emphasize the importance of monitoring environmental quality and adjusting care practices to mitigate health risks in urban tropical settings, supporting equine welfare and management decisions.

Summary

• This study offers the first evidence linking environmental air pollution with alterations in cardiac autonomic regulation in tropical horses.
• Findings highlight age-related parasympathetic decline and a reduced day-night variation in autonomic control in these animals.
• Overall, maintaining good air quality and tailoring care based on age may improve health outcomes for horses living in urban tropical environments.