Heart rate variability parameters as markers of the adaptation to a sealed environment (a hypoxic normobaric chamber) in the horse.
Authors: Muñoz Ana, Castejón-Riber Cristina, Castejón Francisco, Rubio Dolores M, Riber Cristina
Journal: Journal of animal physiology and animal nutrition
Summary
# Editorial Summary Researchers monitoring six horses over a six-day acclimatisation protocol sought to establish whether hypoxic chamber training—a method used in human endurance athletes—could be safely implemented in equines, and critically, what physiological stress markers might indicate readiness. Using heart rate variability (HRV) analysis across multiple parameters (time-domain, spectral and non-linear measures), the team gradually increased chamber exposure from one to six hours daily, documenting sympathetic nervous system activation during the first three days through elevated heart rate, reduced RR intervals and diminished parasympathetic indices. A clear adaptation pattern emerged: whilst days 1–3 showed stress indicators including increased sympathetic tone, parasympathetic activity recovered and stress markers stabilised from day 4 onwards, particularly when chamber time exceeded four hours. The findings suggest a mandatory three-day acclimation period is essential before introducing hypoxic conditions, and that HRV parameters—particularly SDNN, RMSSD and SD1/SD2 ratios—offer reliable, non-invasive biofeedback for practitioners assessing an individual horse's readiness for this training modality. For equine professionals considering hypoxic protocols, this work provides an evidence-based timeline for safe implementation and objective autonomic markers to guide individualised training programmes.
Read the full abstract on PubMed
Practical Takeaways
- •If planning to use hypoxic chambers or sealed environments for performance enhancement in horses, allow a minimum 3-day acclimation period before introducing hypoxic stress to minimize welfare concerns
- •Monitor heart rate variability in the first 24-48 hours of sealed environment exposure as an indicator of stress; normalization of HRV parameters suggests successful adaptation
- •Extended periods (>4 hours) in sealed chambers should only be implemented after full acclimation is confirmed, as this reduces detectable stress markers
Key Findings
- •Horses required at least 3 days of acclimation before tolerating sealed chamber conditions, with sympathetic activation and parasympathetic attenuation on days 1-3
- •Heart rate and SDHR increased during initial exposure (days 1-3) with decreased RR interval duration indicating acute stress response
- •By days 4-5, horses showed increased SDNN, RMSSD, SD1 and SD2 values indicating parasympathetic recovery and successful adaptation
- •Spectral HRV parameters showed minor variations with increased LFpeak and LF% only during later acclimatization days (4-5)