Environmental Factors

Methods

Systematic review and meta-analysis of altitude HRV studies

Key Findings

• Acute altitude markedly reduces time and frequency domain HRV
• LF/HF ratio increases (sympathetic predominance)
• Effects more pronounced above 3,500m
• Trained individuals show better vagal preservation
• Gradual ascent helps maintain autonomic balance

Limitations

Individual acclimatization rates vary significantly

Methods

Review of combined heat and altitude training protocols

Key Findings

• Both heat and altitude stress autonomic system
• HRV helps monitor adaptation progress
• Metrics stabilize as acclimatization occurs
• Heat may provide similar hematological benefits to altitude
• Combined training increasingly common in elite sport

Limitations

Optimal protocols still being refined

Methods

Systematic review and meta-analysis of 24 cold exposure studies

Key Findings

• Cold exposure increased RMSSD and HF power
• Enhanced parasympathetic and reduced sympathetic activity
• 5 minutes post-exercise CWI improves HRV recovery
• Baroreflexes activated by cold water pressure
• Improved subjective sleep quality with regular use

Limitations

Optimal duration and temperature vary by individual

Methods

Panel studies measuring HRV and PM2.5 exposure

Key Findings

• PM2.5 exposure decreases SDNN, RMSSD, and HF power
• Obese individuals show greater HRV decrements
• People with hypertension/diabetes more susceptible
• Inflammation and oxidative stress likely mechanisms
• Blood pressure control may attenuate effects

Limitations

Individual susceptibility varies significantly

Methods

Heat acclimatization studies with HRV monitoring

Key Findings

• Acute heat exposure reduces HRV during exercise
• Sympathetic dominance increases in hot conditions
• 10-14 days of heat exposure improves tolerance
• HRV stabilizes as acclimatization progresses
• Hydration status affects autonomic response

Limitations

Individual heat tolerance varies significantly