Background: High altitude (HA) exposure can lead to changes in resting heart rate variability (HRV), which may be linked to acute mountain sickness (AMS) development. Compared with traditional HRV measures, non-linear HRV appears to offer incremental and prognostic data, yet its utility and relationship to AMS have been barely examined at HA. This study sought to examine this relationship at terrestrial HA.Methods: Sixteen healthy British military servicemen were studied at baseline (800 m, first night) and over eight consecutive nights, at a sleeping altitude of up to 3600 m. A disposable cardiac patch monitor was used, to record the nocturnal cardiac inter-beat interval data, over 1 h (0200–0300 h), for offline HRV assessment. Non-linear HRV measures included Sample entropy (SampEn), the short (α1, 4–12 beats) and long-term (α2, 13–64 beats) detrend fluctuation analysis slope and the correlation dimension (D2). The maximal rating of perceived exertion (RPE), during daily exercise, was assessed using the Borg 6–20 RPE scale.Results: All subjects completed the HA exposure. The average age of included subjects was 31.4 ± 8.1 years. HA led to a significant fall in SpO2 and increase in heart rate, LLS and RPE. There were no significant changes in the ECG-derived respiratory rate or in any of the time domain measures of HRV during sleep. The only notable changes in frequency domain measures of HRV were an increase in LF and fall in HFnu power at the highest altitude. Conversely, SampEn, SD1/SD2 and D2 all fell, whereas α1 and α2 increased (p < 0.05). RPE inversely correlated with SD1/SD2 (r = -0.31; p = 0.002), SampEn (r = -0.22; p = 0.03), HFnu (r = -0.27; p = 0.007) and positively correlated with LF (r = 0.24; p = 0.02), LF/HF (r = 0.24; p = 0.02), α1 (r = 0.32; p = 0.002) and α2 (r = 0.21; p = 0.04). AMS occurred in 7/16 subjects (43.8%) and was very mild in 85.7% of cases. HRV failed to predict AMS.Conclusion: Non-linear HRV is more sensitive to the effects of HA than time and frequency domain indices. HA leads to a compensatory decrease in nocturnal HRV and complexity, which is influenced by the RPE measured at the end of the previous day. HRV failed to predict AMS development.

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Introduction: There is evidence that intermittent hypoxic exposure (IHE) may improve high altitude (HA) performance. In this study the effects of short-term IHE via voluntary apnoea training on HA-related symptoms including acute mountain sickness (AMS) were examined for the first time.Methods: Forty healthy adults were randomised to a self-administered apnoea training (n=19) or to a control group (n=21 no apnoea training) prior to ascent to an altitude of 5100m in the Himalayas over 14 days. The apnoea training was conducted at sea level and consisted of five breath holds per day in week one, seven in week two, followed by 10 per day from weeks 3-6 and until HA exposure. Arterial oxygen saturation (SpO2), heart rate, sleep quality (Insomnia Severity Index, ISI), rating of perceived exertion (RPE), blood pressure and Lake Louise Scores were measured at sea level (in the UK) and at HA at 1400m, 2700m, 3400-3700m, 4050-4200m, 4800m and 5100-5200m. Anxiety (GAD-7) scores were examined at sea level, 1400m and at 5100-5200mResults: Apnoea training led to a significant improvement in the mean longest breathhold times from baseline (80.42±32.49 [median 87.00] seconds) to the end of week six (107.02±43.65 [113.00] seconds) respectively (p=0.009). There was no significant difference in the prevalence of AMS (8/19=42.1% vs 11/21=52.4%; RR 0.80; 95% CI 0.41-1.57: p=0.80) or in GAD-7, ISI and RPE, SpO2, heart rate or blood pressure among the apnoea versus control groups respectively at HA. Conclusions:Apnoea training does not lessen HA-related symptoms in healthy adults travelling up to 5200m. Larger studies using more challenging apnoea protocols and to higher altitudes should be considered.

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