Abstract
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.
Highlights
High altitude (HA) exposure leads to a number of well recognized physiological responses under hypobaric hypoxia (West, 2006)
Ascent to HA ≥2543 m was associated with a significant fall in SpO2, higher rating of perceived exertion (RPE) scores, and an increase in average heart rate, Lake Louise Scoring (LLS) and in the average sleep score component of the LLS (Table 1)
The only notable change in frequency domain measures of heart rate variability (HRV) was an increase in LF power and fall in HFnu at the highest altitude (Table 2)
Summary
High altitude (HA) exposure leads to a number of well recognized physiological responses under hypobaric hypoxia (West, 2006). The influence of HA on the changes in cardiac inter-beat intervals (IBI), known as heart rate variability (HRV), has been an area of significant recent research interest (Huang et al, 2010; Karinen et al, 2012; Boos et al, 2016a, 2017b; Mellor et al, 2017) This attention relates, in part, to the fact many of the factors that are known to affect HRV (e.g., fatigue, stress, insomnia, hypoxia, and cold) are predominant at HA (West, 2006; Kemp et al, 2012; Kiviniemi et al, 2014). Patch monitors can non-invasively and accurately measure the cardiac IBIs, whilst negating the need for intrusive chest straps or electrocardiogram cables that are prone to interference and detachment Despite these advantages, their utility to assess HRV at HA has not been examined. This study sought to examine this relationship at terrestrial HA
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