Effects of acute high-altitude exposure on heart rate variability: a systematic review and meta-analysis.

However, when altitude increases to ≥3,500 m, the decreases in SDNN and the increases in LF/HF become significantly greater, whereas RMSSD, pNN50, HF, and LF, despite showing a numerical trend toward “lower values at higher altitude,” do not exhibit statistically significant differences between subgroups.

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