Objective: To determine the change and relationship of spectral components of heart rate variability (HRV) measurements in subjects with or without acute mountain sickness (AMS) at both low and high altitude. DESIGN: A prospective study. SETTING: A 12-day itinerary by trekking to the Namche Bazaar, 3440 m in Nepal. PARTICIPANTS: A total of 32 subjects were recruited. INTERVENTIONS: The alternations were measured by heart rate (HR), arterial oxygen saturation (SpO2), and spectral analysis of HRV at sea level, 1317 m, 3440 m, 1317 m, and sea level, respectively. MAIN OUTCOME MEASURES: Spectral analysis of HRV. RESULTS: There were statistically significant increases in HR and decreases in SpO2 in all subjects at high altitude. In HRV, the values of R-R interval, total variance, high frequency (HF), low frequency (LF), and HF% were significantly lower at 3440 m than at sea level, respectively (P <0.05). The subjects with AMS had significantly lower total variance, HF, and HF%, respectively, but higher LF:HF ratio (P <0.05) at 3440 m. Subjects with both HF% <20% (nu) and LF:HF ratio > 1.3 measured at 1317 m had odds ratios of 7.00 (95% confidence interval, 1.11 to 44.06; P = 0.047) to get AMS at 3440 m. CONCLUSIONS: The HRV measurements in total variances, HF, and HF% in trekkers with AMS were statistically significantly lower at high altitude. HF% <20% (nu) or LF:HF ratio > 1.3 at lower altitudes could be an important predication parameter of trekkers with AMS at higher altitudes.
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