Reserved higher vagal tone under acute hypoxia in Tibetan adolescents with long-term migration to sea level.

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Tibetans are known as one of the largest and oldest high-altitude natives in the world and are among the best high-altitude-adapted ethnic groups. They exhibit greater vagal tone and less sympathetic stimulation than acclimatized lowlanders at high altitudes. Whether young native Tibetans who had spent long-term residence (more than 3 years) at sea level still reserved their unique autonomic characteristics was the main aim of this study. Heart rate variability (HRV) of 10 native young Tibetan male students and 12 Han counterparts were measured at resting supine position at sea level and 1 h after ascent to 3,700 m in a hypobaric chamber (PO(2) = 13.4 kPa). At sea level, Tibetans showed lower heart rate (HR) and greater HRV. At 3,700 m, the increase of HR was greater in the Hans than in the Tibetans, and the HRV was significantly diminished in the Han group but not in the Tibetan group. The results suggested that Tibetans had a greater parasympathetic dominance over the heart at rest, and acute moderate (3,700 m) hypoxia did not influence their HRV significantly, but it did on the Han subjects. We concluded that the long-term residence of the Tibetans at sea level did not change their unique characteristics of the autonomic systems.