Objective: To investigate the changes of autonomic nervous system （ANS） function during the initial phase at acute high-altitude exposure. Methods: Ninety-nine healthy sea-level male residents were studied in Chengdu plain and then Tibet plateau. Heart rate variability （HRV）

cold pressor test （CPT）

resting heart rate （HR） and blood pressure （BP） were measured at baseline （560 m altitude） and in 2 to 4 d after arriving at Tibet plateau （3 675 m altitude） to assess the ANS function. Results: Compared with baseline

on day 2 in Tibet the standard deviation of normal to normal intervals （SDNN）

high-frequency （HF） power

total power （TP）

root mean square of delta RR （rMSSD）

percentage of delta RR>50 ms （PNN50）

normalized high-frequency （HFnu） and fractal dimension （FD） decreased significantly （SDNN

HF

TP P<0.01

rMSSD

PNN50

HFnu

FD P<0.05）

while the normalized low-frequency （LFnu） and LF/HF increased significantly （P<0.01）. During day 3–4 in Tibet

SDNN

rMSSD

HF

TP and HFnu tended to rebound while LFnu and LF/HF decreased towards baseline day by day. In addition

in Tibet the increase in systolic pressure （SP） and diastolic pressure （DP） during CPT decreased significantly （P<0.01

0.05）

but resting HR increased compared with baseline （P<0.01）. Conclusion: ANS modulation is generally blunted

and the relatively predominant sympathetic control is enhanced originally

then it reverts to the sea level states gradually during the initial days of acute high-altitude exposure.