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Objective: To assess the effects of physical laborers on cardiac function at high altitude and changes in cardiac function after returning to lower altitude.Methods: According to symptomatic scores on acute high altitude reaction (AHAR), 96 male officers and soldiers, who rapidly entered high altitude areas (3700m), and engaged in heavy physical work for 50 day s,were be scored and graded. Serum CK-MB and LDH-1 levels, Tei index, LVEF and LVFS were measured in the 96 servicemen at the 50th day of residing at high altitude, and the 2nd and 15th day after returning to 1500m,and the results were compared with that of 50 healthy controls residing at 1500m.Results: Among the 96 male servicemen, 71 developed AHAR,a nd 24 of them had severe AHAR, 47 mild to moderate AHAR, and the rest 25 had no AHAR. Serum CK-MB,LDH-1 and Tei index were higher significantly in AHAR group than in the no AHAR group. As far as the values of LVEF and LVFS were concerned. Correlation analysis showed that levels of CK-MB and LDH-1 of persons staying at 3700m for 50 days were positively correlated with Tei index, and negatively correlated with LVEF, and also negatively correlated with LVFS. serum CK-MB, LDH-1 and Tei index at 3700m for 50 days were significantly higher than those 2 days and 15 days after returning to 1500m and those in control group. Moreover, the values of LVEF and LVFS were significantly lower than those at 2 and 15 days after returning to 1500m and those in control group (P<0.01).Conclusion: Heavy physical work at high altitude could obviously impair cardiac function. The impairment may aggravate along with increase in severity of AHAR. However, cardiac function may be improved significantly after returning to low altitude for 2 days, and recover to normal status 15 days later.