Abstract: Objective　To study the mechanism of myocardial ischemic and reperfusion injury and its myocardial protection of cardiopulmonary bypass(CPB) among infants.   Methods　The concentrations of complement 3a(C3a)，complement 5a(C5a)，interleukin-6(IL-6)，tumor necrosis factor-alpha(TNF-α)，malonaldehyde(MDA)，surperoxide dismutase(SOD),  creatine kinase MB isoenzyme (CK-MB)，creatine kinase (CK)，troponin (cTnI) and lactic acid dehydrogenase (LDH) were measured before CPB，after CPB 20 min，2 h，6 h and 12 h in 20 cases of infants with congenital heart disease.  Results　Compared with the results before CPB，the levels of C3a and C5a were lower，and the levels of CK and LDH were higher at 20 min，2 h，6 h and 12 h after CPB(P<0.05). At 20 min and 2 h after CPB，the levels of IL-6 increased (P<0.05)，and at 6 h and 12 h after CPB，the levels of IL-6 decreased，but they were still higher than those before CPB (P<0.05). The level of TNF-α increased after CPB 20 min (P<0.05)，and decreased after CPB 2 h. The level of CK-MB increased after CPB 2 h (P<0.05)，and decreased after CPB 6 h  and 12 h，but it was still higher than that before CPB (P<0.05). The level of cTnI increased after CPB 2 h (P<0.05)，and the level of MDA decreased after CPB 20 min and 2 h (P<0.05)，increased significantly after CPB 6 h (P<0.05)，and decreased after CPB 12 h (P<0.05). The level of SOD decreased obviously after CPB 2 h，6 h and 12 h (P<0.05).   Conclusions　The myocardial ischemic and reperfusion injury of CPB at 12 h after CRP are serious among infants，and the mechanism of myocardial ischemic and reperfusion injury may be associated with the damages of myocardial and endothelial cells，due to alexin releasing and the increase of oxyradical.

Key words: Complement 3a, Complement 5a, Malonaldehyde, Superoxide dismutase, Myocardial injury , Infant, Cardiopulmonary bypass