Objective To observe the myocardial protective effect of dexmedetomidine in patients undergoing open-heart surgery under cardiopulmonary bypass (CPB).Methods 50 patients of open-heart surgery under CPB were randomly divided into two groups equally, namely observation group and control group.Observation group was treated with injection of dexmedetomidine at 0.5 μg/kg for 15 min, and then maintained at 0.4 μg/kgoh.The control group was given equal volume of normal saline.Concentrations of IMA and cTnI were determined before anaesthesia (t0), after 30 minutes of CPB (t1) and after surgery (t2).Results IMA and cTnI concentrations of t1 and t2 in the observation group were significantly lower than those in the control group (P<0.05).Conclusion Dexmedetomidine has obvious protective effect on myocardium, which can reduce open-heart surgery of patients with myocardial ischemia reperfusion injury (MIRI).