Dexmedetomidine as a more selective α2-adrenoreceptor agonist than clonidine has gradually gain popularity in anesthetic and ICU settings for its unique pharmacologic characteristics. Dexmedetomidine provides sufficient sedative effects with minimal respiratory effect and then reduces the consumption of other sedative and antinociceptive drugs.Given specific requirements of OPCAB surgery, dexmedetomidine's may be more suitable in varying aspects. Firstly, dexmedetomine can dramatically diminish the level of serum catecholamine (CA). As the investigators know, the diminished level of catecholamine indicates less stress response to surgical stimuli and lower incidence of cardiac events. Secondly, the relatively slower HR by dexmedetomidine can decrease oxygen consumption of myocardium and consequently improve the perfusion of endangered ischemic area. In addition, the decreased HR which is assumably analogous to effects of esmolol or other β-blockers can reduce the occurrence of tachycardia and minimize the motion of heart when "bypass" procedure is performed on the target vessels. Thirdly, it has been reported that dexmedetomidine can alleviate sufferings of renal function during major thoracic surgery .Thus, it is implied that dexmedetomidine might in the similar way ameliorate the damage to renal system caused by hours of stress response and havoc of SIRS ,and then improve clinical prognosis . In view of assumptions the investigators have proposed on the ground of dexmedetomidine's characteristics and specific requirements of OPCAB surgery, the investigators are expecting that dexmdetomidine can fit the situation where patients' limited capacity of coronary artery and sophisticated pathophysiological courses have to be taken into account.