The investigators hypothesized that dexmedetomidine as an anesthetic adjunct can be used in the setting of off-pump coronary bypass procedure and can provide benefits to cardiac function and cognitive performance on the ground of the dexmedetomidine's characteristics.

The emergence of off-pump coronary artery bypass grafting (OPCAB) procedure brings up new challenges to anesthesiologists and urges us to contemplate the choice of anesthetics. In face of these challenges Dexmedetomdine with multiple unique traits can readily fit into this situation.

Off-pump coronary artery bypass (OPCAB) procedure is an emerging procedure which might bring benefit to patients in comparison with conventional CABGs although final words about outcomes haven't been reached on the basis of recent literature. However, OPCAB technique definitely requires more involvement of anesthesiologists especially in cases where hemodynamic compromise is usually inevitable during anastomosis of Lcx or PDA and early extubation in the OR is expected1. Thus, the investigators are encountered with more intraoperative challenges caused by OPCAB than routine CABGs. Concerning about challenges the investigators might take on, the investigators shall not only pay more attention to surgeons' manipulation but also review anesthetics that the investigators have selected to find an optimal plan to minimize adverse effects on heart and other important organs. With this perspective, dexmedetomidine come into our sight with its peculiar traits of hemodynamic effects and assumed cardiac protective effects.

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.

Inclusion Criteria:

• American Society of Anesthesiologists (ASA) scores are between classⅡand Ⅲ
• undergo off-pump coronary artery bypass grafting (OPCAB)

Exclusion Criteria:

• left ventricular ejection fraction <40%
• left ventricular aneurysm
• acute myocardial infarction in latest two weeks
• atrial fibrillation
• associated vascular diseases
• severe systemic diseases involving the renal and hepatic systems
• respiratory disease( forced vital capacity less than 50% of predicted values ) preoperative left bundle branch block .

Patients who are planned to undergo off-pump coronary bypass surgery