<qa>  What is the MAC of desflurane defined as?All patients will be preoxygenated with 100 oxygen for 3 minutes. The patients in group D will receive dexmedetomidine 2 ml diluted in 18 ml of saline IV in a dose of 1 mcgkg over10 minutes through infusion pump prior to induction. Group P patients will receive similar volume of normal saline over 10 min. The study drug and placebo infusions will be prepared by an anesthesiologist who is not involved in the study and the anesthesiologist recording the details is unaware of the type of infusion patients received. Anesthesia will be induced with IV fentanyl 1.5 μgkg and propofol in a dose of 10 mg every 5 seconds until the BIS level dropped below 60 and confirmed with loss of response to verbal commands. In both groups, Atracurium 0.5 mgkg IV will be administered, and trachea will be intubated. Fresh gas flow oxygen in air 30-40 at a rate of 3 Lmin for 10 minutes will be administered then the flow will be decreased at a rate of 1 Lmin using a closed system Fabius GS, Dräger, Lübeck, Germany and ventilation will be adjusted to maintain end-tidal carbon dioxide at 30-35 mmHg. Patients in study groups will receive the volatile agent at a concentration of 1 MAC Minimum Alveolar Concentration. The MAC of desflurane is defined as an expired fraction of 6.0 vol. Then the dial-up desflurane percentage will be adjusted to establish a BIS value between 40- 50. A maintenance dose of Dexmedetomidine infusion at 0.5 mcgkghour made to 20 mL with normal saline will be administered in group D patients and a similar volume of normal saline will be administered in Group P, till the end of surgery. Top up doses of Atracurium will be given as needed. When heart rate and blood pressure exceeds 20 of baseline in spite of adequate level of anesthesia guided by BIS, boluses of 0.5 μgkg will be given. While decrease in heart rate and blood pressure more than 20 of baseline, will be treated by ephedrine 5 mg. 