Effect of Dexmedetomidine on Brain Homeostasis and Neurocognitive Outcome

Each participant will receive standard monitoring (ECG, SpO2, SBP, BIS, urine output, temperature). More detailed hemodynamic monitoring will be obtained by Edwards Lifesciences ClearSight system (CO, CI, SV, SVI, SVV, SVR, SVRI). TCI Propofol and Remifentanil will be the agents of choice for induction and maintenance in anesthesia and cisatracurium will be used for neuromuscular blockade for intubation. Protective mechanical ventilation will be chosen (7ml/kg IBW) with a respiratory rate to obtain a PaCO2 of 35-40 mmHg. PEEP will be changed for the best PaO2/FiO2 ratio and FiO2 of choice will be 0.5. The radial artery catheterization will be applied for direct blood pressure measurement and arterial blood gas sampling (pH, PaO2, PaCO2, HCO3, BE, osmolality, lactic acid, Hb, glucose, Na and K will be measured). The jugular bulb ipsilateral to the craniotomy site will be catheterized for receiving blood samples for blood gas analysis. The following oxygenation and metabolic parameters / derivates will be measured or calculated: SjvO2, pH, PjvO2, PjvCO2, HCO3, BE, Osmolality, Lactic acid jv, Hb, Glucose, Na, K, AjvDO2, AjvCO2, O2ERbr, eRQbr, AjvDL, and LOI. Dexmedetomidine or normal saline (placebo) administration will start 10 minutes after anesthesia induction and maintained throughout the surgical procedure. Phases * T0: 5 minutes before administration of either DEX or placebo * T15: 10 minutes after administration of either DEX or placebo * T30: 30 minutes after administration of either DEX or placebo * T60: 60 minutes after administration of either DEX or placebo * T120: 120 minutes after administration of either DEX or placebo * T240: 240 minutes after administration of either DEX or placebo * End of surgical procedure Blood samples for measuring S-100b, NSE, cortisol, TNF-a and IL-6 will be obtained at phases T0, end of surgery and 24 hours after administration of either DEX or placebo. Neurocognitive testing will be performed before surgery, 1 week and 1 month later using Karnofsky Performance Status (KFS), Mini Mental State Exam (MMSE), Μontreal Cognitive Assessment (MoCA) and Addenbrooke's Cognitive Exam (ACE III). Intraoperative consumption of propofol and remifentanil will also be recorded