Effects of Intraoperative Magnesium Sulfate on Perioperative Pain Relief After Spine Surgery

Each participant will receive standard monitoring (ECG, SpO2, capnography, SBP, oesophageal temperature, accelerography) and an intravenous access will be established. The level of anaesthesia will be monitored with the bispectral index (BIS), targeting to a BIS level 40-50. Group M patients will receive intravenous magnesium sulfate 20 mg/kg over a 15-min period before induction of anaesthesia and 20 mg/kg/h by continuous i.v. infusion during the operation. Group C will be given isotonic solution of 0.9% in the same volume as the study drug. A standard anesthesia protocol will be applied involving propofol 2mg/kg (iv) and fendanyl 2 μg/kg (iv). Cis-atracurium 0.2 mg/kg (iv) will be given to facilitate endotracheal intubation. Anaesthesia will be maintained with air 50% and oxygen 50%, and desflurane adjusted to achieve a target BIS between 40 and 50. Remifentanil will be added to the anesthesia regimen as needed. Hemodynamic parameters will be recorded upon * Baseline: Before the administration of the tested drug * T5: 5 minutes after the administration of the tested drug * T10: 10 minutes after the administration of the tested drug * T15: 15 minutes after the administration of the tested drug * Ts: surgical incision * T30: 30 minutes after the administration of the tested drug * T45: 45 minutes after the administration of the tested drug * T60: 60 minutes after the administration of the tested drug * T90: 90 minutes after the administration of the tested drug * T120: 120 minutes after the administration of the tested drug * T180: 180 minutes after the administration of the tested drug Low arterial blood pressure during surgery defined as a mean blood pressure value \< 50 mmHg will be treated by a bolus of 5 mg ephedrine administered intravenously or phenylephirne civ for persistent hypotension. Also, time to accelerography recording indicating the appropriateness of neuromuscular block for intubation, mean expired desflurane concentration (from 30 min after skin incision to the end of surgery), boluses of ephedrine and total intraoperative remifentanil consumption will be recorded. Postoperative analgesic protocol will involve paracetamol 1 mg (iv), lornoxicam 8mg (iv) and morphine 3 mg (upon request). Postoperatively pain assessment will be performed by Visual Analogue Scale (VAS), Verbal Rating Scale (VRS) and Numerical Rating Scale (NRS) at emergence from anesthesia and 2, 4, 6, and 24 h in the study period. Time to first analgesic request and total analgesics consumption postoperatively (morphine equivalents) will be recorded. Episodes of shivering, as well as episodes of nausea and vomiting (PONV), will be recorded at emergence and thereafter, throughout the study period. Finally, patients' global satisfaction will be assessed the first day after surgery using a 5-grade scale (1= worst discomfort ever experienced in their life and 5= totally satisfied during the immediate postoperative period).