Comparison of the Hemodynamic Effect of Three Positions During Induction of Anesthesia

Upon arrival to the operating room, routine monitors (ECG, pulse oximetry, and non-invasive blood pressure monitor) will be applied; intravenous line will be secured, and routine pre-medications (ranitidine 50 mg and midazolam 3-5 mg) will be administrated. Before induction of anesthesia, patients will be randomly allocated into:supine group, head-down group or leg elevation group Baseline mean arterial blood pressure will be obtained as average of 3 reading before induction of anesthesia at supine position. Induction of anesthesia will be achieved using fentanyl (2 mcg/Kg), propofol (2 mg/Kg), and atracurium (0.5 mg/Kg). Endotracheal tube will be inserted after 3 minutes of mask ventilation. Anesthesia will be maintained by isoflurane (1-1.5%) and atracurium 10 mg increments every 20 minutes. Ringer lactate solution will be infused at a rate of 2 mL/Kg/hour. Any episode of hypotension (defined as mean arterial pressure \< 80% of the baseline reading) will be managed by 5 mcg norepinephrine. If the hypotensive episode persisted for 2 minutes, another bolus of norepinephrine will be administered.