Our aim, in this study, is to elucidate the optimal dosing range (ED50 and ED95) of bolus remifentanil in order to counter the painful stimuli resulting from direct laryngoscopy and tracheal intubation by using the NOL index. In order to maximize the clinical relevance of our findings, induction of general anesthesia will be done with standard medications and doses and laryngoscopy will be done with the Macintosh laryngoscope, the most common tool used for intubation in the operating room

Background: There are no study so far that determined the ED50 and ED95 of remifentanil to perform tracheal intubation by using the NOL index as the criterion measured to determine the failure (painful intubation) or the success (pain free intubation) of the assessed procedure.

Objectives: To determine the ED50 and ED95 of remifentanil bolus to perform tracheal intubation without NOL index variating over threshold for nociceptive response (determined as a NOL index going over 20 - peak value of maximum 20) using the Macintosh blade when obtaining good conditions of intubation in anesthetized patients. Good conditions of intubation will be determined as obtaining a Cormack-Lehane grade I and II.

Inclusion Criteria:

• ASA status I or II
• Mallampati class I or II
• Patients > 18 years old and <65 years (defining elderly patients)
• Elective general, gynecological, orthopedic, plastic or urological surgery under general anesthesia
• Type of surgery that usually necessitate endotracheal intubation and controlled ventilation

Exclusion Criteria:

• Patient refusal
• History of psychiatric diseases or psychological problems (including mental retardation); inability to give consent; language barrier.
• Anticipated difficult airway (Mallampati class III and IV, thyromental distance < 6 cm, mouth opening < 3 cm, neck extension <80° and neck flexion <35°, inability to prognath, meaning bringing lower mandibular before the upper maxilla)
• Patient with history of neck rigidity or instability
• BMI > 30 kg/m2
• Anticipated difficult ventilation (patients with beard, obesity with BMI more than 30 kg/m2)
• Patient with history of oropharyngeal or tracheal surgery (excluding adenoidectomy, tonsillectomy and teeth removal)
• Severe coronary artery disease
• Serious cardiac arrhythmias (including atrial fibrillation)
• Use of β-blockers (all types)
• History of opioid or illicit drug substance abuse
• Chronic use of psychotropic and/or opioid drugs
• Allergy to remifentanil or propofol
• Pregnancy
• Contraindications to mask ventilation (gastrointestinal tract obstruction, pregnancy, active not-treated gastroesophageal reflux disease, non-fasting patients)
• Unexpected difficult airway requesting excessive, possibly painful airway manipulations
• Requiring hemodynamic support with vasopressors or inotropes 2 min before laryngoscopy to 5 min after insertion of endotracheal tube

remifentanil will be given by a dosage of 0.5-1.0-1.5-2.0 µg/kg of adjusted body weight bolus according to randomization, by an infusion pump over 30 seconds