The purpose of this study is to compare the sedation effect of dexmedetomidine and propofol when they are both combined with midazolam and remifentanil during awake endotracheal intubation.

Awake intubation is one of the best strategy guaranteed by the American Society of Anesthesiologists (ASA) guidelines for the management of patients with anticipated difficult airways. Hemodynamic stability, optimal intubating conditions, patients' comfort, amnesia and preservation of patents' spontaneous respiration are critical for awake intubation. Sedation is one of the key elements for this technique. Intravenous midazolam, propofol, dexmedetomidine and remifentanil are commonly used as sedatives during awake intubation. These agents are not preferred to be used alone but in combination with each other for the purpose of minimizing their respective side effects. The aim of this randomized controlled trial is to compare the safety and effectiveness of dexmedetomidine versus propofol for sedation during awake endotracheal intubation when they are both combined with midazolam and remifentanil.

Inclusion Criteria:

1. Age 18-65 Years old;
2. scheduled for elective surgery under general anesthesia with oral tracheal intubation;
3. The American Society of Anesthesiologists(ASA) grade is I or II, and the cardiac function is 1-2;
4. Body mass index (BMI) 18-30 kg/m2.

Exclusion Criteria:

1. Patients have severe cardiac diseases (cardiac function grading greater than grade 3/arrhythmia including sick sinus syndrome, atrial fibrillation, atrial flutter, atrioventricular block, frequent ventricular premature, multiple ventricular premature, ventricular premature R on T, ventricular fibrillation and ventricular flutter/acute coronary syndrome) or respiratory failure or hepatic failure or renal failure;
2. body mass index (BMI) ≥30 kg/m2 or <18 kg/m2;
3. Patients with poor blood pressure control (receive regular antihypertensive medical treatment but still have systolic blood pressure > 150 mmHg and/or diastolic blood pressure > 90 mmHg);
4. Patients with a higher risk of reflux and aspiration, such as full stomach, gastrointestinal obstruction, gastroparesis, and pregnant women;
5. Patients have schizophrenia, epilepsy, Parkinson's disease, intellectual disability, hearing impairment.;
6. Patients who take sedative and analgesic drugs for a long time;
7. Patients who are allergic to propofol, dexmedetomidine, midazolam or remifentanil
8. Patients who are expected to be difficult to intubate;
9. Patients who are participating in other clinical trials, or who refuse to sign informed consent.