Orthognathic surgery, one kind of Oro-maxillofacial surgery, is a complicate surgery that undergoes several hours with nasotracheal intubation general anesthesia. To limit blood loss during operation, the patients are often under intentional hypotension. However, the intentional hypotension may confuse with hypovolemic induced low blood pressure. The hypothesis is using flotrac (to measure stroke volume variation) to keep the patients hemodynamics stable under Tridil and propofol infusion and avoid over-infusion of crystalloid or colloid and prevent hypovolemia induced postoperative nausea and vomiting.

Orthognathic surgery, one kind of oro-maxillofacial surgery, is a complicated surgery that undergoes several hours with nasotracheal intubation general anesthesia. Patients without premedication undergo orthognathic surgery with nasotracheal intubation general anesthesia.

Patients without premedications Monitoring of patients with entropy for consciousness, ECG, with noninvasive BP and invasive blood pressure through redial artery, train-of-four monitor, target controlled infusion of propofol, infusion Tridil, and sevoflurane for mainly anesthesia maintenance. Ventilator setting as volume control (8-12ml/kg), frequency respiratory rate (8-12/min), I/E ratio: 1/2, positive end expiratory pressure: 4 mmHg, Fraction inspiratory O2: 60%.

To maintain the MAP >60-55 mmHg, urine output 0.5-1ml/kg, and body core temperature > 36 Celsius degree.

Closely observation of patients postoperatively for adverse events or complications.

Inclusion Criteria:

• patients undergo orthognathic surgery, ASA I~III, 20 to 65 years old, no mouth limitation

Exclusion Criteria:

• ankylosing spondylitis, limited mouth opening < 3 cm, liver or renal disease, obese patients (BMI>35kg/m2), patients refused

patients who need to operate for abnormal face or abnormal teeth biting