Deliberate hypotension is defined as lowering the systolic blood pressure to 80-90mmHg, or the mean blood pressure to 50-65mmHg. This technique is usually employed for operations that have a high risk of intraoperative hemorrhage, such as orthognathic surgery. Several different regimens are used to lower the patient's blood pressure, such as vasodilators, autonomic nervous system inhibitors, opioids and inhalation anesthetics. However, the effects that these agents have on the QTc and Tp-e intervals during deliberate hypotension is not known. Virtually every kind of anesthetic is reported to have some effect on the QTc and Tp-e intervals. Because orthognathic surgery usually takes 3-4 hours to complete, the amount of anesthetics and drugs used to maintain low blood pressure is not small. Therefore, the effect that these agents may have on the QTc and Tp-e intervals may not be negligible. The investigators of the present study found that the high dose of commonly used hypotensive agents tend to prolong these variabilities to some extent. This study will be able to provide insight as to which hypotensive anesthesia regimen has the least effect on the QTc and Tp-e intervals, and therefore will be helpful in minimizing cardiovascular risks of deliberate hypotensive anesthesia.

Inclusion Criteria:

1. ASA class 1
2. Adults over the age of 20
3. Patients requiring deliberate hypotensive anesthesia for orthognathic surgery

Exclusion Criteria:

1. Patient refusal
2. Patients with arrhythmia
3. Patients with cardiac anomalies or past history of cardiac surgery
4. Patients with abnormal electrolyte levels
5. Patients taking medications that are known to prolong QT intervals
6. Patients with QTc intervals greater than 440ms
7. Illiterate patients