The investigators propose a new wash-in technique for sevoflurane low flow anesthesia with fresh gas flow of O2:N2O or O2:air 1:1 L/min with sevoflurane 8%.

The objective of this study is to identify time to achieve alveolar concentration of sevoflurane at 1, 1.5, 2, 2.5, 3, and 3.5%

The benefits of low flow anesthesia are economic, less pollution, more heat and humidity preservation but it requires appropriate wash-in phase. The current wash-in protocols for sevoflurane are complicated and time consuming. The investigators propose a new 1-1-8 sevoflurane wash-in protocol which is more simple and predictable.

The objective of this study is to identify time to achieve alveolar concentration of sevoflurane at 1, 1.5, 2, 2.5, 3, and 3.5% with 1-1-8 wash-in protocol.

All patients will receive iv propofol 2 mg/kg as induction agent and succinyl choline 1.5 mg/kg for intubation. Then the ventilation will be controlled to achieve end-tidal CO2 30-35 mmHg with O2:N2O or O2:air 1:1 L/min with sevoflurane 8%. Time to achieve alveolar concentration of sevoflurane at 1, 1.5, 2, 2.5, 3, and 3.5% will be recorded and analyzed. After that, the concentration of sevoflurane and anesthetic technique will be adjusted to requirement of anesthesiologist.

Inclusion Criteria:

• ASA classification I-II
• age 18-65 years

Exclusion Criteria:

• BMI > 35
• contraindicated for N2O
• • with pulmonary or cardiac disease