The use of 100% oxygen during the induction of general anesthesia is always preferable to have enough time to secure the airway by endotracheal intubation, because preoxygenation with a low oxygen concentration may reduce the safe time for apnea. However, using a low oxygen concentration during preoxygenation might prevent the formation of atelectasis. There is still no clear conclusion about the best oxygen concentration for preoxygenation. Our study is designed to evaluate the safety of preoxygenation with 80%, 60%, 40%, 30% and 21% oxygen by the safe time for apnea during the induction of general anesthesia.

We decide to add 3 groups (80%, 60%, and 40% oxygen) in our study to fully evaluate the safety of preoxygenation with different oxygen concentration.

Zijia Li 2018.3.2

Inclusion Criteria:

• Nonsmoking patients aged 18 years to 60 years
• Undergoing general anesthesia for elective surgery
• Pulse oxygen saturation ≥95% when breathing air quietly in bed
• American Society of Anesthesiologists(ASA) physical status class I-II
• Informed consent can be obtained

Exclusion Criteria:

• A suspected difficult airway
• Be allergic to midazolam, propofol , fentanyl or Cisatracurium
• Severe disease of respiratory system or cardiovascular system,or obstruction of the digestive tract
• Oxygen reserve is suspected low,or patient who does not tolerate hypoxemia well
• A history of mental disorder or patient who can not collaborate well
• Body mass index more than 30 kg/㎡