Background: Atelectasis occurs in the majority of patients with general anesthesia, which can lead to postoperative pulmonary complications (PPCs) and affect the postoperative outcome. However, there is still no exact method to prevent it though agents. S-ketamine may prevent atelectasis due to its airway smooth muscle relaxation and anti-inflammatory effects. Lung ultrasound is a portable and reliable bedside imaging technology for diagnosing anesthesia-induced atelectasis. Here, we intend to assess whether small-dose of S-ketamine can reduce the incidence of atelectasis after intubation under general anesthesia using lung ultrasound, in order to prevent the early formation of perioperative atelectasis and perioperative pulmonary complications.

Methods: The trial is a single institution, prospective, randomized controlled, parallel grouping, double-blind study. From October 2021 to March 2022, 100 patients (18-60 years old) scheduled for elective surgery will be recruited from Beijing Tiantan Hospital, Capital Medical University, and randomly assigned to the S-ketamine group (Group S) and the normal saline group (Group N) at a ratio of 1:1. The label-masked agents will be given 5 minutes before induction and all patients will undergo a standardized general anesthesia protocol. Related data will be collected at three time point: after radial artery puncture (T1), 15 minutes after tracheal intubation (T2) and before extubation (T3). The primary outcome will be the total LUS scores at T2. Secondary outcomes include LUS scores in 6 chest regions at T2; total LUS scores at T3; arterial blood-gas analysis results (PaCO2, PaO2) and PaO2/FiO2 at T2 and T3, plateau pressure(Pplat) and dynamic lung compliance (Cdyn) at T2 and T3; the incidence of postoperative complications associated with S-ketamine and postoperative pulmonary complications (PPCs) 2 and 24 hours after surgery；.

Discussion: This trial aims to explore whether a simple and feasible application of S-ketamine before the induction of general anesthesia can prevent atelectasis. The results of this study may provide new ideas and direct clinical evidence for the prevention and treatment of perioperative pulmonary complications during anesthesia.

Key words: atelectasis, S-ketamine, ultrasound, RCT, protocol

Inclusion Criteria:

1. Age of 18 - 60 years;
2. American Society of Anesthesiologists (ASA) physical status of I - II;
3. Patients scheduled for urinary, obstetrics and gynecology surgery in supine position or lithotomy position under general anesthesia, and the expected operation time is more than 1 hour;
4. Patients and their authorized surrogates have signed the informed consent form.

Exclusion Criteria:

1. Patients who have difficulty in applying pulmonary ultrasound, such as chest fractures, surgical history, etc;
2. Patients who have a history of upper respiratory tract infection one month before surgery, or a history of smoking more than 6 packs/year;
3. Preoperative chest X-ray or CT abnormalities, including atelectasis, pneumothorax, pleural effusion or pneumonia;
4. BMI>30kg/㎡;
5. Expected difficulty in intubation or mask ventilation;
6. Patients with significant increase in intraocular pressure and intracranial pressure before surgery;
7. Patients allergic to S-ketamine, propofol and opioids.
8. Patients with a positive history of psychiatric disorders.