The goal of this clinical trial is to learn whether low-flow anesthesia reduces perioperative atelectasis compared to normal-flow anesthesia in adults undergoing laparoscopic bariatric surgery. The main questions it aims to answer are:

Does low-flow anesthesia result in a smaller increase in lung ultrasound score from baseline to postoperative hour 1? Does low-flow anesthesia result in better preservation of pulmonary function at postoperative hour 48?

Researchers will compare low-flow anesthesia (fresh gas flow 0.5 L/min) to normal-flow anesthesia (fresh gas flow 2 L/min) to see if low-flow anesthesia reduces the extent of perioperative atelectasis as measured by lung ultrasound.

Participants will:

Be randomly assigned to receive either low-flow or normal-flow inhalational anesthesia with sevoflurane during surgery Undergo lung ultrasound assessments before surgery and at postoperative hours 1 and 24 Undergo spirometric testing one day before surgery and at postoperative hour 48

This prospective, single-center, randomized controlled trial investigates the effect of low-flow anesthesia on perioperative atelectasis in patients undergoing elective laparoscopic sleeve gastrectomy. Morbidly obese patients are at increased risk of perioperative atelectasis due to reduced functional residual capacity, diaphragmatic elevation, and the additional physiological burden of pneumoperitoneum and reverse Trendelenburg positioning during laparoscopic surgery.

Eligible patients are randomized in a 1:1 ratio to one of two groups. In the normal-flow group (Group N), anesthesia is maintained with sevoflurane at a fresh gas flow of 2 L/min. In the low-flow group (Group D), anesthesia is initiated at 4 L/min to achieve rapid equilibration and then reduced to 0.5 L/min once 1.0 minimum alveolar concentration (MAC) is reached. All patients receive standardized lung-protective mechanical ventilation and an identical anesthetic protocol otherwise.

Atelectasis is quantified using the modified Lung Ultrasound Scoring System (mLUSS), in which each of 12 lung zones is scored 0-3 based on aeration loss, yielding a total score of 0-36. Assessments are performed preoperatively, at postoperative hour 1, and at postoperative hour 24 by a single investigator blinded to group allocation. Pulmonary function tests (FVC, FEV₁, FEV₁/FVC, PEF, FEF₂₅-₇₅) are performed preoperatively and at postoperative hour 48 by a separate blinded investigator.

Secondary outcomes include changes in spirometric parameters, intraoperative hemodynamic event incidences, mean intraoperative FiO₂ and plateau pressure, sevoflurane and remifentanil consumption, postoperative nausea and vomiting incidence, and postoperative analgesic requirements.

Inclusion Criteria:

Age 18-65 years ASA physical status III Scheduled for elective laparoscopic sleeve gastrectomy under general anesthesia Able to provide written informed consent and cooperate with study procedures

Exclusion Criteria:

Severe respiratory comorbidities (advanced COPD, restrictive lung disease, or severe obstructive sleep apnea syndrome) Advanced heart failure (ejection fraction < 20%) Chronic renal failure Hepatic insufficiency Uncontrolled diabetes mellitus Uncontrolled hypertension Limited capacity to cooperate with study procedures

Individual participant data will not be shared. This is a single-center academic study; data are available from the corresponding author upon reasonable request.