Morbid obese patients undergoing sleeve gastrectomy due to morbid obesity after 01.01.2019 will be randomly divided into 2 groups with controls. To prevent selection bias in the study, numbers will be produced at random. The produced numbers will be determined as 0: control and 1: experiment groups and patients will be divided into groups as such. Random numbers will be generated by the MedCalc 18.2.1. program (MedCalc Statistical Software MedCalc Software bvba, Ostend, Belgium; http://www.medcalc.org; 2018). Group 1 is determined as patients to be administered high-flow anesthesia, while Group 2 will be administered low-flow anesthesia. After anesthesia induction, Group 1 will have 4 liters/minute (50% O2 50%) flow administered, while patients in Group 2 will have 1 liter/minute (50% O2, 50% air) flow administered. The study included adult patients who signed the voluntary consent form aged from 18-65 years, American Society of Anesthesiologists (ASA) III, and body mass index (BMI) \>40. Inclusion criteria for the study are no alcohol or drug addiction or diagnosis of chronic obstructive pulmonary disease (COPD) during routine preoperative assessment by chest diseases, FEV1/FVC ratio, FEV1 and FVC values within normal limits and no previous abdominal surgery.

Those with stop-bang score below 4 will be included in the study. On the day of surgery, all patients had FEV1, FVC value and FEV1/FVC ratio examined by a single anesthesia technician using a manual RST device without the knowledge of the anesthesia expert.

Pressure tests and calibration of the anesthesia device is performed each morning for every surgery. After calibration the mechanical ventilator alarm limits for ins O2 are lower limit 40%, EtCO2 35-45 mmHg, min Vol (tidal volume in 1 min x frequency) according to the patient ± 0.5. After patient is placed on the operating table in ramp position, all patients are monitored with triple route ECG, pulse oximetry and pressure cuff. The clinical protocol for all patients undergoing sleeve gastrectomy includes BIS (bispectral index) and TOF (train of four) monitoring. Before beginning induction, first 3 mg midazolam and fentanyl 150 mvq IV were administered. According to ideal weight and BIS score for 40-60, 2-5 mg propofol and 0.5 mg rocuronium according to true weight were administered. Patients were ventilated with 6 L/min 60% O2 for 2 minutes. Patients were endotracheally intubated with a Macintosh laryngoscope. In addition to assessment of respiratory sounds linked to obesity with auscultation, EtCO2 capnography was used to confirm the efficacy of intubation. After intubation 50 mg ranitidine and 8 mg ondansetron were routinely administered. Fixing the remifentanil dose to 0.1 mcg/kg/min, infusion was administered. Sevoflurane percentage was changed to ensure MAC (minimum alveolar concentration of 0.6-1.1 for BIS score of 40-60 for maintenance. During ventilation in PRVC mode, Group 1 had 4 liter/minute (50% O2, 50% air) flow administered, while Group 2 had 3 L/min 4% for Sevoflurane the first 3 minutes as wash-in. Then flow was administered at 1 L/min (50% O2, 50% air). According to ideal weight, mechanical ventilator settings were 6-10 ml tidal volume, frequency 12/min (increasing, if necessary, for etCO2 35-45 mmHg), PEEP 5-10 cm/H2O and inspirium/expirium ratio ½. In Group 1, sevoflurane percentage was set to correlate with BIS monitoring. In Group 2, after intubation the sevoflurane percentage was set to a higher percentage to reach the desired MAC value. Every 5 minutes during surgery, MAC, BIS, mean arterial pressure, peak heart rate, sPO2, etCO2, inSO2, frequency and PEEP values were recorded. In Group 2, 5 minutes before the end of surgery, flow was increased from 1 L to 4 L for wash-out. Both groups had remifentanil ended 10 minutes before the end of surgery. Later 1 g paracetamol and 100 mg tramadol IV were administered. Five minutes before the end of the operation patient-controlled SIMV+VE mode with trigger setting of 6 was organized. At the end of surgery, patients were administered 2 mg/kg sugammadex according to ideal weight. When the patients trigger setting reaches muscle power it was lowered to zero. When trigger is zero and there is sufficient tidal volume to reach TOF (train of four) value \>90% and BIS score is 80-100, patients were extubated. They were sent to the recovery room. In recovery the ARISCAT risk index was calculated. Later when modified Aldrete scoring system is ≥9, patients were sent to the ward.

After surgery, mobilization and respiratory physiotherapy were provided in the 2nd hour. Vital signs including mean arterial pressure (MAP) were recorded in the postoperative period with 20 mg tenoxicam I.V. administered in the 1st hour. The PCA device was set with tramadol 300 mg/100 ml, bolus 10 mg, lock time 12 minutes without basal infusion. Patients with numeric rating scale (NRS) scores ≥4 had a rescue dose of 4 mg morphine I.V. administered. Each patient began oral intake (water) in the postoperative 24th hour, with early mobilization. A second dose of tenoxicam I.V. was administered in the postoperative 8th hour.

In the 24th hour postoperative, patients had analgesia to ensure VAS score is below 4, with RFT repeated by the same doctor who performed RFT preoperatively.