Pneumoperitoneum (PNP) and the position of the patient required for laparoscopic surgery lead to pathophysiological changes that complicate anesthesia. PNP is characterized by an increased intra-abdominal pressure (IAP), the cranial displacement of the diaphragm that can lead to the formation of intraoperative atelectasis and decrease end-expiratory lung volume (EELV). At the same time, PNP can reduce respiratory system compliance by 30-50% in healthy patients. During elective abdominal surgery under general anesthesia, atelectasis forms in almost 90% of patients and can become a focus of postoperative pneumonia. The negative effect of PNP is more prominent in Trendelenburg position. And one of the methods to avoid the effects of PNP and Trendelenburg position on lung tissue is to apply positive end-expiratory pressure (PEEP). PEEP is acknowledged as a component of lung protective ventilation (LPV) along with low tidal volume (TV) 6-8 ml/kg. On the other hand, excessive PEEP can lead to the overdistension of lung tissue and cause volutrauma and hemodynamic instability. It is necessary to use sufficient PEEP to minimize atelectasis, improve respiratory biomechanics and maintain oxygenation.

Electrical impedance tomography shows changes in ventilation and perfusion during mechanical ventilation with the different PEEP levels.

The study aimed to select optimum PEEP level based on optimum ventilation-to-perfusion match based on electrical impedance tomography measurements.

Electrical impedance tomography shows changes in ventilation and perfusion during mechanical ventilation with the different PEEP levels. The investigators will measure the following variables: resistivity of low and high pass band and end-expiratory lung index in 4 regions of interest and globally, global inhomogeneity index, global lung-heart index, global regional ventilation delay, compliance win, compliance loss, plateau pressure, and driving pressure.

The investigators will measure abovementioned variables in the following conditions:

• PEEP 5 mbar with the patient in a horizontal supine position (initial measurement, Baseline),
• PEEP 5 mbar in Trendelenburg position in carboxyperitoneum conditions (after reaching the set abdominal pressure of 12-14 mbar) (reference measurement, Ref),
• PEEP 8 mbar in Trendelenburg position under carboxyperitoneum conditions (abdominal pressure 12-14 mbar),
• PEEP 10 mbar in Trendelenburg position in carboxyperitoneum conditions (abdominal pressure 12-14 mbar),
• PEEP 12 mbar in Trendelenburg position in carboxyperitoneum conditions (abdominal pressure 12-14 mbar),
• PEEP 14 mbar in Trendelenburg position in carboxyperitoneum conditions (abdominal pressure 12-14 mbar),
• PEEP 16 mbar in Trendelenburg position in carboxyperitoneum conditions (abdominal pressure 12-14 mbar)
• PEEP 5 mbar with the patient in a horizontal supine position after deflation of the carboxyperitoneum.

After 5 minutes of carboxyperitoneum in Trendelenburg position the investigators will assess ventilation, perfusion and their relationship by the "Analysis" tab in comparison with the initial one in the intubated patient in the supine position (Baseline): improvement of ventilation (CW - compliance win, in %) and deterioration of ventilation (CL - compliance loss, in %), global homogeneity of ventilation (GI - homogeneity index, in %), regional ventilation delays (RVD, in %), ventilation compliance index and perfusion (LHI - lung heart index, in %).

After all stages have been completed, a comparative analysis of the influence of different levels of PEEP on ventilation, perfusion and their ratio will be carried out by using the "Analysis" tab at each stage in comparison with the reference (Ref): CW and CL, GI, RVD, LHI.

Inclusion Criteria:

• Patients who undergo gynecological surgery under conditions of carboxyperitoneum in the Trendelenburg position.

Exclusion Criteria:

• Pregnancy,
• Hypoxemia before surgery (SpO2 < 94%),
• body mass index more than 35 kg/m2,
• Unstable hemodynamics and/or life-threatening arrhythmia,
• Primary or secondary lung diseases (COPD, interstitial lung diseases, metastatic lung disease)
• Presence of an implantable pacemaker and/or defibrillator
• Chronic diseases in the stage of decompensation with the development of extrapulmonary organ dysfunction (liver cirrhosis, progression of cancer, chronic heart failure).

Gynecological surgery

Patients who underwent elective gynecological operations in conditions of carboxyperitoneum with Trendelenburg position