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However, PEEP trials were only conducted after the induction of general anesthesia and not after the establishment of capnoperitoneum. In that study, PEEP trials were performed for EIT-guided identification of PEEP levels in a subgroup of ten patients undergoing laparoscopic surgery.
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In a randomized trial, Pereira and colleagues showed that ventilation with EIT-guided individualized PEEP resulted in a reduction of postoperative atelectasis in comparison to ventilation with a fixed PEEP of 4 cmH 2O. This information can be used for identifying individualized PEEP levels that minimize both overdistension and atelectasis formation. Analysing tidal impedance changes during MV with decremental PEEP titration allows assessment of regional alveolar collapse and overdistension. Electrical impendence tomography (EIT) is a noninvasive and radiation-free imaging modality based on the measurement of electrical potentials at the chest wall surface that can be used for direct monitoring of regional ventilation. Unfortunately, standard monitoring parameters used during mechanical ventilation, such as peripheral oxygen saturation (SpO 2), are not suitable for identifying regional phenoma such as atelectasis formation and pulmonary overdistension during mechanical ventilation. The situation is even more complicated in case of laparoscopic surgery in Trendelenburg position due to high pleural pressure caused by positioning and capnoperitoneum and may require higher PEEP values for preventing alveolar collapse in comparison to ventilation in supine position without capnoperitoneum. Additionally, possible hemodynamic effects of higher PEEP levels requiring more intravenous fluids and vasoactive medications should be taken into consideration while choosing the most appropriate PEEP value for lung protective ventilation. While low tidal volume became an established component of lung protective ventilation, the optimal PEEP value for preventing alveolar collapse and regional hypoventilation without causing overdistension during general anesthesia remains controversial. Therefore, adequate lung protective ventilation should be applied in order to reduce the risk of PPC. Further damage to lung parenchyma may be caused by cyclic opening and closing of alveolar units during inspiration and expiration (atelectrauma). Increased pressure (barotrauma) or high volumes (volutrauma) during mechanical ventilation may damage lung parenchyma by causing high transpulmonary pressures that exceed the elastic capacity of the lungs. Postoperative pulmonary complications (PPC) are common after general anesthesia with mechanical ventilation and may lead to prolonged hospitalisation, increased mortality and morbidity. This study was prospectively registered at German Clinical Trials registry (DRKS00016974). Even with high PEEP levels, alveolar overdistension was negligible during MV in Trendelenburg position with capnoperitoneum. The PEEP levels required for preventing alveolar collapse and for balancing collapse and overdistension in Trendelenburg position with capnoperitoneum were significantly higher than those required for achieving the same goals in supine position without capnoperitoneum. Open-Lung-PEEP and Best-Compromise-PEEP were positively correlated with body mass index during MV in supine position but not in Trendelenburg position. In Trendelenburg position with capnoperitoneum, Open-Lung-PEEP was 18 (18–20) cmH 2 O ( p < 0.0001 vs supine position) with 1.8 (0.5–3.9)% of overdistension and 0 (0–1.2)% of collapse and Best-Compromise-PEEP was 18 (16–20) cmH 2O ( p < 0.0001 vs supine position) with 1.5 (0.7–3.0)% of overdistension and 0.2 (0–2.7)% of collapse. In supine position, the median Open-Lung-PEEP was 12 (8–14) cmH 2O with 8.7 (4.7–15.5)% of overdistension and 1.7 (0.4–2.2)% of collapse.
TRENDELENBURG POSITION TRIAL
In 30 patients undergoing laparoscopic gynecological surgery, relative overdistension and alveolar collapse were assessed with electrical impedance tomography (EIT) during a decremental PEEP trial ranging from 20 to 4 cmH 2O in supine position without capnoperitoneum and in Trendelenburg position with capnoperitoneum. We sought to identify the positive end-expiratory pressure (PEEP) levels necessary to counteract atelectasis formation (“Open-Lung-PEEP”) and to provide an equal balance between overdistension and alveolar collapse (“Best-Compromise-PEEP”). Laparoscopic surgery in Trendelenburg position may impede mechanical ventilation (MV) due to positioning and high intra-abdominal pressure.