The investigators hypothesize that the level of positive end-expiratory pressure (PEEP) is often incorrectly applied in certain operative conditions, such as in laparoscopy with head down (Trendelenburg) positioning. This can result in excessive levels of lung stress and postoperative pulmonary complications.

Incorrect intra-operative ventilator management can be harmful for the patient, potentially leading to postoperative pulmonary complications and ventilator-induced lung injury. During routine anesthesia procedures, most anesthetists will set the ventilator by rule of thumb with a PEEP of 4-6 cmH2O, a tidal volume of 6-8 ml/kg of ideal body weight and a frequency of 10-15 breaths per minute in order to provide lung protective ventilation. However, due to recent advances in surgical practice, patients are more frequently placed in non- physiological states, such as Trendelenburg position up to 30° with concurrent pneumoperitoneum and intra-abdominal pressures of 15mmHg or higher, as in for example robot-assisted radical prostatectomy or gynecological procedures. This extreme positioning and increased intra-abdominal pressure can have a significant effect on respiratory mechanics and can potentially result in excessive lung stress. The changes in applied positive pressure ventilation will result in changes of regional ventilation: both an increased amount of atelectasis and an increased amount of regional hyperinflation are observed in this setting. The ideal PEEP level balances the recruitment of atelectasis versus excessive hyperinflation. These changes in regional ventilation can be assessed by lung ultrasound. The lung ultrasound score can distinguish atelectasis from normal aeration in the different lung regions of interest.

This project is designed as a single center cohort study. Non-obese (BMI \< 30kg/m2), lung-healthy non-pregnant, non-smoking individuals without right sided heart failure, scheduled for elective laparoscopy of the lower abdomen, will be recruited. Standardized induction and maintenance with propofol TCI (3-6μg/l plasma concentration as calculated by the Marsh model), sufentanil (0.2μg/kg) and rocuronium (0.6mg/kg) will be provided. Neuromuscular blockade will be monitored using a train-of-four (TOF) monitor and kept with a TOF count \< 1 throughout the study using additional doses if indicated. A radial arterial line will be placed. Mechanical ventilation will be provided in volume control mode with a tidal volume of 4-6 ml/kg of ideal body weight (IBW) aiming for a driving pressure ≤ 15cmH2O, a starting PEEP of 5cmH2O, a frequency of 12-18 breaths per minute titrated to the end-tidal CO2 measurement and an initial FiO2 of 0.4. An esophageal balloon catheter with pressure sensor will be used to calculate transpulmonary pressures. The balloon and pressure sensor will be calibrated as per manufacturers guideline. Respiratory parameters will be recorded and saved for later evaluation using the FluxMed GrT monitor and software (MBMED, Argentina). After inflation of the pneumoperitoneum, lung ultrasound will be performed bilaterally at the midclavicular line between the second and third ribs, at the posterior axillary line above the level of T4 and at the posterior axillary line closely superior to the diaphragm, thus retaining 6 ultrasound loops which will be saved for post-hoc lung ultrasound scoring. The lung ultrasound measurements will be repeated at different decremental levels of PEEP: 15, 10, 5 and 0 cmH2O respectively. Arterial blood gas analysis will be performed before insufflation of the pneumoperitoneum and repeated at each level of PEEP. A minimum of 4 minutes equilibration time will be provided after changing PEEP.