After obtaining written informed consent, 23 patients were included in the study. After primary recruitment and preoperative evaluation, the patients received routine monitoring (electrocardiography, SpO
2, noninvasive blood pressure measurement; Infinity Delta XL, Dräger Medical, Lübeck, Germany) and a 18–20-G intravenous catheter was established. After preoxygenation to an fraction of expired oxygen of 0.8, anesthesia was induced with 0.3–0.5 μg∙kg
− 1 predicted body weight [
10] iv sufentanil (Janssen-Cilag, Neuss, Germany) and 2–3 mg∙kg
− 1 actual body weight iv propofol (Fresenius Kabi, Bad Homburg vor der Höhe, Germany). Tracheal intubation was facilitated with 0.6 mg∙kg
− 1 predicted body weight iv rocuronium (Fresenius Kabi). If the patient required a rapid sequence induction, neuromuscular blockage was performed by the administration of 1.0 mg∙kg
− 1 predicted body weight iv rocuronium. Neuromuscular blockage was monitored with a mechanomyograph (TOFscan; Dräger Medical). For tracheal intubation, we used tracheal tubes with low pressure cuffs (internal diameter of 7.0–7.5 mm for women and 8.0 mm for men; Mallinckrodt Hallo-Contour; Covidien, Neustadt an der Donau, Germany). After adequate placement of the tracheal tube, iv propofol was administered continuously (110–150 μg∙kg
− 1∙min
− 1). Potential hypotension (defined as mean arterial pressure < 65 mmHg) was treated with a continuous infusion of iv noradrenaline (0.03–0.2 μg∙kg
− 1∙min
− 1). Perioperative volume requirements were addressed with a crystalloid solution (8 ml∙kg
− 1∙h
− 1, Jonosteril; Fresenius Kabi). According to our local standard, mechanical ventilation was started as volume-controlled baseline ventilation (Fabius Tiro, Dräger Medical) with a tidal volume of 7 ml∙kg
− 1 predicted body weight, inspiration-to-expiration ratio of 1:2, a positive end-expiratory-pressure (PEEP) of 9 cmH
2O and ventilation frequency set to maintain an end-tidal carbon dioxide partial pressure between 4.7 and 5.1 kPa. These ventilation settings were based on our study protocol and in accordance with our clinical routine in obese patients. After 7 min of baseline ventilation, all patients were randomly allocated to one of two crossover groups to receive ventilation sequences either VCV-FCV or FCV-VCV for 7 min per ventilation mode. To avoid irritations due to the surgical procedure (e.g. impaired respiratory mechanics by the capnoperitoneum and electrical irritations of the measurement of Electrical Impedance Tomography), our study was performed prior to the surgical intervention. For adequate allocation, a computer generated randomization in blocks was used. Disclosure of the randomization was requested right after induction of anesthesia. A study related anesthesiologist conducted the randomization in blocks, enrolled participants and assigned participants to the interventions. During the study protocol, ventilation variables were kept constant as set during the baseline measurements. To prevent from the risks of extubation and reintubation, FCV was performed by introducing the narrow-bore tracheal tube (Tribute, Ventinova Medical B.V.) into the standard tracheal tube. Blocking the cuff of the Tritube in the lumen of the tracheal tube provided a sufficient seal. By controlling both tube’s markings, placement of the Tritube’s tip exceeding that of the standard tracheal tube by 2–5 mm was ensured, and the potential risk of bronchial intubation was avoided. Respiratory data were collected from both ventilators via the respective serial communication interface and analyzed offline. Electrical impedance tomography (EIT) was performed with PulmoVista 500 (Dräger Medical) in all patients to measure regional ventilation, changes in relative thoracic electrical impedance during the different ventilation phases, relative end-expiratory lung volume (ΔEELV) and to compare the expiratory decrease in intrapulmonary air [
11‐
13].