Positional Effects on Lung Ventilation and Perfusion in Obesity

This will be a observational, physiological study involving healthy volunteers divided into two groups: Obese group (BMI 30 - 40 kg/m²) and non-obese group (BMI 18.5 - 24.9 kg/m²). Each participant will serve as their own control across body positions. Baseline measurements will be obtained in upright sitting, followed by randomized order of supine, Trendelenburg (30°), prone, left lateral, and right lateral. Participants' demographic and anthropometric data, including height, weight, chest circumference, waist circumference, and hip circumference, will be recorded. Medical history, current medication use, smoking status, and cumulative smoking exposure (pack-years) will be documented. Electrical Impedance Tomography (EIT) measurements will be performed using the Infivision ET1000 device. All study procedures will take place in a private room within the General Intensive Care Unit of Koç University Hospital, with privacy ensured throughout the protocol. After removal of upper body clothing, an appropriately sized EIT belt, selected according to chest circumference, will be positioned circumferentially around the thorax at the level of the axillae. A pulse oximetry probe will be applied, and continuous monitoring of heart rate and peripheral oxygen saturation will be maintained during all measurements. Following a 5-minute stabilization period in the upright sitting position, baseline EIT measurements will be obtained. The following parameters will be recorded: percentage of ventral ventilation, percentage of dorsal ventilation, percentage of ventral perfusion, percentage of dorsal perfusion, Match Index (MI), Dead Space Index (DI), Shunt Index (SI), Global Inhomogeneity Index (GI), and Center of Ventilation (CoV). To minimize potential carry-over effects, the sequence of positions will be randomized individually for each participant. Randomization will be performed using computer-generated random sequences (randomizer.org), with predefined randomization sets created prior to study initiation. Each position will be maintained for approximately 5 minutes. In each position, end-expiratory lung impedance difference (ΔEELI) will be measured using the upright sitting position as the reference. The total duration of the study protocol is expected to be approximately 30 minutes per participant. The study will be completed once all measurements have been obtained in all positions.