Electrical Impedance Tomography (EIT) is a non-invasive, bedside monitoring tool that provides real-time information on regional ventilation and perfusion. In particular, EIT has the potential to guide individualized mechanical ventilation in obese patients by revealing how gravitational and positional factors alter regional lung behavior. Assessing the effects of different horizontal positions on both ventilation and perfusion may help optimize respiratory management strategies tailored to body habitus. By comparing obese and non-obese healthy participants across different positions, this study aims to provide novel insights into the postural effects on lung aeration and perfusion distribution, and to highlight the role of EIT in tailoring individualized ventilation strategies.

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.

Inclusion Criteria:

-BMI between 18-25 kg/m² or 30-40 kg/m²

Exclusion Criteria:

• Individuals with a body mass index (BMI) between 25 and 30 kg/m²,
• Pregnancy
• Respiratory tract infection within the previous two weeks
• Anatomical abnormalities of the chest wall that may interfere with Electrical Impedance Tomography (EIT) measurements.

Healthy non-obese and obese volunteers

Data collected for the study, including de-identified individual participant data will be made available to other within 6 months after the publication of this trial, as will additional related documents (study protocol, statistical analysis plan and informed consent form), for academic purposes (e.g , meta-analyses) upon request to the corresponding author, and with a signed data access agreement.

6 months after the publication

Academic purposes