Acute respiratory distress syndrome is characterized by heterogeneous lung injury, with dependent regions often collapsed and non-dependent regions relatively well-aerated, forming the so called "baby lung." Mechanical ventilation, while essential, can induce additional lung injury (VILI) via overdistension (baro/volutrauma) or repetitive alveolar collapse (atelectrauma). Protective ventilation strategies with low tidal volumes (VT 6-8 mL/kg predicted body weight, PBW) reduce mortality, but their efficacy relies on accurate PBW estimation, which is derived from patient height. In critical care, height measurement is often challenging, and common methods such as visual estimation or tape measurement can be inaccurate, leading to inappropriate VT settings and increased risk of lung stress and VILI. Alternative methods, including heel-to-knee distance and laser measurement, may offer more precise PBW estimation, yet their impact on lung mechanics in ARDS remains unexplored. This study addresses the knowledge gap by evaluating whether differences in height measurement methods significantly affect lung stress, tidal volume distribution, and ventilatory mechanics in ARDS patients.

Patients' heights will be measured using five methods (stadiometer, visual, tape, laser, heel-to knee). Corresponding PBW and tidal volumes (VT = 6 mL/kg PBW) will be calculated and applied in randomized order, each for 30 minutes. Lung stress, ventilatory mechanics, gas exchange, and regional ventilation distribution will be assessed for each VT setting, preceded by a short alveolar recruitment maneuver. During the study, continuous monitoring of hemodynamics and oxygenation will be performed.

Ventilatory parameters including plateau pressure, driving pressure, and transpulmonary pressures will be recorded. Electrical impedance tomography will assess regional tidal volume distribution. Each patient's participation is limited to approximately two hours with no further follow-up.

Inclusion Criteria:

• Adult patient
• Mechanical ventilation on volumetric mode
• Sedated and curarized patient
• Diagnosis of ARDS according to the Berlin criteria
• Diagnosis of ARDS < 48 hours
• Signed informed consent
• Affiliation with a social security or health insurance scheme

Exclusion Criteria:

• Patient opposing the use of his/her data for research purposes.
• Contraindication to placement of an esophageal balloon catheter (e.g., esophageal fistula, esophageal varices).
• Contraindication to strict supine positioning for height measurement (e.g., proven or suspected intracranial hypertension with intracranial pressure >18 mmHg).
• Impossibility of measuring height with the reference method (stadiometer) due to deformity or amputation of both lower limbs.
• Patient under guardianship or curatorship, or deprived of liberty.
• Pregnant or postpartum patient, or patient currently breastfeeding.