This multicenter, physiological, observational study hypothesizes that in moderate to severe ARDS, trunk inclination unloads the chest wall, but its impact on lung mechanics depends on PEEP levels and lung recruitability.

There is near-universal agreement among caregivers that head-up positioning is beneficial for mechanically ventilated patients. In most intensive care units, a semi-recumbent position (head of bed elevated 30-45°) has therefore become standard practice, except when absolutely contraindicated. This widespread adoption is driven primarily by robust clinical evidence showing that trunk inclination reduces the incidence of ventilator-associated pneumonia. In patients under general anesthesia, physiological studies showed a clear mechanistic benefit: the vector of abdominal weight shifts caudally, increasing resting lung volume and thereby decreasing the tendency for atelectasis formation.

In patients with acute respiratory distress syndrome (ARDS), however, the physiological consequences of trunk inclination remain undecided. Here, the descent of the diaphragm in the head-up position increases transpulmonary pressure (PL) at end-expiration, which tends to recruit previously collapsed lung units. Yet the "baby lung" of ARDS, the markedly reduced aerated lung volume, operates on widely different segments of its pressure-volume curve (i.e. the lower flat portion, the steep linear portion, or the upper flat portion). Consequently, the net effect of the rise in end-expiratory PL depends on whether recruitment of additional units outweighs overdistension of those already open.

Theoretically, for example, in patients with high lung recruitability but insufficient PEEP, trunk inclination should tilt the balance toward recruitment; in the same patients receiving excessive PEEP, the same maneuver may instead promote overdistension. To date, however, neither the overall effect of trunk inclination nor the modulating roles played by PEEP level and lung recruitability have been adequately assessed. Previous studies have almost invariably assessed trunk inclination at a single fixed PEEP without quantifying lung recruitability, thereby limiting the generalizability of their findings and leaving unresolved the complex interactions among posture, PEEP, chest-wall mechanics, and recruitability.

To address these critical gaps, the investigators designed this multicenter, physiological, observational study. The investigators hypothesized that, in moderate to severe ARDS, trunk inclination unloads the chest wall and that its net impact on lung mechanics is fundamentally determined by the prevailing PEEP level and the individual level of lung recruitability.

Inclusion Criteria:

• Intubated moderate and severe ARDS according to the Berlin definition (PaO2/FiO2 ratio <= 200 mmHg)
• Under continuous sedation with or without paralysis

Exclusion Criteria:

• Age <18 years
• Bronchopleural fistula
• Pure COPD exacerbation
• Contraindication to EIT monitoring (e.g. burns, pacemaker, thoracic wounds limiting electrode belt placement)
• Hemodynamic instability (Systolic BP < 75 mmHg or MAP < 60 mmHg despite vasopressors and/or heart rate < 55 bpm)
• Contraindications to mobilization (e.g., intracranial hypertension, spinal cord injury)
• Intra-abdominal hypotension (IAP≥12mmHg)
• Pregnancy
• Attending physician deems the transient application of high airway pressures to be unsafe

Intubated mechanically ventilated ARDS patients will be considered for enrollment