An observational study in Acute Respiratory Distress Syndrome (ARDS), sponsored by Zhongda Hospital. Recruiting at 3 sites in China. Open to participants aged 18 Years and older. Per ClinicalTrials.gov, last updated 2026-09-03.
Sponsored by Zhongda Hospital · Observational
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.
Intubated mechanically ventilated ARDS patients will be considered for enrollment
Exclusion Criteria:
Intubated mechanically ventilated patients with moderate to severe ARDS according to the Berlin definition
Other: Specific lung recruitment maneuvers and decremental PEEP steps
Specific lung recruitment maneuvers will be performed to measure the potential for lung recruitment. Followed by a decremental PEEP steps to determine lung mechanics at different PEEP levels. These process will be repeated when patients change to another position. Electrical impedance tomography signals, synchronized signals of airway pressure and flow, esophageal pressure will be recorded continuously.
Transpulmonary driving pressure (ΔPL)
Physiological parameter calculated as end-inspiratory transpulmonary pressure minus end-expiratory transpulmonary pressure. Transpulmonary pressure is monitored continuously in real time using an esophageal balloon catheter.
Time frame: 2 hour
Percentage of overdistension and collapse
Global and regional information, derived from electrical impedance tomography (EIT) through a decremental PEEP trial from 24cmH2O to 6cmH2O. Collapse and overdistention were calculated assuming zero collapse at PEEP = 24 cm H2O (or lower if not tolerated) and zero overdistention at PEEP = 6 cm H2O, according to a method proposed by Costa. Therefore, the reported percentages of collapse and overdistension refer to relative percentages of modifiable collapse and overdistension.
Time frame: 2 hours
Lung compliance (Clung)
Physiological parameter calculated as the tidal volume divided by the transpulmonary driving pressure (ΔPL). Global lung compliance uses tidal volume measured by the ventilator. Regional lung compliance uses regional tidal volume derived from electrical impedance tomography (EIT) regional ventilation distribution. Transpulmonary pressure is monitored continuously in real time using an esophageal balloon catheter.
Time frame: 2 hours
Respiratory system compliance (Crs)
Physiological parameter calculated as tidal volume divided by the driving pressure (driving pressure = plateau airway pressure minus total positive end-expiratory pressure). Tidal volume is measured by the ventilator. Plateau pressure and total PEEP are obtained during end-inspiratory and end-expiratory occlusive pauses on the ventilator, respectively.
Time frame: 2 hours
Chest wall compliance (Ccw)
Physiological parameter calculated as tidal volume divided by chest wall driving pressure, where chest wall driving pressure is the change in esophageal pressure between end-inspiration and end-expiration. Tidal volume is measured by the ventilator. Esophageal pressure is monitored continuously in real time using an esophageal balloon catheter.
Time frame: 2 hours
Recruitment-to-inflation (R/I) ratio
Physiological parameter calculated as the ratio of the compliance of the recruited lung (Crec) to the respiratory system compliance measured at low PEEP through a single-breath method according to Chen et.al. Crec is derived from the recruited volume (difference between the actual exhaled tidal volume after a PEEP change maneuver and the volume predicted by low-PEEP compliance) divided by the change in PEEP. All measurements are performed on ventilator.
Time frame: 2 hours
Lung recruitability (ΔCollapse24-6)
Physiological parameter defined as the absolute reduction in the percentage of lung collapse (ΔCollapse24-6) when comparing PEEP 6 cmH₂O (at the start of the protocol) to PEEP 24 cmH₂O. The percentage of lung collapse is measured by electrical impedance tomography (EIT).
Time frame: 2 hours
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Respiratory Distress Syndrome→
Zhongda Hospital