This is a single-center, prospective, randomized, three-period crossover physiological study in adult patients with moderate-to-severe acute respiratory distress syndrome (ARDS, PaO₂/FiO₂ ≤150 mmHg). Each enrolled subject will receive all three APRV ventilation conditions in random sequence: low-level spontaneous breathing (spontaneous minute-ventilation proportion 10%-30%), high-level spontaneous breathing (31%-50%), and fully suppressed spontaneous breathing as control. A 30-minute wash-out period is set between each condition to restore unified physiological baseline. Primary outcome is ventilation-perfusion mismatch percentage (mismatch%) measured by Electrical Impedance Tomography (EIT). Secondary outcomes include EIT-derived pulmonary parameters, respiratory mechanics, hemodynamics, diaphragmatic function and safety profiles. Only short-term, reversible ventilator and sedation adjustments will be performed; routine clinical management remains unchanged throughout the trial.
1. Age ≥18 and ≤80 years old. 2. Diagnosed with moderate-to-severe ARDS according to the 2023 global ARDS definition.
1. Refractory shock defined as persistent hyperlactatemia and/or prolonged capillary refill time, fluid-resistant, requiring norepinephrine ≥ 0.5 μg/kg/min.
Crossover
APRV ventilation with low-level spontaneous breathing
DeviceAPRV ventilation mode, spontaneous-contributed minute-ventilation proportion maintained at 10%-30%. 30-minute stabilization before outcome measurement.
APRV ventilation with high-level spontaneous breathing
DeviceAPRV ventilation mode, spontaneous-contributed minute-ventilation proportion maintained at 31%-50%. 30-minute stabilization before outcome measurement.
APRV ventilation with fully suppressed spontaneous breathing
DeviceAPRV ventilation mode, spontaneous breathing fully suppressed by deep sedation (RASS-4 to -5). 30-minute stabilization before outcome measurement.
Ventilation-perfusion mismatch percentage (Mismatch%)
Ventilation-perfusion mismatch percentage (Mismatch%), calculated as the sum of dead-space percentage and shunt percentage measured by electrical impedance tomography (EIT). A composite index reflecting overall ventilation-perfusion mismatch, assessed at baseline and under three APRV experimental conditions. Unit: %
Match percentage(match%)
Match percentage derived from electrical impedance tomography (EIT). Unit: %.
Pendelluft amplitude
Pendelluft amplitude derived from electrical impedance tomography (EIT). Unit: %.
Dead-space percentage
Dead-space percentage derived from electrical impedance tomography (EIT), assessed at baseline and under three APRV experimental conditions. Unit: %.
Shunt percentage
Intrapulmonary shunt percentage derived from electrical impedance tomography (EIT), assessed at baseline and under three APRV experimental conditions. Unit: %.
Incidence and severity of adverse events
Incidence and severity of adverse events during study interventions, focusing on respiratory events, circulatory events and procedure-related adverse events. Unit: unitless.
Rate of premature intervention discontinuation for safety concerns
Rate of premature discontinuation of study interventions due to safety-related reasons. Unit: %.
Inspired oxygen fraction (FiO₂)
Set inspired oxygen fraction, obtained directly from ventilator readings, recorded at baseline (prior to intervention, after stabilization) and at the end of each 30-minute APRV experimental condition (after stabilization). Unit: %.
Arterial pH
Arterial blood pH measured from arterial blood gas analysis, measured at baseline (prior to intervention, after stabilization) and at the end of each 30-minute APRV experimental condition (after stabilization). Unit: unitless.
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