Adult (>18 yo) ICU patients undergoing PSV ventilation Inspiratory support level (PS) between 8 and 15 cmH2O and positive end-expiratory pressure (PEEP) between 5 and 12 cmH2O,  Presence of acute circulatory failure defined as:  Systolic blood pressure (SBP) ≤90 mmHg or mean arterial pressure (MAP) ≤70 mmHg or requiring vasopressors to maintain SBP >90 mmHg or MAP >70 mmHg or decline of SBP > 50 mm Hg in known hypertensive patients, along with one or more of the following: urinary flow ≤0.5 mL/kg/min for ≥2 hours heart rate ≥100 beats per minute presence of skin mottling and 4) blood lactate concentration ≥4 mmol/L.  Exclusion criteria  left ventricular ejection fraction <30% or severe valvular dysfunction atrial fibrillation severe acute respiratory distress syndrome (ARDS) abdominal compartment syndrome air leakage through chest drains artifacts in arterial waveform pathological respiratory patterns due to neurological diseases signs of fatigue or respiratory distress