Sigh35 and End-expiratory Occlusion Test (EEOT) for Assessing flUid Responsiveness in Critically Ill Patients Undergoing Pressure Support Ventilation

* 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