Distensibility Index of Inferior Vena Cava in Assessing Fluid Responsiveness: The Impact of Spontaneous Respiratory Effort

Inclusion Criteria: 1. defined as shock who has the presence of systolic blood pressure (SBP) ≤ 90 or a \> 40 mmHg decline of SBP in patients known to be hypertensive or mean arterial pressure (MAP) ≤ 70 mmHg or requiring vasopressors to maintain SBP \> 90 mmHg or MAP \> 70 mmHg, along with signs of hypoperfusion (urinary flow \< 0.5 ml/kg/min for \> 2 h, or presence of skin mottling or blood lactate concentration ≥ 2.0 mmol/L); 2. received mechanical ventilation; 3. had the Pulse Indicator Continuous Cardiac Output (PiCCO) catheter for hemodynamic monitoring； 4. had the sign of fluid administration which include: (a) MAP \< 65 mmHg and/or SBP \< 90 mmHg; (b) the need for vasopressors; (c) urinary flow \< 0.5 ml/kg/min for \>2 h; (d) tachycardia; (e) lactic acidosis; or (f) delayed capillary refilling. Exclusion Criteria: 1. were pregnant; 2. were younger than 18 years of age; 3. could not have spontaneous respiratory drive or effort measured; 4. had intra-abdominal pressure ≥ 16 cmH₂O \[15\]; 5. had conditions affecting dIVC measurement, such as ECMO cannulation; 6. had factors precluding clear ultrasound imaging; 7. defined as right ventricular dysfunction who has (a) tricuspid annular plane systolic excursion \< 16 mm; (b) moderate or greater tricuspid regurgitation; (c) moderate or greater pulmonary artery hypertension\[16\]; 8. expected to die within 24 hours; 9. had family members of the patient declined enrollment.