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.