This study explored how inspiratory effort affects the accuracy of the distensibility index of inferior vena cava (dIVC) in assessing fluid responsiveness (FR).This prospective observational study was conducted in an intensive care unit of a university hospital and included shock patients receiving ventilation with spontaneous breathing activity. Hemodynamic parameters were collected before and after fluid challenge by the pulse indicator continuous cardiac output. dIVC was calculated by ultrasound, and respiratory effort was assessed using airway occlusion pressure (P0.1) and end-expiratory occlusion pressure (Pocc) before fluid administration. Responders were defined by a ≥10% increase in cardiac output (CO) after 250 mL saline infused in 10 minutes. The dIVC > 18% was considered indicative of FR. Predictive performance was evaluated using area under receiver operating characteristic curves (AUROC), stratified by P0.1 and Pocc.

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.

This prospective observational study was conducted in an intensive care unit of a university hospital and included shock patients receiving ventilation with spontaneous breathing activity