This prospective single-center observational study aims to evaluate whether the ultrasound-measured femoral vein-to-femoral artery diameter ratio can predict fluid responsiveness in mechanically ventilated adult intensive care patients with acute circulatory failure or shock. Only patients who already have PiCCO monitoring as part of routine clinical care and for whom a fluid challenge is clinically indicated will be included. No additional invasive procedure will be performed for study purposes. Cardiac index will be measured with PiCCO before and after administration of 500 mL balanced crystalloid over 10 minutes, and femoral vein and femoral artery diameters will be measured by bedside ultrasonography. Fluid responsiveness will be defined as an increase of more than 10% in cardiac index after fluid administration. The study will assess the diagnostic performance of the femoral vein-to-femoral artery diameter ratio as a practical bedside parameter to support hemodynamic decision-making in critically ill patients.

Acute circulatory failure and shock are common conditions in the intensive care unit, and fluid administration remains a key component of hemodynamic management. However, unnecessary fluid loading may worsen tissue edema, oxygenation, and organ function. Therefore, practical bedside tools that help identify fluid-responsive patients are clinically important.

The aim of this study is to evaluate the clinical utility of the ultrasound-measured femoral vein-to-femoral artery diameter ratio in predicting fluid responsiveness in mechanically ventilated adult ICU patients with acute circulatory failure or shock. This is a prospective, single-center, observational study.

No additional invasive intervention will be performed for research purposes. PiCCO monitoring will not be initiated for study reasons; only patients who already have PiCCO monitoring as part of routine ICU management and who are scheduled to undergo a clinically indicated fluid challenge will be enrolled.

Before the fluid challenge, cardiac index will be recorded using PiCCO, and femoral vein and femoral artery diameters will be measured by bedside ultrasonography at the same anatomical level proximal to the femoral artery bifurcation. Each ultrasound measurement will be repeated at least three times, and the mean value will be recorded. A fluid challenge consisting of 500 mL balanced crystalloid administered over 10 minutes will then be performed according to routine clinical care. After the fluid challenge, cardiac index and the femoral vein-to-femoral artery diameter ratio will be reassessed using the same methods.

Fluid responsiveness will be defined as an increase of more than 10% in cardiac index after fluid administration. Responders and non-responders will be compared, and the diagnostic performance of the femoral vein-to-femoral artery diameter ratio will be analyzed.

Demographic data, baseline clinical findings, hemodynamic measurements, and ultrasound measurements will be collected and analyzed. Appropriate descriptive statistics, group comparisons, and diagnostic performance analyses will be used to evaluate the discriminative ability of the femoral vein-to-femoral artery diameter ratio. All data will be de-identified and handled in accordance with patient confidentiality principles.

Inclusion Criteria:

• Adult patients aged 18 years or older
• Admission to the intensive care unit where the study is being conducted
• Receiving invasive mechanical ventilation
• Clinical indication for a fluid challenge due to acute circulatory failure or shock, as determined by the responsible intensive care physician
• PiCCO monitoring already in place as part of routine clinical care
• Clinical conditions allowing collection of pre- and post-fluid challenge cardiac index and ultrasonographic measurements
• Technical feasibility of femoral vein and femoral artery diameter measurements by ultrasonography

Exclusion Criteria:

• Age younger than 18 years
• Failure to obtain written informed consent from the patient or legally authorized representative before enrollment
• Absence of PiCCO monitoring as part of routine clinical care or no clinical plan for fluid challenge
• Inability to complete pre- and post-fluid challenge measurements (e.g., early death, urgent surgery, transfer)
• Local conditions preventing reliable femoral ultrasonographic assessment, including open wound, infection, dressing, major anatomic obstacle, poor image quality, known femoral vein thrombosis, or severe vascular pathology on the relevant side
• Conditions likely to significantly impair the reliability of thermodilution measurements (e.g., severe tricuspid regurgitation or significant intracardiac shunt)
• Major treatment changes during the fluid challenge that preclude comparable measurements (e.g., major vasopressor dose change, major ventilator setting change, new arrhythmia)
• Missing or inadequate data judged by the investigators to compromise data integrity, including absence of measurements required for the primary outcome

The study population consists of adult intensive care unit patients with acute circulatory failure or shock who are receiving invasive mechanical ventilation and for whom the responsible intensive care physician has determined a clinical indication for a fluid challenge. Only patients who already have PiCCO monitoring in place as part of routine clinical care and in whom femoral vein and femoral artery ultrasonographic measurements can be obtained will be enrolled.