Prediction of Fluid Responsiveness Using the Femoral Vein-to-Artery Diameter Ratio

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