Fluid responsiveness is a critical determinant guiding fluid therapy in critically ill patients. However, excessive fluid administration can result in fluid overload, leading to venous congestion and worse clinical outcomes. Venous congestion, a marker of impaired fluid clearance, is increasingly recognized as a significant contributor to poor prognosis. Previous studies have demonstrated the coexistence of fluid responsiveness and venous congestion in critically ill patients. Notably, these studies were limited by the absence of fluid challenge-the gold standard for assessing fluid responsiveness-leaving the dynamic relationship between fluid responsiveness and venous congestion incompletely understood. This study aims to investigate the interplay and temporal evolution of fluid responsiveness and venous congestion following a standardized fluid challenge in critically ill patients.

Inclusion Criteria:

• Age ≥ 18 years
• Hospitalized in the intensive care unit (ICU)
• Hemodynamic monitoring in place, with a pulse wave contour analysis- derived estimation of cardiac output (either calibrated or uncalibrated)
• Decision made by clinicians to perform volume expansion through intravenous infusion of crystalloid fluid

Exclusion Criteria:

• Pregnancy
• Refusal to participate by relatives of the patient or the patient himself

Critically ill patients with evidence of inadequate tissue perfusion, who are already monitored using a pulse wave contour analysis-derived estimation of cardiac output (either calibrated or uncalibrated)，and for whom clinicians have decided to administer volume expansion through intravenous crystalloid infusion.