Characterizing Perfusion and Congestion Responses to Fluid Loading in Critically Ill Septic Patients (FLUID-IMPACTS)

Sepsis is a leading cause of mortality worldwide and a major contributor to deaths in intensive care units. Early hemodynamic resuscitation, particularly fluid loading, is a cornerstone of septic shock management. However, the benefit-risk balance of fluid administration is difficult to assess in routine practice. Insufficient fluid resuscitation may result in persistent tissue hypoperfusion, organ ischemia, and multiorgan failure, whereas excessive fluid administration is associated with increased mortality, mainly due to systemic venous congestion and organ edema.

The concept of fluid tolerance, defined as the ability of a patient to receive fluids without developing harmful consequences related to fluid overload, is increasingly recognized. Nevertheless, its evaluation remains challenging because of the lack of validated tools and consensual thresholds. In addition, although several markers of tissue perfusion and systemic venous congestion have been described, their combined clinical relevance and prognostic value following fluid loading in septic shock have not been specifically evaluated.

This study aims to assess perfusion and congestion responses to fluid loading in patients with septic shock. The primary objective is to compare patients according to changes in tissue perfusion markers (lactate concentration, mottling score, capillary refill time, venous-to-arterial CO₂ gradient, and central venous oxygen saturation) and systemic venous congestion markers (central venous pressure and hepatic and portal vein Doppler indices) after fluid administration. Secondary objectives include evaluating the evolution of venous congestion markers and their association with organ dysfunction within 48 hours, the relationship between post-fluid loading congestion dynamics and 28-day mortality, and identifying pre-fluid loading predictors of patients who fail to improve tissue perfusion while exhibiting worsening venous congestion.

This is a prospective, multicenter, non-interventional observational cohort study conducted in five intensive care units. Eligible patients are adult patients with septic shock, mechanically ventilated, equipped with arterial and central venous catheters, and presenting a positive passive leg-raising test defined as an increase greater than 10% in cardiac output or left ventricular outflow tract velocity-time integral. All patients receive standard care in accordance with international guidelines, and fluid administration is entirely at the discretion of the treating physician.

Clinical, biological, hemodynamic, and echocardiographic data are collected before and after fluid loading. Patients are retrospectively classified into four groups based on the presence or absence of improvement in tissue perfusion and worsening of venous congestion. Follow-up continues until ICU discharge or day 28.

Approximately 280 patients are expected to be screened to include 170 patients. The results may help identify patients who are fluid responsive in terms of cardiac output but at risk of harmful venous congestion, supporting more individualized fluid resuscitation strategies in septic shock.