Preload responsiveness and venous congestion have largely been investigated independently in recent literature. However, recent data report a similar incidence of venous congestion regardless of fluid responsiveness status, challenging the concept of a linear continuum between preload independence and fluid intolerance. These findings support the need for a more individualized hemodynamic management strategy that takes venous congestion risk into account.

The right ventricle plays a central role in this framework. Its function is to maintain an adequate venous return pressure gradient to ensure cardiac output while limiting upstream venous congestion, under strong dependence on its afterload. In physiological conditions, the right ventricle adapts to changes in afterload by increasing contractility to preserve right ventriculo-arterial coupling and optimize its performance.

In chronic cardiopulmonary diseases, right ventriculo-arterial uncoupling is a well-established prognostic factor, including the presence of occult uncoupling revealed by fluid loading. In critically ill patients, right ventricular systolic dysfunction associated with venous congestion-defining right heart failure-is strongly associated with increased mortality, as is right ventriculo-arterial uncoupling itself.

To support the concept of fluid tolerance, the investigators hypothesize that impairment of right ventriculo-arterial coupling may exist or occur during fluid loading in critically ill patients, independently of preload responsiveness, and may be associated with worsening upstream venous congestion.

This is a prospective observational study designed to evaluate right ventriculo-arterial coupling dynamics during fluid loading in critically ill patients with acute circulatory failure. The study aims to characterize alterations in coupling, their relationship with venous congestion, and the performance of different echocardiographic indices used to quantify right ventricular function and coupling.

Study Population:

Critically ill adult patient in intensive care unit who require fluid loading as part of routine clinical management, based on the attending physician's discretion. Eligible patients may present with hypotension requiring vasopressor support, low mean arterial pressure, or other signs of hemodynamic instability. Patients are prospectively and consecutively included, reflecting standard ICU practice.

Study Procedures:

Fluid loading is administered according to current clinical guidelines. Echocardiographic assessment of the right ventricle is performed immediately before and after fluid administration. The primary measure of RVPA is the TAPSE/TRV ratio. Additional echocardiographic parameters will be collected and other validated indices to define RVPA to allow comparisons across methods.

Secondary Analyses:

Secondary analyses will stratify patients based on fluid responsiveness, defined as a ≥10% increase in cardiac output after fluid loading, and on the presence of venous congestion assessed by VExUS score and portal vein pulsatility. Baseline hemodynamic and echocardiographic parameters will be analyzed to identify predictors of RVPA deterioration. The relationship between RVPA and upstream venous congestion, organ dysfunction, ICU length of stay, and mortality will also be explored. Reproducibility of right ventricule parameters measurements will be assessed by calculating intra- and inter-observer variability.

Technical Considerations:

Echocardiography will be performed by trained operators using standardized acquisition protocols.

Measurements will include TAPSE, tricuspid regurgitation velocity, tricuspid S-wave velocity, PASP, FAC, and IVC diameter.

Data will be recorded before fluid loading and immediately after completion of the fluid challenge, typically within 30 minutes.

Fluid responsiveness will be assessed according to standard dynamic parameters or maneuvers selected by the attending physician.

Patients with poor echogenicity, acute respiratory distress, ongoing myocardial infarction, acute cor pulmonale, or other exclusion criteria will not be included.

Rationale:

The right ventricle is a key determinant of venous return and cardiac output, and its function is highly dependent on afterload. RVPA reflects the efficiency of coupling between right ventricular contractility and pulmonary arterial load. Understanding RVAC dynamics during fluid loading may inform individualized fluid management strategies, potentially minimizing venous congestion and organ dysfunction. Comparing different echocardiographic indices allows evaluation of their relative performance and reliability in detecting RV uncoupling in critically ill patients.

Data Management and Analysis:

All echocardiographic and hemodynamic data will be collected prospectively and recorded in a secure database. Changes in RVAC before and after fluid loading will be analyzed descriptively and in association with fluid responsiveness and venous congestion parameters. Agreement between different RVAC indices will be assessed using correlation and Bland-Altman analyses. Predictive models may be developed to identify baseline factors associated with RVAC deterioration.

Ethical Considerations:

Fluid loading is part of routine clinical care. No experimental interventions are applied. All patients or their legally authorized representatives will provide informed consent in accordance with institutional and national regulations.

Inclusion criteria:

• Critically ill hospitalized patients
• Age ≥ 18 years
• Patients undergoing fluid loading at the discretion of the attending physician, following prediction of fluid responsiveness using any recommended maneuver or dynamic parameter, in the setting of acute circulatory failure requiring vasopressor support and/or mean arterial pressure < 65 mmHg (or a decrease of ≥ 30 mmHg from baseline in patients with chronic hypertension), and/or other signs of hemodynamic instability (tachycardia, mottling, oliguria, hyperlactatemia)
• Affiliated with a national health insurance system

Exclusion criteria :

• Formal refusal from the patient or legally representative after information
• Patients transferred from another intensive care unit
• Pregnant or postpartum patients
• Acute respiratory distress (defined as respiratory rate ≥ 35 breaths/min and/or signs of increased work of breathing)
• Ongoing acute coronary syndrome
• Acute or pulmonale (defined by right ventricular dilation associated with paradoxical septal motion related to an abrupt increase in right ventricular afterload)
• Primary pulmonary arterial hypertension
• Intra-abdominal hypertension (intravesical pressure > 15 mmHg)
• Poor echogenicity precluding adequate echocardiographic assessment of the right ventricle
• Severe valvular heart disease or early postoperative period following valvular surgery

The study population consists of adult critically ill patients with acute circulatory failure. Eligible patients are those for whom the attending physician independently decides to perform fluid loading as part of standard care, in accordance with current European recommendations. Patients may present with hypotension requiring vasopressor support, low mean arterial pressure, or other signs of hemodynamic instability.

Participants are prospectively and consecutively included at the time of fluid loading decision, and undergo standardized echocardiographic assessment of right ventricular function and venous congestion before and after fluid administration. The population reflects routine clinical practice in the intensive care unit.