The hemodynamic effects of proning and PEEP may vary depending on pre- existing cardiac function, lung mechanics, and volume status, and the available evidence remains conflicting. The impact on the VExUS score of these manoeuvres is unclear. This study aims to assess the effects of PEEP in supine and prone position on the VExUS score as well as echocardiographic parameters in a cohort of healthy volunteers.

Venous congestion: characterized by elevated venous pressure and impaired flow through the organs towards the right heart, contributes to organ dysfunction and is associated with adverse outcomes in critically ill patients. Historically, hemodynamic assessment has focused on the arterial side of circulation, giving less attention to venous hemodynamics. However, there is growing evidence that venous congestion is a mediator of adverse outcomes and contributes to end-organ damage. With increasing venous congestion, subsequent venous distension in the splanchnic circulation occurs and the pressure waveform of the right atrium is retrogradely transmitted into the distal venous circulation. The more venous congestion is present, the more pulsatility is transmitted into the periphery. This represents the backbone of the venous excess ultrasound (VExUS) score assessment. The VExUS score assessment therefore represents a multi-organ Doppler approach evaluating venous congestion by measuring the inferior vena cava (IVC) diameter, flow pattern of hepatic veins, portal vein and intra-renal veins by ultrasound.

Heart lung interactions and implications on VExUS: the VExUS score as well as femoral and jugular vein flows are expected to be affected by positive pressure ventilation. The VExUS score correlates with the Mean systemic filling pressure (MSFP) as a surrogate for stressed volume, in other words the proportion of blood which contributes to creating pressure and blood flow. As right atrial pressure is influenced by Positive End-Expiratory Pressure (PEEP), one would expect elevated VExUS Scores with higher PEEP levels.

Influence of position on hemodynamics: conflicting effects on the circulatory system mediated by prone positioning have been described; no data about VExUS Score assessment in prone position is available.

Inclusion Criteria:

• Voluntary participants ≥ 18 years in good overall health with no relevant organ dysfunction in their medical history and signed informed consent form.

Exclusion Criteria:

• Non-French or non-German speakers
• Unable to provide informed consent
• Body mass index > 40kg/m
• Self-reported pregnancy
• History of pneumothorax
• Previously participated in the study