The aim of the VeMic study is to explore if venous congestion is linked with microcirculatory impairment in elective cardiac surgery patients in the postoperative ICU stay.

Cardiac surgery is associated with a significant risk of postoperative complications, including organ dysfunction such as acute kidney injury (AKI). Traditionally, clinical management in the intensive care unit (ICU) has focused on monitoring macrocirculatory parameters-such as blood pressure, cardiac output, and central venous pressure (CVP)-to guide treatment. However, adequate systemic perfusion does not always translate into sufficient tissue oxygenation at the microcirculatory level. Microcirculation, which occurs in the smallest vessels such as capillaries, is where oxygen and nutrients are actually delivered to the cells. Impairment in microcirculatory function can occur even when macrocirculatory values appear normal, leading to what is known as loss of hemodynamic coherence. Despite growing evidence of the clinical importance of microcirculation, routine bedside assessment remains limited due to technical challenges and the lack of easily applicable tools.

While elevated CVP and signs of venous congestion have been linked to worse outcomes such as AKI, the underlying mechanisms remain poorly understood. CVP alone is an imperfect marker for venous stasis and does not reliably predict microcirculatory impairment. Newer ultrasound-based tools-such as the Vexus score and femoral vein Doppler (FVD)-are emerging as promising, non-invasive methods to assess venous congestion at the bedside. However, it is currently unclear whether these sonographic signs of venous congestion are actually associated with impaired microcirculatory function. This knowledge gap limits clinicians' ability to interpret ultrasound findings in the context of microvascular health and to make informed decisions about fluid management and organ perfusion.

The VeMic study aims to investigate whether ultrasound-based indicators of venous congestion correlate with objectively measured microcirculatory impairment in patients admitted to the ICU after non-emergency cardiac surgery. By combining ultrasound assessments with advanced analysis of sublingual microcirculation using handheld vital microscopy and automated software, the study seeks to bridge the gap between macro- and microcirculatory monitoring. The findings may help determine whether these easily accessible ultrasound tools can be used to detect early signs of microvascular dysfunction and guide more targeted, physiology-based interventions in the postoperative setting.

Inclusion Criteria:

• Age ≥ 18 years
• Elective or urgent (need for definitive procedure during hospitalisation, but not emergency intervention) cardiac surgery

Exclusion Criteria:

• Age < 18 years
• Pregnant women
• Known severe chronic kidney disease (estimated glomerular filtration rate <15 mL/min per 1.73 m2 or dialysis)
• Renal or liver transplantation
• Any known condition interfering with Doppler evaluation of the portal system (including known or suspected cirrhosis or portal vein thrombosis or huge abdominal emphysema).
• Inability to consent to study
• Emergency cardiac surgery

Patients presenting for elective or urgent cardiac surgery. Urgent patients are defined as patients who have not been electively admitted for operation but who require intervention or surgery on the current admission for medical reasons. These patients cannot be sent home without a definitive procedure but do not require immediate emergency surgery.