People with sepsis who are treated in the intensive care unit (ICU) often have impaired blood flow to the skin and other tissues. These alterations in tissue perfusion may be associated with disease severity and clinical outcomes.

The aim of this study is to see whether simple signs of blood flow to the skin, such as capillary refill time and blood test measurements, are linked to outcomes in people with sepsis treated in the ICU.

Adults with sepsis who are admitted to the ICU will be invited to take part. Researchers will collect routine clinical data and blood test results during the first day of ICU treatment. No extra procedures or treatments will be performed as part of the study.

The results of this study may improve understanding of early signs of impaired circulation in sepsis and their relation with recovery and survival.

Sepsis is a life-threatening condition caused by a dysregulated host response to infection and is associated with high morbidity and mortality in intensive care units (ICUs). Early detection of impaired tissue perfusion is essential for risk stratification and may help predict clinical outcomes.

Peripheral perfusion can be assessed using a combination of bedside clinical signs and physiological and biochemical markers. Capillary refill time (CRT) is a simple, non-invasive bedside measure of peripheral perfusion. In addition, respiratory and blood gas-derived variables, such as end-tidal carbon dioxide (ETCO₂) and the veno-arterial carbon dioxide difference (ΔCO₂), may reflect alterations in tissue blood flow and microcirculatory dysfunction. However, the relationship between these markers and clinical outcomes in patients with sepsis is not fully established.

This is a prospective observational cohort study conducted in adult ICU patients with sepsis or septic shock. Eligible participants will be enrolled after ICU admission. Sepsis will be defined according to Sepsis-3 criteria, with a Sequential Organ Failure Assessment (SOFA) score of 2 or higher at inclusion. Baseline disease severity will be further assessed using the Acute Physiology And Chronic Health Evaluation (APACHE) IV score.

Peripheral perfusion markers will be recorded at predefined time points: at inclusion (T0), six hours (T6), and twenty-four hours (T24). CRT and ETCO₂ will be measured at the bedside, using standardized methods. Arterial and venous blood gas samples will be obtained at the same time points to determine PaCO₂, PvCO₂, lactate levels, and the veno-arterial carbon dioxide difference (ΔCO₂). Blood sampling will be performed through existing arterial or venous catheters whenever possible.

Additional routinely collected clinical data will include hemodynamic variables, ventilatory parameters, vasopressor use, and organ support. Outcomes will include SOFA score on day three, ventilator-free days and vasopressor-free days within the first seven days, and vital status at day 7 and day 28.

No experimental treatments or protocol-driven changes to standard clinical management will be introduced. This study aims to evaluate the association between early peripheral perfusion markers-combining CRT, ETCO₂, and blood gas-derived variables-and short-term outcomes in adults with sepsis treated in the ICU.

Inclusion Criteria:

• adults aged 18 years or older
• admission to the intensive care unit
• sepsis or septic shock, defined according to Sepsis-3 criteria
• SOFA score ≥ 2 at inclusion
• presence of an arterial line and a central venous catheter suitable for blood sampling
• possibility to perform peripheral perfusion assessment (CRT) at predefined time points

Exclusion Criteria:

• age under 18 years
• pregnancy
• expected ICU stay less than 24 hours
• limitations of therapy at ICU admission (e.g., DNR or comfort-only care)
• improper central venous catheter position (catheter tip not located in the superior vena cava)
• absence of arterial or central venous access required for paired blood gas sampling
• refusal of consent, when applicable according to local ethics requirements

The study population comprises adult patients admitted to the intensive care unit with sepsis or septic shock, defined according to Sepsis-3 criteria, with a Sequential Organ Failure Assessment (SOFA) score of ≥ 2 at the time of inclusion. Participants will be enrolled early after ICU admission and managed according to standard care and local treatment protocols.

The study population consists of critically ill patients requiring invasive monitoring, enabling the assessment of peripheral perfusion using capillary refill time and paired arterial and venous blood gas analyses during the first 24 hours of ICU admission.