CO2 Gap Changes Versus Inferior Vena Cava Collapsibility in Relation to Cardiac Index as a Prognostic Value in Septic Shock

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In 2016, sepsis and septic shock was redocumented as fatal organ dysfunction caused by infection-induced host response disorders (Singer et al. 2016). Infectious shock is a subtype of sepsis; its circulation abnormalities significantly increase the mortality rate. The definition was updated to facilitate rapid identification and timely treatment. Despite the continuous progress of awareness and intervention, the mortality rate of septic shock is approaching 40% or more (Gasim et al. 2016, Karampela et al. 2022). Infectious shock exists in the presence of imbalance of oxygen supply and demand as well as tissue hypoxia, early improvement of tissue hypoperfusion is key to the treatment, a specific cluster treatment program was recommended in the guidelines of sepsis rescue action (Rhodes et al. 2017). Severe sepsis remains associated with high mortality, and the early recognition of the signs of tissue hypoperfusion is crucial in its management. The effectiveness of oxygen-derived parameters as resuscitation goals has been questioned, and the latest data have failed to demonstrate clinical advantage (Rudd et al. 2020). Prompt diagnosis and appropriate treatment of sepsis are of ulmost importance and key to survival. However, routinely used biomarkers, such as C-reactive protein and procalcitonin, have shown moderate diagnostic and prognostic value. Of note, the recent consensus definition for sepsis is based on clinical criteria, implying the paucity of reliable sepsis biomarkers. The new diagnostic criteria also incorporate the use of the SOFA score, a composite prediction tool, which is derived by a combination of clinical signs and biomarkers of organ dysfunction, leaving aside classic inflammatory biomarkers (Pierrakos et al. 2020, Karampela et al. 2022). The venous oxygen saturation (SvO2) is \<70% in the majority of patients with severe sepsis on admission to the intensive care unit (ICU). The central venous-to-arterial carbon dioxide difference or only carbon dioxide gap (PCO2 gap) has gained relevance as a measure of assessment of several parameters (Mallat et al. 2015). The balance of dioxide carbon (CO2) production by the tissues and its elimination through the lungs can be reflected by the difference between the mixed venous content (CvCO2) and the arterial content (CaCO2). This venous-arterial difference in CO2 content (CCO2) can be estimated by the following equation: ΔPCO2 = PvCO2 - PaCO2, denominated PCO2 gap and in physiological conditions it ranges from 2 to 5 mmHg. In a few words, it indicates the difference between partial pressure of carbon dioxide in central venous blood (PvCO2) and arterial blood (PaCO2) (Janotka et al. 2021). The venous-to-arterial carbon dioxide difference (Pv-aCO2) can indicate the adequacy of microvascular blood flow in the early phases of resuscitation in sepsis (Ospina-Tascon et al. 2016, de Sá 2022). Hence, other resuscitation goals, such as PCO2 gap, have been suggested, due to their ability to predict adverse clinical outcomes and simplicity in patients achieving normal oxygen derived parameters during the early phases of resuscitation in septic shock. The PCO2 gap can be a marker of cardiac output adequacy in global metabolic conditions that are less affected by the impairment of oxygen extraction capacity (Bitar et al. 2020).

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