[Research progress of diagnostic and therapeutic value of carbon dioxide-derived indicators in patients with sepsis]

Abstract

Effectively assessing oxygen delivery and demand is one of the key targets for fluid resuscitation in sepsis. Clinical signs and symptoms, blood lactic acid levels, and mixed venous oxygen saturation (SvO₂) or central venous oxygen saturation (ScvO₂) all have their limitations. In recent years, these limitations have been overcome through the use of derived indicators from carbon dioxide (CO₂) such as mixed veno-arterial carbon dioxide partial pressure difference (Pv-aCO₂, PCO₂ gap, or ΔPCO₂), the ratio of mixed veno-arterial carbon dioxide partial pressure difference to arterial-mixed venous oxygen content difference (Pv-aCO₂/Ca-vO₂). Pv-aCO₂, PCO₂ gap or ΔPCO₂ is not a purely anaerobic metabolism indicator as it is influenced by oxygen consumption. However, it reliably indicates whether blood flow is sufficient to carry CO₂ from peripheral tissues to the lungs for clearance, thus reflecting the adequacy of cardiac output and metabolism. The Pv-aCO₂/Ca-vO₂ may serve as a marker of hypoxia. SvO₂ and ScvO₂ represent venous oxygen saturation, reflecting tissue oxygen utilization. When oxygen delivery decreases but tissues still require more oxygen, oxygen extraction rate usually increases to meet tissue demands, resulting in decreased SvO₂ and ScvO₂. But in some cases, even if the oxygen delivery rate and tissue utilization rate of oxygen are reduced, it may still lead to a decrease in SvO₂ and ScvO₂. Sepsis is a classic example where tissue oxygen utilization decreases due to factors such as microcirculatory dysfunction, even when oxygen delivery is sufficient, leading to decrease in SvO₂ and ScvO₂. Additionally, the solubility of CO₂ in plasma is approximately 20 times that of oxygen. Therefore, during sepsis or septic shock, derived variables of CO₂ may serve as sensitive markers for monitoring tissue perfusion and microcirculatory hemodynamics. Its main advantage over blood lactic acid is its ability to rapidly change and provide real-time monitoring of tissue hypoxia. This review aims to demonstrate the principles of CO₂-derived variables in sepsis, assess the available techniques for evaluating CO₂-derived variables during the sepsis process, and discuss their clinical relevance.