21 Mar 08
Mixed vs central venous oxygen saturation may be not numerically equal, but both are still clinically useful
By E Rivers
Mixed venous oxygen saturation (SvO2) in sepsis is commonly referred to as an end point of low impact on clinical decisions in sepsis patients because of the following common refrain: “SvO2 is always increased in septic ICU patients.” However, there are fundamental principles that render this modality clinically useful when applying it to the supply-dependent phase of sepsis (ie, global tissue hypoxia). The presence of global tissue hypoxia not only has pathologic significance in vitro, but there is a pathologic link among the clinical presence of global tissue hypoxia (ie, low SvO2 and cardiac index), the generation of inflammatory mediators, and mitochondrial impairment of oxygen utilization that is seen in septic ICU patients. Furthermore, identifying sudden episodes of supply dependency in septic ICU patients (ie, sudden decreases in SvO2) has diagnostic and prognostic significance. With this background, the rationale for using central venous oxygen (ScvO2) saturation as a surrogate for SvO2 to detect and treat global tissue hypoxia in the most proximal phase of sepsis management (supply dependency) was the basis for its use in the Early Goal Directed Therapy in Severe Sepsis and Septic Shock Study (EGDT).