Oxygen-derived variables such as central venous oxygen saturation (SvO2) or mixed venous oxygen saturation(SmvO2) may also provide evidence of tissue hypoxia. These measures, however, represent the algebraic sum of oxygenation in all venous return, and may therefore not reflect relative distributive hypo-perfusion of certain organs. Nonetheless,saturations below 70% correlate with global tissue hypoxia and a poor prognosis in severe sepsis and septic shock(4)