Mixed venous saturation can help assess tissue oxygen delivery.
By using the derivation of the Fick equation, the mixed venous saturation can be determined by:
SvO2 = SaO2 – [(VO2) / (Hb x 1.36 x Q)]
SvO2: mixed venous saturation SaO2: arterial oxygen saturation VO2: oxygen consumption Hg: hemoglobin
Q: cardiac output
From this equation, one can see that SvO2 varies directly with SaO2, Hg, cardiac output and inversely with VO2.
The normal SvO2 is 65-75%, which denotes tissue oxygen extraction to be 25-35%. Normal PvO2 is 35-45mmHg.
Of note, an accurate sampling the mixed venous blood must be done by drawing from the PA port of the Swan-Ganz catheter.