CV physiology: mixed venous oxygen

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.