Mixed venous oxygen saturations

Regarding mixed venous oxygen saturations, which one of the following statements is CORRECT?

a) In septic shock, SvO2 is unlikely to be normal or supranormal

b) With a ventricular septal defect, a reduction in SvO2will be observed

c) If oxygen flux is fixed, elevated oxygen consumption results in increased SvO2

d) If arterial oxygen saturation, haemoglobin and oxygen consumption are constant,

SvO2 varies directly with cardiac output

e) Cyanide toxicity causes a reduction in SvO2

Answer: d

Explanation

True mixed venous oxygen saturations are those found in the pulmonary artery,

measured with a pulmonary artery flotation catheter either by sampling the blood or

fibreoptic oximetry incorporated into the device. The oxygen content of venous blood

is a function of how much oxygen the body is using (oxygen consumption – V̇ O2)

balanced against how much oxygen it was supplied with (oxygen delivery – ḊO2).

Oxygen flux is oxygen delivery per unit time (i.e. oxygen content of arterial blood x

cardiac output). The law of mass action tells us essentially ‘what goes in, must come out

or have been used up’. This is the basis of Fick’s law, which when applied here gives:

Q ¼ V_ O2=ðCaO2 _ CvO2Þ

Now, ignoring the dissolved component, oxygen content of blood is 1.34×(Sats/

100)×Hb

so; CaO2 ¼ 1:34 _ ðSats=100Þ _ SaO2 and CvO2 ¼ 1:34 _ ðSats=100Þ _ SvO2

If this is substituted into the Fick equation and rearranged we find:

SvO2 ¼ SaO2 _ 100VO2=ð1:34 _ Hb _ QÞ

Thereby proving the statement in Option (d)

A VSD causes a left-to-right shunt, so oxygenated blood is contributed to that blood

ultimately exiting the right side of the heart, thus SvO2 is elevated. Cyanide toxicity

causes reduced oxygen consumption via disruption of the cellular cytochrome oxidase

system, so SvO2 will rise. In septic shock it is not unusual to find elevated SvO2 because

of the direct cellular toxic effect of the physiological insult and response causing

reduced tissue oxygen consumption. Despite this it is recommended that we aspire

to maximal oxygen delivery in our goal-directed therapy in sepsis with targeted SvO2

or ScvO2 as a guide (65% or 70% respectively).