[DrSeddik]

What's the mixed venous blood oxygen content and what is its significance?

I know it's abnormal in septic shock, but I still don't know why...

 

[Sabio]

Mixed Venous Oxygen Content = Venous Oxygen Content

Shock is a state in which there's inadequate tissue perfusion that's inadequate to meet the metabolic cellular demands. Septic shock follows the same principle.

The oxygen content of the pulmonary arterial circulation is what's called mixed venous oxygen content as you have mixed the systemic and heart venous blood in the pulmonary trunk.

Simple rearrangement of the Fick principle yields this equation;
Venous Oxygen content = Arterial Oxygen content - Oxygen consumption / Cardiac output.

Therefore, mixed venous O2 content is directly related to arterial oxygen content (therefore related to hemoglobin concentration and partial pressure of Oxygen) and cardiac output (therefore it will increase in cases of low cardiac output) and oxygen consumption which means tissue perfusion (therefore you have increased mixed venous O2 content in shock and in any case of impaired tissue perfusion).

Another scenario likely tested in USMLE is when you have arteriovenous shunts which of course will lead to increased mixed venous O2 content as the Oxygen takes a U turn and is not consumed in the tissue.

References:
Harrison's Principles of Internal Medicine
Kaplan Physiology book
http://www.springerlink.com/content/n42u68q7p84lhm1k/

 

[Laurentiu]

mixed venous blood oxygen content/significance.

Measurement of mixed venous oxygen content have been used to assess the adequacy of tissue oxygenation and in steady states may provide information on changes in the cardiac output.
The average value is 150 ml/L in healthy resting individuals. The lowest value was 120ml/L.

To understand the importance of mixed venous oxygen content it is helpful to consider its significance in the estimation of cardiac output from Fick equation.
CVO2 = CaO2 - VO2/Q.

CVO2 & CaO2 are the mixed venous & arterial blood contents of O2 respectively

VO2 is the O2 uptake.
Q is the cardiac out put.

As the mixed venous content is dependent on all the terms on the right side of equation it can provide a meaningful assessment of cardiac output or changes in cardiac output only if O2 up take and arterial content remain nearly constant.

This does not happen in septic shock.

 

[khanar]

....
This does not happen in septic shock.

It's related to any kind of shock
As Sabio pointed out, any state of inadequate tissue perfusion will ultimately decreases your mixed venous oxygen content, even your equation supports that.

 

[DrSeddik]

cardiac output (therefore it will increase in cases of low cardiac output)

Thank you for the elaborated but very much needed explanation.

I think from the modulated Fick's principle (which took a lot of time accepting the new version ) low cardiac output will decrease the Venous oxygen content.

I read further about the subject and what I got to in conclusion is that in hypovolemic shock, cardiac output will decrease, and become unable to keep up with the rate of oxygen uptake by the tissue, which further increase the "oxygen consumption/cariac output" figure, increaseing venous oxygen content. While in septic shock oxygen cosumption (tissue extraction) is impaired, and cardiac output is initially high, making the venous oxygen content high, much like the Artervenous shunts you have talked about.. that's why I saw it frequently in questions about septic shock.

 

[indigo]

So Sabio, does it basically mean as Septic shock has vasodilation blood will not reach most of the peripheral tissues (peripheral runoff) and hence the extraction of oxygen will be low giving a high mixed venous oxygen and vice versa in the case of cardiogenic shock,hypovolemic shock.