Pulmonary shunt

[Nick8]

If I want to assess a lung shunt in a patient who is on a ventilator, do I have to set FiO2=100% or I can assess the shunt on any FiO2?

 

[Doctor4Life1769]

I thought it was 100% O2, but my memory fades me at this point.... I'll relearn it for Step 3/internship/residency.

 

[PMPMD]

If you're talking about the calculation of shunt fraction from the forumla below, then yes.

Qs/Qt = (CcO2 - CaO2) / (CcO2 - CvO2)

FiO2 is set to 1.0 to make the calculation of CcO2 easier.

 

[Nick8]

If you're talking about the calculation of shunt fraction from the forumla below, then yes.

Qs/Qt = (CcO2 - CaO2) / (CcO2 - CvO2)

FiO2 is set to 1.0 to make the calculation of CcO2 easier.

Thank you for your answer.
However does the shunt depend on FiO2? If yes, Iwill have different shunts in different sets of FiO2 in one patient.

 

[Paseo Del Norte]

Thank you for your answer.
However does the shunt depend on FiO2? If yes, Iwill have different shunts in different sets of FiO2 in one patient.

Traditionally, I believe the thinking is that the shunt equation, when calculated with an FiO2 of less than 1.0 will be a mix of both shunt and venous admixture as opposed to shunt only.

 

[fakin' the funk]

If you're talking about the calculation of shunt fraction from the forumla below, then yes.

Qs/Qt = (CcO2 - CaO2) / (CcO2 - CvO2)

FiO2 is set to 1.0 to make the calculation of CcO2 easier.

Which is to say, FiO2 is set to 1.0 to assume a ScO2 (sat post-ventilated capillary) of 100%

Unless you have crazy diffusion impairment, you can simplify this equation to:

Shunt % = (1-SaO2)/(1-SvO2)

The shunt fraction does not depend on the FiO2, but the PO2 and SaO2 and SvO2 will