Need help with the alveolar air equation

[Becky423]

Ok guys, I need some help with the alveolar air equation:

I know it goes something like PAO2 = PIO2 - (PACO2/R) + F.

Scenario is: pt breathing room air @ sea level, PaO2 = 100, PaCO2 = 40. If you double the alveolar ventialtion for 10 minutes, what is the new PO2 & PCO2 assuming R=0.8?

Answer is PO2 = 125 & PCO2 = 20

Rationale is:

- increase alveolar ventilation --> increase PA02

- double alveolar ventilation --> no double in PAO2 in person whose PA)2 is already 104 b/c highest PAO2 one could possibly achieve at sea level

First of all, I can't see how the above #'s go into this equation. Second, I though that if you double ventilation, you can double O2 up to 150... But I may have it all wrong, everything's starting to look blurry to me.

Thanks in advance if anyone could enlighten me!!!

- Becky

[loveanesthesia]

Ok guys, I need some help with the alveolar air equation:

I know it goes something like PAO2 = PIO2 - (PACO2/R) + F.

Scenario is: pt breathing room air @ sea level, PaO2 = 100, PaCO2 = 40. If you double the alveolar ventialtion for 10 minutes, what is the new PO2 & PCO2 assuming R=0.8?

Answer is PO2 = 125 & PCO2 = 20

Rationale is:

- increase alveolar ventilation --> increase PA02

- double alveolar ventilation --> no double in PAO2 in person whose PA)2 is already 104 b/c highest PAO2 one could possibly achieve at sea level

First of all, I can't see how the above #'s go into this equation. Second, I though that if you double ventilation, you can double O2 up to 150... But I may have it all wrong, everything's starting to look blurry to me.

Thanks in advance if anyone could enlighten me!!!

- Becky

PAO2=FiO2(PB-PH2O) - PaCO2/RQ

At sea level PB = 760, and water vapor pressure at body temp = 47

so if breathing room air and PaCO2 = 40 the equation is:

PAO2 = .21(760-47) - 40/.8

PAO2 = 150-50 = 100

If PaCO2 = 20

PAO2 = .21(760-47) - 20/.8

PAO2 = 150 - 25 = 125

I think the rational is better stated by explaining that the increase in oxygen in the alveoli is secondary to the reduction in the level of carbon dioxide.

The only way alveolar oxygen content on room air could be 150 is if there was no carbon dioxide being produced (the patient is dead), or you could quickly increase alveolar oxygen by increasing the FiO2

[FLSRNA79]

this is how we understand that alveolar gas equation at our academic facility:

first you have to calculate the "alveolar gas equation"

fio2 x (patm-ph20)- paco2/r

* ph2o = 47mmhg

*r = vq mismatch = 0.8

then you have to come up with the [color=#e46c0a]alveolar to arterial gradient (a-a gradient)

equation* pao2 - pao2

note: the pao2 is figured from the "alveolar gas equation"; whereas the pao2 is obtained from the patient's abg results

*normal a-a gradient = 5-15mmhg or (age/4)+4

the a-a gradient looks @ the physiology of gas exchange across the alveolar-capillary membrane

increases with: v/q mismatch, r to l shunting, or diffusion abnormality

*the higher your a-a gradient, the worse the pulmonary disease process is (i.e. v/q mismatch; r to l shunting; diffusion abnormality)

hope that helps!

flsrna79 *wine