Thread by @medrants

1/
#UncleBob was puzzled last night by a very low A-a gradient during an @UnremarkableLab session

Patient had a pCO2 of 55 and pO2 of 76 which calculates as a gradient of 5. That is probably impossible so something must be off.

@anandiyermd @DxRxEdu

2/ Two numbers in the A-a gradient are variable: atmospheric pressure and RQ. The atmospheric pressure in Birmingham is around 745 rather than the 760 we normally use. If you plug that into the equation the A-a gradient decreases to ~2. So that does not explain it.

3/ The big variable is the respiratory quotient. What is the respiratory quotient: "Respiratory quotient, also known as the respiratory ratio (RQ), is defined as the volume of carbon dioxide released over the volume of oxygen absorbed during respiration. "

4/ One source suggests that RQ varies between 0.7 & 1. Thus our A-a gradient calculation is always an estimate unless we could measure RQ. If we assumed RQ of 1 and atmospheric pressure of 745, then A-a for our patient would have been 14 - which makes sense.

5/ RQ is a measure of basic metabolic rate and changes with diet. I
So as much as I love A-a gradient, I learned to adjust slightly for Birmingham being 800-900 feet above sea level
and RQ being a variable. I hope this discussion is accurate
@anandiyermd