My wife watches the Food Network a lot and I occasionally watch it with her but I can only take so much of it before I go off and read or work on one of my projects. I’ve noticed however in the various cooking contests that sometimes a chef will deconstruct a familiar recipe. This more or less means they break the recipe down into its components and present them as separate pieces or perhaps by putting what goes inside on the outside instead.
I’ve discussed the DLCO test with numerous people and have found that many know and understand (or at least remember) the ATS/ERS criteria for test quality. At the same time however, there seems to be very few people that understand the formula used to calculate the single-breath DLCO and I suspect this is probably because most of us didn’t like the mathematics classes we had to attend in high school or college (and tried to forget what we learned as quickly as we could afterwards).
The DLCO formula isn’t that complicated however, and more importantly all the components of the DLCO test and the reasons for the ATS/ERS quality criteria are embedded within it. All this seems to be a good reason to de-construct the DLCO “recipe” and try to explain it’s various pieces.
As a reminder the single-breath DLCO formula is:
VA = alveolar volume in ml
BHT = breath holding time in seconds
Pb = barometric pressure
PH2O = partial pressure of water vapor in the lung
FITrace = fractional concentration of tracer gas in the inspired DLCO mixture
FATrace = fractional concentration of tracer gas in the alveolar sample
FICO = fractional concentration of CO in the inspired DLCO mixture
FACO = fractional concentration of CO in the alveolar sample
I think the part that bothers everybody the most is:
and that’s because there’s two different things going on here. First, the part within the brackets:
is intended to correct the initial CO concentration for the dilution that occurs when the DLCO test gas mixture is inhaled and mixes with the gas that was within the lung at the start of the inhalation. The whole point of the DLCO test is to measure CO uptake but the initial concentration for this measurement is not what’s in the tank, it’s what’s in the lungs after it has been diluted by the lung’s residual volume and deadspace gas.