The effect of errors in Inspiratory Volume on DLCO.

Yesterday while reviewing reports I ran across an interesting error in the Inspiratory Volume (VI) from a DLCO test. I’ve probably seen this before but this time I realized what effect it could have on DLCO. Specifically, what I saw was that at the start of the DLCO test the subject had not finished exhaling and although the technician had started the test, the subject continued to exhale.

What makes this interesting is that the software used the subject’s volume at the start of the test as the initial volume. This means that the software measured the VI from the initial volume to the end of inspiration, not from the point at which the subject stopped exhaling to the end of inspiration. This also means that the VI was underestimated by 0.20 L and this affects both VA and the calculated DLCO.

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DLCO, de-constructed

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.
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COHb and Pulse Oximetry

I was reviewing a report recently that included the results for walking oximetry. These showed that the individual has a resting SaO2 of 97% and desaturated significantly to 86% after walking a couple hundred yards. This was curious since a DLCO had also been performed and the results for that test were 94% of predicted. It’s unusual for somebody with a normal DLCO to have that low of an SaO2 but I have seen it before in individuals who were unable to ventilate adequately because of a paralyzed diaphragm. I’ve also seen it happen sometimes when somebody has a peripheral vascular disease like Reynaud’s that produces a poor quality oximeter signal. Buried in the technician’s notes however, was an additional piece of information that called into question both the resting and the exercise SaO2 readings. Specifically, the notes mentioned that an ABG had been performed and that the subject’s COHb was 9%.

Oxygen saturation is measured spectrophotometrically. The different forms of hemoglobin, i.e. oxyhemoglobin (O2Hb), deoxyhemoglobin, methemoglobin (MetHb) and carboxyhemoglobin (COHb) absorb the frequencies of red and infrared light differently.

from Hampson NH. Pulse oximetry in severe carbon monoxide poisoning. Chest 1998; 114: 1036-1041

Although non-invasive oximetry was first developed during the 1930’s and 1940’s (in 1935 by K. Mathes in Germany and independently in 1942 by G. Milliken in the USA), current pulse oximeter technology dates from 1972 (by Takuo Aoyagi, researcher for Nihon Koden in Japan). The original pulse oximeters were large, bulky and generally stationary pieces of equipment. Oximeters underwent progressive miniaturization during the 1980’s and 1990’s and rapidly evolved into the handheld and fingertip units we see today and the only “stationary” oximeters that remain are those used in ICU-type monitoring systems.

Modern laboratory CO-oximeters measure the absorption of light in a blood sample at up to 128 wavelengths, spread across the entire hemoglobin absorption spectrum. Using mathematical analysis they can report total hemoglobin concentration and oxygen saturation in addition to fractional deoxyhemoglobin, COHb, and MetHb.

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