My wife had a pulmonary function test performed in May. She has the CREST variety of scleroderma and has been diagnosed with early pulmonary hypertension. We got a copy of her report that stated her DLCO was 17.7, which was 60 percent of predicted value. Yet when we got a summary from her pulmonary doctor, he said the DLCO was corrected (for alveolar volume) to 84 percent of predicted.
This is confusing us, since her total lung capacity (TLC) was 111 percent of predicted. My understanding of the reason for correcting a DLCO for alveolar volume (DLCO/VA) is to get a clearer picture of how much of the available capillary surface membrane is actually lost in patients with diminished lung volume. If a person has a TLC of 111 percent, how can you correct the DLCO to 84 percent based on alveolar volume when, in fact, there is increased lung volume? In fact, shouldn't the DLCO be adjusted downward instead? (Using TLC as an equivalent to AV, I came up with a figure of 54%)
We've left messages for the doctor asking this question, but he is apparently too busy to return our calls. My wife is coming to the NIH in a couple weeks for an evaluation, and I would like to have this question answered in advance if possible. Thank you!
Total lung capacity (TLC) is not the same as alveolar volume (VA). TLC is a measure of the lungs, when fully expanded. VA is a compartment of TLC. The volume of lung that actually participates in ventilation is the VA. Individuals with advanced emphysema, for example, have an elevated TLC with a reduced VA.
If a lung is removed, the TLC and VA are reduced in tandem. However with diffuse lung disease of any type, the TLC and VA may not even move in the same direction. With lung disease, portions of the lung that comprise the TLC may be poorly ventilated or even unventilated. The purpose of the DLCO/VA is to establish the diffusing capacity (DLCO) of only the lung area that is ventilated. You correctly surmised that correction for a reduced VA results in an increase in the percent of predicted value. The results you cite suggest that, while the TLC is elevated, the VA is reduced. The TLC is not equivalent to VA.
I hope this answers your question. By the way, a TLC of 111% may not be abnormal. The range of normal DLCO is wide. However a DLCO of 60% is low. A more complete explanation would involve talking about the relationship between ventilation and perfusion. Blood flowing to the lung to be oxygenated is perfusion.
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