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Correct Diffusing Capacity of Lung for Carbon Monoxide and Carbon Monoxide Transfer Coefficient in Considering Respiratory Function in Patients With Stable Anorexia Nervosa FREE TO VIEW

Douglas Clark Johnson, MD
Author and Funding Information

From the Spaulding Rehabilitation Hospital.

Correspondence to: Douglas Clark Johnson, MD, Spaulding Rehabilitation Hospital, 125 Nashua St, Boston, MA 02114; e-mail: djohnson5@partners.org


Financial/nonfinancial disclosures: The author has reported to CHEST that no potential conflicts of interest exist with any companies/organizations whose products or services may be discussed in this article.

Reproduction of this article is prohibited without written permission from the American College of Chest Physicians (www.chestpubs.org/site/misc/reprints.xhtml).


© 2010 American College of Chest Physicians


Chest. 2010;137(5):1252-1253. doi:10.1378/chest.09-2908
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To the Editor:

There is another explanation for the low diffusing capacity of the lung for carbon monoxide (Dlco) values of patients with moderate-to-severe anorexia reported by Gardini Gardenghi and colleagues1 in CHEST (November 2009) other than “the progressive enlargement of peripheral lung units without relevant alveolar septa destruction.” Because Dlco varies with hematocrit, and patients with anorexia nervosa often have anemia,2 it is particularly important to adjust predicted Dlco and carbon monoxide transfer coefficient (Kco) for hematocrit, which was not done in their study.

The authors mistakenly refer to Kco as “lung diffusion capacity corrected for alveolar ventilation.” In fact, Kco = Dlco/VA, where VA is the alveolar volume, which equals the volume of distribution of the tracer gas (usually helium) minus predicted dead space.3 Kco changes much more with lung volume than does Dlco, so Kco does not “correct for VA.”

Both Dlco and Kco have known changes with lung volume, as would be expected with membrane conduction (DM) varying linearly with VA and blood conduction (θVC) not changing.4 Because the surface area for diffusion is less at lower lung volume, Dlco decreases with lung volume. Because Dlco decreases less than lung volume, Dlco/VA or Kco increases at lower lung volumes. One can correct Dlco (Daco) and Kco (Kaco) for VA. The predicted Daco = predicted Dlco (0.58 + 0.42 VA/VAtlc) and Kaco = Kco (0.42 + 0.58/[VA/VAtlc]), where VA is the measured VA and VAtlc is the predicted VA at total lung capacity (predicted total lung capacity [TLC] minus predicted dead space).4 Percent predicted DACO equals percent predicted Kaco and provides a good indication of the lung’s diffusion correcting for lung volume.

The data demonstrate a problem with the European Community for Coal and Steel equations for predicted Kco. For control subjects, Dlco was 95% predicted and TLC 105% predicted, yet Kco was only 82% predicted. This occurred because of inconsistent equations for predicted Kco and Dlco. Instead, predicted Kco (Dlco/VA) should be calculated as predicted Dlco divided by predicted VA, with predicted VA equal to predicted TLC minus predicted dead space.3 Because their patients had normal TLC and should not have elevated dead space, their predicted Kco should nearly equal predicted Dlco.

The authors should recalculate their data and report Dlco and Kco as percent predicted, adjusting predicted values for hemoglobin. They should also report percent predicted VA. My guess is that anemia will explain some, but not all, of the reduction in Dlco. Finally, they should correct Dlco and Kco for lung volume, by reporting Daco (Dlco as percent predicted adjusted both for hemoglobin and for VA) and Kaco. By adjusting for hemoglobin and properly correcting Dlco for lung volume (Daco), their study can provide further evidence for an impairment of gas exchange in patients with moderate-to-severe anorexia nervosa.

Gardini Gardenghi G, Boni E, Todisco P, Manara F, Borghesi A, Tantucci C. Respiratory function in patients with stable anorexia nervosa. Chest. 2009;1365:1356-1363. [CrossRef] [PubMed]
 
Miller KK, Grinspoon SK, Ciampa J, Hier J, Herzog D, Klibanski A. Medical findings in outpatients with anorexia nervosa. Arch Intern Med. 2005;1655:561-566. [CrossRef] [PubMed]
 
Macintyre N, Crapo RO, Viegi G, et al. Standardisation of the single-breath determination of carbon monoxide uptake in the lung. Eur Respir J. 2005;264:720-735. [CrossRef] [PubMed]
 
Johnson DC. Importance of adjusting carbon monoxide diffusing capacity (DLCO) and carbon monoxide transfer coefficient (KCO) for alveolar volume. Respir Med. 2000;941:28-37. [CrossRef] [PubMed]
 

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References

Gardini Gardenghi G, Boni E, Todisco P, Manara F, Borghesi A, Tantucci C. Respiratory function in patients with stable anorexia nervosa. Chest. 2009;1365:1356-1363. [CrossRef] [PubMed]
 
Miller KK, Grinspoon SK, Ciampa J, Hier J, Herzog D, Klibanski A. Medical findings in outpatients with anorexia nervosa. Arch Intern Med. 2005;1655:561-566. [CrossRef] [PubMed]
 
Macintyre N, Crapo RO, Viegi G, et al. Standardisation of the single-breath determination of carbon monoxide uptake in the lung. Eur Respir J. 2005;264:720-735. [CrossRef] [PubMed]
 
Johnson DC. Importance of adjusting carbon monoxide diffusing capacity (DLCO) and carbon monoxide transfer coefficient (KCO) for alveolar volume. Respir Med. 2000;941:28-37. [CrossRef] [PubMed]
 
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