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Carl R. O’Donnell, ScD; Alexander A. Bankier, MD; Leopold Stiebellehner, MD; John J. Reilly, MD; Robert Brown, MD; Stephen H. Loring, MD
Author and Funding Information

Correspondence to: Carl R. O’Donnell, ScD, Division of Pulmonary, Critical Care and Sleep Medicine, Beth Israel Deaconess Medical Center, E/Dana 717, 330 Brookline Ave, Boston, MA 02215; e-mail: codonne1@bidmc.harvard.edu


Financial/nonfinancial disclosures: The authors have 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;138(1):233-234. doi:10.1378/chest.10-1008
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To the Editor:

We wish to thank Scarlata et al for their thoughtful correspondence regarding our article in CHEST.1 We agree that the functional consequences of lung hyperinflation in the setting of airflow obstruction are more directly related to elevated operating volume (the range of volume excursion over which ventilatory work is performed and gas exchange takes place) than to the total lung capacity (TLC). Because the differences between TLC and functional residual capacity (FRC) or residual volume (RV) are assessed spirometrically by volume displacement at the airway opening (as inspiratory capacity [IC] or vital capacity [VC], respectively), measurements of FRC and RV are subject to the same sources of error as TLC, and any absolute error in the estimation of TLC would be added to the FRC and RV with the result that FRC/TLC and RV/TLC ratios are disproportionately elevated.

We have speculated about the influence of erroneously elevated volume ratios on the interpretation of published COPD outcome studies. For example, Fessler et al2 developed a model in which plethysmographically measured RV/TLC ratios predicted improvement in FEV1 following lung volume reduction surgery (LVRS). Because any plethysmographic error would be proportionately greater for RV, the RV/TLC ratio would be artifactually elevated. Among LVRS candidates in our sample, average plethysmographic RV/TLC was 64.9%, whereas average helium dilution RV/TLC was only 58.4%. When incorporated into the model of Fessler and colleagues, this difference in RV/TLC yields a twofold difference in predicted post-LVRS improvement in FEV1 (30% for plethysmographic vs 15% for He estimates). Also, the data of Casanova et al3 indicated an approximate 5% annual increase in mortality for each 1% decrement of IC/TLC ratio in COPD. Among our subjects, plethysmographic IC/TLC averaged 20.7%, whereas helium IC/TLC averaged 27.6%. This difference in the estimate of IC/TLC ratio yields an approximate 30% difference in predicted annual mortality. We believe these examples demonstrate potential consequences of plethysmographic error for predicting clinical outcomes and indicate the need to take a new look at an accepted measure.

With respect to the observed lack of significant association between TLC and degree of airflow obstruction in the study of Dykstra et al,4 we point out two things. First, the authors used a conservative criterion of P < .01 for inclusion of TLC predictors in the statistical model: The actual P value for the association of TLC and FEV1 in their multivariate model was 0.04. Second, we observed a threshold effect in the relationship between FEV1 and plethysmographic overestimation of TLC such that the magnitude of apparent error became much greater at FEV1 values < 30% of predicted. Analyses that treat the relationship between FEV1 and plethysmographic TLC as a linear continuum may be prone to underestimating its magnitude.

Finally, the authors state that “TLC in COPD patients varies as a function of phenotype.” Unfortunately, no citation is provided, and we are unaware of any good evidence for the relationship of TLC to COPD phenotype (emphysema vs chronic bronchitis) that is not potentially biased by the reported limitations in plethysmographic volume measurement.

O’Donnell CR, Bankier AA, Stiebellehner L, Reilly JJ, Brown R, Loring SH. Comparison of plethysmographic and helium dilution lung volumes: which is best in COPD? Chest. 2010;1375:1108-1115. [CrossRef] [PubMed]
 
Fessler HE, Scharf SM, Permutt S. Improvement in spirometry following lung volume reduction surgery: application of a physiologic model. Am J Respir Crit Care Med. 2002;1651:34-40. [PubMed]
 
Casanova C, Cote C, de Torres JP, et al. Inspiratory-to-total lung capacity ratio predicts mortality in patients with chronic obstructive pulmonary disease. Am J Respir Crit Care Med. 2005;1716:591-597. [CrossRef] [PubMed]
 
Dykstra BJ, Scanlon PD, Kester MM, Beck KC, Enright PL. Lung volumes in 4,774 patients with obstructive lung disease. Chest. 1999;1151:68-74. [CrossRef] [PubMed]
 

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References

O’Donnell CR, Bankier AA, Stiebellehner L, Reilly JJ, Brown R, Loring SH. Comparison of plethysmographic and helium dilution lung volumes: which is best in COPD? Chest. 2010;1375:1108-1115. [CrossRef] [PubMed]
 
Fessler HE, Scharf SM, Permutt S. Improvement in spirometry following lung volume reduction surgery: application of a physiologic model. Am J Respir Crit Care Med. 2002;1651:34-40. [PubMed]
 
Casanova C, Cote C, de Torres JP, et al. Inspiratory-to-total lung capacity ratio predicts mortality in patients with chronic obstructive pulmonary disease. Am J Respir Crit Care Med. 2005;1716:591-597. [CrossRef] [PubMed]
 
Dykstra BJ, Scanlon PD, Kester MM, Beck KC, Enright PL. Lung volumes in 4,774 patients with obstructive lung disease. Chest. 1999;1151:68-74. [CrossRef] [PubMed]
 
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