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Carl R. O’Donnell, ScD; Alexander A. Bankier, MD; Leopold Stiebellehner, MD; John J. Reilly, MD, FCCP; Robert Brown, MD; Stephen H. Loring, MD
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From the Department of Pulmonary and Critical Care Medicine (Dr O’Donnell), the Department of Radiology (Dr Bankier), and the Department of Anesthesia and Critical Care (Dr Loring), Beth Israel Deaconess Hospital; the Department of Pulmonology (Dr Stiebellehner), Medical University of Vienna; Pulmonary and Critical Care Medicine (Dr Reilly) Brigham and Women’s Hospital; and the Pulmonary and Critical Care Medicine Unit (Dr Brown), Massachusetts General Hospital.

Correspondence to: Carl O’Donnell, ScD, Beth Israel Deaconess Medical Center, 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 (http://www.chestpubs.org/site/misc/reprints.xhtml).


© 2010 American College of Chest Physicians


Chest. 2010;138(5):1281-1282. doi:10.1378/chest.10-1607
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To the Editor:

We thank Dr Stănescu for his correspondence regarding our recent publication in CHEST (May 2010).1 We have presented data that challenge conventional ideas about the plethysmographic method for measuring the resident volume of gas in the chest. Dr Stănescu’s critique is based largely on the traditional arguments defending that method over the helium (He) dilution method, and our rebuttal follows.

Unfortunately, the first sentence in his letter belies a careful reading of our report. Contrary to his claim that we studied 132 patients with airflow obstruction, we actually studied patients undergoing thoracic CT scan for a variety of indications, 79 with spirometrically documented obstruction and 53 with no evidence of obstruction.

He goes on to state, “The CT scan is a new method for measuring lung volume. One would expect that the authors, before applying it to patients with airflow obstruction, would compare it with established methods in healthy subjects.” As we report in the Abstract and “Results” section, “…there were significant within-subject differences in TLC [total lung capacity] by measurement technique among subjects with airflow obstruction, but not among those without airflow obstruction.”

Thus, we have established the comparability of CT scan with established techniques in subjects with normal lung function. Furthermore, radiographic techniques, including CT imaging, have been used for several decades to assess lung volume.2 The CT scan is not a “new method for measuring lung volume” as Dr Stănescu suggests.

The concern is raised that our “findings may be explained by an underestimation of lung volumes using the CT scan method resulting from submaximal inspiration in the supine position.” We did discuss the potential influence of the supine position on lung volume measurements. As we reported, at H1 (where CT imaging was performed during spirometric monitoring) we compared upright vs supine vital capacities, and the mean difference was approximately 80 mL (upper 95% CI <200 mL). Two other pieces of evidence suggest that supine position alone does not explain the observed differences in TLC by technique. The first is lack of by-technique difference in TLC among healthy individuals. Thus, for the supine position to explain our data, it would require that TLC from supine position be less than that from a seated position only in those with obstruction and that the magnitude of the difference increase with severity of obstruction. We do not know of any evidence that this is the case, nor does Dr Stănescu cite any such evidence in defense of his criticism. The second is the very close correspondence between He dilution and CT scan lung volume among subjects with COPD (mean difference = 150 mL). To argue that our results in those with obstruction are explained in one instance by underestimation of TLC by CT scan, measured with subjects supine, and in another instance underestimation by He, measured with subjects seated, would require the highly unlikely coincidence that the two underestimations are equal and independent of the severity of the obstruction. Dr Stănescu does not provide us with a hypothesis or data supporting the justification for his suggestion that this coincidence exists.

It is true that we did not attempt mutually standardized quality control among the three participating laboratories. However, all three sites were in university-affiliated hospitals and each laboratory adhered to American Thoracic Society/European Respiratory Society guidelines for pulmonary function test administration. We did monitor quality indicators, such as adequate He dilution time, plethysmographic panting frequency, linearity of plethysmographic box-pressure vs mouth-pressure plots, and test reproducibility. We are not sure how “one [laboratory] across the ocean” influences interlaboratory reliability, as Dr Stănescu seems to imply. To the extent that interlaboratory differences might affect results, we would expect less precision (more noise); therefore, a decreased probability of identifying any existing differences in line with our hypothesis (a conservative bias).

It is argued by Dr Stănescu that “In patients with moderate to severe airflow obstruction lung spaces are closed. The volume of these spaces is measured using plethysmography …[and]…CT scans, but not using He dilution.” However, patients with severe airways obstruction do not routinely have atelectasis; therefore, there must be ventilation of the regions subtended by closed airways, such as by collateral pathways. This ought to allow equilibration of He throughout the lung, if sufficient time is allowed. Furthermore, because closing volume increases with age and is in the range of functional residual capacity in people in their 60s, Dr Stănescu’s argument would predict that TLC measured by helium would be progressively less than TLC measured by plethysmography with aging. To our knowledge, this has not been observed.

We point out that “It was not until 1978 that Brown and Slutsky demonstrated frequency-dependent discrepancies in the measurement of thoracic gas volume among patients with asthma, a result suggesting that reported increases in TLC in asthma may have been largely an artifact of Pleth.” Thus we do not “omit mention that this conclusion was reached before 1982.”

In response to the statement, “Then and later, it was shown that plethysmography systematically overestimates lung volume in patients with airflow obstruction when they are panting at  = 2 Hz. However, panting at  = 1 Hz corrects the overestimation of lung volume,” we acknowledged research3,4 demonstrating panting frequency dependence of plethysmographic TLC in airflow obstruction and the apparent accuracy of measures made at 1 Hz panting frequency.5 However, we went on to point out that none of the work on which the acceptability of 1-Hz panting is based included subjects with very severe obstruction. Many of our subjects did have an FEV1 substantially less than that reported for subjects in previous research on the effects of panting frequency. The largest by-technique differences we observed occurred among subjects with the most severe airflow obstruction.

O’Donnell CR, Bankier AA, Stiebellehner L, Reilly JJ, Brown R, Loring SH. Comparison of plethysmographic and helium dilution lung volumes: which is best for COPD? Chest. 2010;1375:1108-1115. [CrossRef] [PubMed]
 
Denison DM, Morgan MD, Millar AB. Estimation of regional gas and tissue volumes of the lung in supine man using computed tomography. Thorax. 1986;418:620-628. [CrossRef] [PubMed]
 
Rodenstein DO, Stănescu DC, Francis C. Demonstration of failure of body plethysmography in airway obstruction. J Appl Physiol. 1982;524:949-954. [PubMed]
 
Rodenstein DO, Stănescu DC. Frequency dependence of plethysmographic volume in healthy and asthmatic subjects. J Appl Physiol. 1983;541:159-165. [PubMed]
 
Shore SA, Huk O, Mannix S, Martin JG. Effect of panting frequency on the plethysmographic determination of thoracic gas volume in chronic obstructive pulmonary disease. Am Rev Respir Dis. 1983;1281:54-59. [PubMed]
 

Figures

Tables

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 for COPD? Chest. 2010;1375:1108-1115. [CrossRef] [PubMed]
 
Denison DM, Morgan MD, Millar AB. Estimation of regional gas and tissue volumes of the lung in supine man using computed tomography. Thorax. 1986;418:620-628. [CrossRef] [PubMed]
 
Rodenstein DO, Stănescu DC, Francis C. Demonstration of failure of body plethysmography in airway obstruction. J Appl Physiol. 1982;524:949-954. [PubMed]
 
Rodenstein DO, Stănescu DC. Frequency dependence of plethysmographic volume in healthy and asthmatic subjects. J Appl Physiol. 1983;541:159-165. [PubMed]
 
Shore SA, Huk O, Mannix S, Martin JG. Effect of panting frequency on the plethysmographic determination of thoracic gas volume in chronic obstructive pulmonary disease. Am Rev Respir Dis. 1983;1281:54-59. [PubMed]
 
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