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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
Author and Funding Information

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; the Division of Pulmonary and Critical Care Medicine (Dr Reilly), University of Pittsburgh; and the Pulmonary and Critical Care Medical Unit (Dr Brown), Massachusetts General Hospital.

Correspondence to: Carl R. O’Donnell, ScD, Beth Israel Deaconess Medical Canter, Division of Pulmonary and Critical Care Medicine, E/Dana717, 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(2):459-460. doi:10.1378/chest.10-1093
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To the Editor:

We thank Dr Tantucci for the interesting points raised in his letter regarding our article in CHEST.1 We agree with the “doubts” he expresses: “…that CT scan may be adopted as the gold standard for measuring [total lung capacity] TLC....” In fact, we clearly state in our discussion: “Measurement of absolute lung volume lacks any gold standard for comparison.” We address the potential limitations of CT scan measurements and include several more paragraphs to detail the strengths and limitations of plethysmography and helium dilution volume measurement as well. We thus take exception to the statement that our efforts in this regard seem marginal, especially as the criticisms contained in his communication are not well supported by citation to relevant literature.

We acknowledge a lack of spirometric monitoring during CT image acquisition at two institutions (H2, H3). However, we point out that subjects at H2 received breathing instruction from a prepared script and practiced breath-hold maneuvers prior to imaging. The effectiveness of this approach to improve patient performance has been validated.2 Furthermore, patients at H3 were evaluated as part of a longitudinal study involving repeated chest CT scans and were thus very familiar with the procedure.

The author supposes that unconscious loss of air and substantial unidirectional volume signal drift during breath hold among subjects at H1 may result in underestimation of “corrected” TLC. Volume was monitored using a dry-seal, volume-displacement spirometer. The device was calibrated and leak tested prior to each study. While wearing nose clips, subjects breathed through a large flanged rubber mouthpiece held in place by one of the investigators (C. O.), and breath hold at full inhalation was monitored by real-time observation of direct writing pen on paper. Following image acquisition, while still connected to the device, subjects were instructed to inhale maximally and slowly exhale all the way. The resulting vital capacity measure was compared with the largest vital capacity obtained independently in the upright sitting position with a volume displacement spirometer during pulmonary function testing. The mean difference of these upright vs supine vital capacities was approximately 80 mL with upper 95% CI < 200 mL.

The longest time between pulmonary function testing and CT scan was < 2 months. In fact, 80% of subjects were tested on the same day, 90% within 1 week, and only six subjects (< 5%) at > 2 weeks.

Our CT scan volume estimates did not include the extrathoracic airways, intrathoracic trachea, or main stem bronchi. This exclusion may have contributed to the reported average 240 mL difference between CT scan and helium dilution estimates of TLC, but not to observed differences between helium and plethysmography.

The author wonders whether an inappropriate density threshold limit was applied during our CT scan analysis and cites results published by Zompatori et al3 that suggest a 230-mL increase of measured air volume per 100 Hounsfield units (HU) of change in the higher limit of the density mask. First, we referenced tracheal air on each individual scan and used this value to identify tissue density on each section. Thus, we did not assume that any given mask density was uniformly accurate. Second, Zampatori et al3 studied 17 patients with airflow obstruction by comparing CT scan-derived volume estimates at masks densities of 200, 300, and 400 HU with volumes measured plethysmographically. In effect, they assumed that plethysmography was the gold standard and concluded that CT scan volume estimates obtained with a 200-HU density mask were most accurate (“The upper threshold was more correct…”) because they most closely corresponded with those measured by body box. Their CT scan-derived TLC estimates obtained with a 200-HU mask density averaged 873 mL less than those obtained by plethysmograph. This is precisely equivalent to the mean plethysmographic-to-CT scan volume difference we report in Table 2.

Regarding upright to supine change in TLC, the author references Denison et al4 for evidence of a 500-mL change in TLC because of blood shift from legs to thorax. Denison et al4 did not measure upright supine differences and merely included references within table legends that refer to two other studies, one of which was conducted > 50 years ago.5,6 We are unaware of currently accepted evidence that there is such a substantial reduction in TLC while supine due to a central shift of blood volume.

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]
 
Bankier AA, O’Donnell CR, Bosielle PM. Respiratory instructions for CT examinations of the lungs: a hands-on guideline. Radiographics. 2008;284:919-931. [CrossRef] [PubMed]
 
Zompatori M, Fasano L, Pacilli AM, Battista G, Cavina M, Pezzi S. Automatic evaluation of total lung capacity and of emphysema involvement with spiral computerized tomography (CT) in obstructive pneumonia [In Italian]. Radiol Med (Torino). 2001;1011-2:18-24
 
Denison D, Morgan M, Millar A. Estimation of regional gas and tissue volumes of the lung in supine man using computed tomography. Thorax. 1986;418:620-628. [CrossRef] [PubMed]
 
Allen SM, Hunt B, Green M. Fall in vital capacity with posture. Br J Dis Chest. 1985;793:267-271. [CrossRef] [PubMed]
 
Svanberg L. Influence of posture on the lung volumes, ventilation and circulation in normals; a spirometric-bronchospirometric investigation. Scand J Clin Lab Invest. 1957;9suppl 25:1-195. [CrossRef] [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]
 
Bankier AA, O’Donnell CR, Bosielle PM. Respiratory instructions for CT examinations of the lungs: a hands-on guideline. Radiographics. 2008;284:919-931. [CrossRef] [PubMed]
 
Zompatori M, Fasano L, Pacilli AM, Battista G, Cavina M, Pezzi S. Automatic evaluation of total lung capacity and of emphysema involvement with spiral computerized tomography (CT) in obstructive pneumonia [In Italian]. Radiol Med (Torino). 2001;1011-2:18-24
 
Denison D, Morgan M, Millar A. Estimation of regional gas and tissue volumes of the lung in supine man using computed tomography. Thorax. 1986;418:620-628. [CrossRef] [PubMed]
 
Allen SM, Hunt B, Green M. Fall in vital capacity with posture. Br J Dis Chest. 1985;793:267-271. [CrossRef] [PubMed]
 
Svanberg L. Influence of posture on the lung volumes, ventilation and circulation in normals; a spirometric-bronchospirometric investigation. Scand J Clin Lab Invest. 1957;9suppl 25:1-195. [CrossRef] [PubMed]
 
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