From the University of Brescia, Scienze Mediche e Chirurgiche.
Correspondence to: Claudio Tantucci, MD, University of Brescia, Scienze Mediche e Chirurgiche, 1a Medicina, Piazzale Spedali Civili 1, Brescia, Italy 25123; e-mail: email@example.com
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
I read the article by O’Donnell and coworkers1 recently published in CHEST (May 2010). Their conclusions about the accuracy of two functional methods (plethysmographic and dilutional) to measure lung volumes (in this case, total lung capacity [TLC]) in patients with severe airway obstruction are based on the assumption that CT scan is the reference method.
Several doubts exist that CT scan may be adopted as a gold standard for measuring TLC, especially in the above-mentioned study, for the following reasons.
1. No spirometric control of lung volume was performed during the CT image acquisition in H2 and H3 centers where two-thirds of patients with airflow obstruction were recruited.
2. At H1, center lung volume was spirometrically controlled, but for an average acquisition time of 20 s, the unconscious loss of air could be substantially high at TLC (without a shutter); the correction made by the authors for this unavoidable problem is unclear and possibly inaccurate. It should be stressed that measuring correctly a slow vital capacity after 20 s of breath-hold time implies the use of a pneumotachograph (or spirometer) with a prolonged zero stability of the flow (or volume) signal. For most of the commercially available instruments for lung function testing, a threshold value of ± 25 mL/s is accepted as zero,2 meaning a potential drift of ± 500 mL for 20 s, if this problem is not adequately checked and fixed.
3. There was a relatively long time interval (up to 2 months) between the TLC measurements performed by CT scan and those obtained during pulmonary function testing.
4. There was no mention of the measurement of anatomic dead space when computing TLC by CT scan (“from apex to base of each lung”). Anatomic dead space (that encompasses also upper airways and extrathoracic trachea) amounts to 150 to 200 mL and even more at TLC, and it is included in the plethysmographic and dilutional measurements of TLC.
5. There was no technical description of the CT scanners that were used (kilovolt peak, milliamperes) and no mention of the density threshold limits prefixed to perform the CT scan volumetric measurements of the lung. It has been shown that in a three-dimensional model of the lung the gas volume increases about 230 mL for each 100 Hounsfield units of change in the higher limit of the density mask.3
6. The effect of body position on TLC has been mentioned by the authors. It should be clearly stressed that in the supine position TLC can be reduced up to 500 mL,4 essentially because of the blood shift from legs to thorax.
7. Finally, it must be remembered that TLC by CT scan is measured just once, whereas TLC by plethysmographic or dilutional method is always the mean of two or, even better, three measurements.
Taking account of these sources of error (which all potentially contribute to underestimating TLC obtained by CT scan) is mandatory in this kind of study. Such effort seems only marginally accomplished in the work of O’Donnell and colleagues.1 Therefore, the mean difference between the plethysmographic and CT scan measurements of TLC reported by the authors in the patients with more severe airflow obstruction ( > 1.07 L) could be substantially less and very likely not significant. In this case, any judgment about the plethysmographic method as systematically overestimating TLC in the presence of airway obstruction would be highly questionable and unwarranted.
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