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Communications to the Editor |

Spirometry in the Diagnosis of Small Airways Obstruction FREE TO VIEW

Ravichandran Theerthakarai, MD; M. Anees Khan, MD, FCCP
Author and Funding Information

Affiliations: St. Joseph’s Hospital & Medical Center Paterson, NJ School of Graduate Medical Education, Seton Hall University South Orange, NJ,  Université Catholique de Louvain Cliniques Universitaires Saint-Luc, Brussels, Belgium

Correspondence to: Ravichandran Theerthakarai, MD, Pulmonary Division, St. Joseph’s Hospital & Medical Center, Paterson, NJ



Chest. 2000;117(3):922-923. doi:10.1378/chest.117.3.922
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To the Editor:

In the article titled “Small Airways Obstruction Syndrome” (July 1999),1 Dr. Stănescu reported on four patients with the functional pattern of decreased vital capacity (VC) and FEV1, normal FEV1/VC ratio and total lung capacity, and increased residual volume (RV). In these patients, Dr. Stănescu alleges, the obstructive pattern would be overlooked on routine spirometry unless RV also was measured.

While a low VC or a low FEV1 may result from either a restrictive or an obstructive defect, the FEV1/VC ratio should be the primary guide to distinguish between the two patterns.2 Three of the four subjects in Dr. Stănescu’s study have FEV1/VC ratios < 70%, which would, therefore, place them in the appropriate category of obstructive defect without the need for RV measurement. Additionally, the study did not provide other spirometric data such as on the midexpiratory phase of forced expiratory flow (FEF25–75%), which may also be useful in confirming the presence of airways obstruction in the presence of a borderline FEV1/VC ratio.,2

During a review of our laboratory data, we discovered this pattern in several subjects who had correctly had obstruction diagnosed based on the spirometric assessment alone, which showed a disproportionate decrease in FEF25–75% and a normal FEV1/VC ratio (confirmed, of course, by the presence of a high RV and RV/TLC ratio).

References

Stănescu, D (1999) Small airways obstruction syndrome.Chest116,231-233. [PubMed] [CrossRef]
 
American Thoracic Society.. Lung function testing: selection of reference values and interpretative strategies.Am Rev Respir Dis1991;144,1202-1218. [PubMed]
 
To the Editor:

I thank Drs. Theerthakarai and Khan for their interest in my article (July 1999).1

Alluding to the American Thoracic Society (ATS) statement,2 the authors write that the FEV1/vital capacity (VC) ratio should be the primary guide to distinguish between a restrictive and an obstructive defect. Of course, the authors are correct. But the problem is how to define the defect, when FEV1/VC ratio is within normal limits, and both FEV1 and VC are decreased, ie, the syndrome we have described.,1

Drs. Theerthakarai and Khan assert that three of our four patients have FEV1/VC ratios <70%, which would therefore place them in the appropriate category of obstructive defect without the need for residual volume (RV) measurement. I was surprised by this affirmation. According to the ATS statement2they quote,“ defining a fixed FEV1/VC [vital capacity] ratio as a lower limit of normal [my emphasis] is not recommended in adults because FEV1/VC is indirectly related to age and height.” All our four subjects had percentage of predicted FEV1/VC ratios within normal limits, as shown in Table 1. We also computed in our subjects expected values for the FEV1/VC ratio using two well-known American formulas.4 According to subjects’ ages and heights, the FEV1/VC ratios of our subjects were also within expected limits. Therefore, the FEV1/VC ratio of the patients we presented did not place them in the category of obstructive defect!

The authors also write that they have “discovered this pattern in several subjects who were correctly diagnosed as obstructive, based on the spirometric assessment alone, showing a disproportionate decrease in FEF25–75% [forced expiratory flow after 25 to 75% of VC has been expelled] and normal FEV1/VC ratio (confirmed, of course, by a high RV and RV/TLC [total lung capacity] ratio)”. One might ask why the authors needed another measurement (FEF25–75%) when the defect was correctly diagnosed by a high RV and RV/TLC and normal FEV1/VC?

The ATS guidelines2 suggest that, indeed, in the presence of a borderline FEV1/VC, maximal expiratory flow rates may help confirm the presence of airway obstruction. But FEV1/VC ratios in our subjects were not borderline. They were within normal limits. Furthermore, the guidelines,2 emphasize that “when FEV1/VC ratio is within the expected range, abnormalities in flow occurring late in the maximal expiratory flow-volume curve, should not be graded as to severity, and, if mentioned, interpretations of their clinical significance should be guarded.”

References
Stănescu, D Small airways obstruction syndrome.Chest1999;116,231-233. [PubMed] [CrossRef]
 
American Thoracic Society.. Lung function testing: selection of reference values and interpretative strategies.Am Rev Respir Dis1991;144,1202-1218. [PubMed]
 
Crapo, RO, Gardner, RM Reference spirometric values using techniques and equipment that meet ATS recommendations.Am Rev Respir Dis1981;123,659-664. [PubMed]
 
Hankinson, JL, Fedan, KB Spirometric reference values from a sample of the general U.S. populationAm J Respir Crit Care Med1999;159,179-187. [PubMed]
 

Figures

Tables

References

Stănescu, D (1999) Small airways obstruction syndrome.Chest116,231-233. [PubMed] [CrossRef]
 
American Thoracic Society.. Lung function testing: selection of reference values and interpretative strategies.Am Rev Respir Dis1991;144,1202-1218. [PubMed]
 
Stănescu, D Small airways obstruction syndrome.Chest1999;116,231-233. [PubMed] [CrossRef]
 
American Thoracic Society.. Lung function testing: selection of reference values and interpretative strategies.Am Rev Respir Dis1991;144,1202-1218. [PubMed]
 
Crapo, RO, Gardner, RM Reference spirometric values using techniques and equipment that meet ATS recommendations.Am Rev Respir Dis1981;123,659-664. [PubMed]
 
Hankinson, JL, Fedan, KB Spirometric reference values from a sample of the general U.S. populationAm J Respir Crit Care Med1999;159,179-187. [PubMed]
 
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