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Defining the Lower Limit of Normal for FEV₁/ FVC

We thank Dr. Miller and colleagues for their comments on our recent article (1). In this study, we found that the lower limit of the normal postbronchodilator FEV₁/FVC exceeded 70% across all ages among healthy never-smokers. Miller and colleagues question the reliability of this result. According to two studies published in 1983, there is a relatively slow cooling of gases within the Vitalograph, and correcting spirometric indexes to BTPS assumes that the spirometer has a short cooling time constant (2, 3). Thus, Miller and colleagues suggest that applying the BTPS correction gives too large of an FEV₁/FVC. They furthermore propose that the FEV₁/FVC is overestimated because the Vitalograph stopped recording expiratory volume after 6 seconds.

We acknowledge that temperature conversion of volumes measured by the Vitalograph may not be linear. However, a significant problem of overestimation has been shown to be most prominent at more extreme temperatures (4). In our study, room temperature ranged from 19 to 24°C. Furthermore, the linearity of our spirometers was checked by using a 1.00-L syringe equipped with a one-way valve. The Vitalograph has been the most commonly used spirometer in Norway since 1965, both in general practice and in scientific research (5–7). The Vitalograph is still in widespread use and is, as far as we know, not considered a weakness by peers when reviewing scientific manuscripts.

Previous studies have found an FEV₁/FEV₆ cutoff of 73% to be the best substitution for an FEV₁/FVC cutoff of 70% based on general populations. However, we do not have any reason to suspect a similar difference among the asymptomatic never-smokers in our study. Such healthy subjects will most likely have reached their FVC within the 6-second limit. Previous studies have shown that the difference between FEV₆ and FVC increases with smoking and degree of airway obstruction (8). Furthermore, although the Vitalograph chart stopped moving after 6 seconds, the subjects were asked to continue to exhale beyond 6 seconds to reach their FVC. The highest volume was then registered. Determining the number of healthy never-smokers who had to expire longer than 6 seconds to reach FVC would indeed be a study of interest. So far we do not have data that indicate that our measurements are not representative for asymptomatic never-smokers.

Ane Johannessen

Haukeland University Hospital, Bergen, Norway

Sverre Lehmann

Haukeland University Hospital and University of Bergen, Bergen, Norway

Ernst Omenaas

Haukeland University Hospital, Bergen, Norway

Geir Egil Eide

Haukeland University Hospital and University of Bergen, Bergen, Norway

Per Bakke

Haukeland University Hospital, Bergen, Norway

Amund Gulsvik

University of Bergen, Bergen, Norway

FOOTNOTES

Conflict of Interest Statement: None of the authors has a financial relationship with a commercial entity that has an interest in the subject of this manuscript.

REFERENCES

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