Background: Maximal expiratory flows (MEFs) depend on the elastic recoil pressure in the alveoli, airway resistance and bronchial collapsibility. MEFs at lower levels of vital capacity [MEFs at x% FVC (MEF(x))] would indicate the patency of peripheral airways. In Japan, a ratio of MEF(50) to MEF(25) (MEF(50)/MEF(25)) greater than 4.0 is used as an index of injury to the small airways in subjects without airflow limitation. However, to date there have been no epidemiological investigations relating to this index. The aim of this study was to evaluate the impact of cigarette smoking on MEFs in a general population, and to assess the validity of using this index to evaluate injury to the small airways.
Methods: Subjects aged 40 years or older (n=2,917), who had participated in a community-based annual health-check in Takahata, Japan, were enrolled in the study. MEF(75), MEF(50) and MEF(25) were measured in these subjects.
Results: In smokers, as compared with never-smokers, the percentage predicted MEFs (%MEFs) decreased according to the aging of the population, except in the case of %MEF(25) in females. In males, but not in females, %MEFs decreased significantly with an increase in cigarette consumption. In both genders, MEF(50)/MEF(25) was slightly, but significantly, elevated with aging of the population. In addition, 36.5% of subjects who participated in this health-check had MEF(50)/MEF(25) values greater than 4.0. No difference in MEF(50)/MEF(25) was observed between smokers and never-smokers.
Conclusion: Cigarette smoking enhanced the age-related decline in MEFs. Since many healthy subjects aged 40 years or older have MEF(50)/MEF(25) values greater than 4.0, the use of this criterion may over-estimate the presence of small airway disease.