Air trapping at CT: high prevalence in asymptomatic subjects with normal pulmonary function

To determine the degree and extent of air trapping at computed tomography (CT) in subjects with normal pulmonary function test results.

The study group consisted of 50 subjects with normal pulmonary function, including 26 nonsmokers and 24 smokers (14 current and 10 ex-smokers; 11 mild and 13 heavy smokers). All 50 subjects underwent thin-section CT at which images were obtained during deep inspiration and expiration at three lung levels. The mean expiratory increase in lung attenuation was measured at each level. Air trapping was visually classified into four degrees (none, lobular, mosaic, or extensive), and the extent of air trapping was also semiquantitatively calculated. The visual grade and semiquantitative ratio of air trapping were compared among nonsmokers, current smokers, and ex-smokers and among nonsmokers, mild smokers, and heavy smokers by using the Kruskal-Wallis rank test and the Fisher protected least significant difference test, respectively.

The mean increase in lung attenuation in the three levels at expiration was 111.9 HU +/- 46.3 (SD). The overall frequency of air trapping was 64%. Lobular, mosaic, and extensive air trapping were seen in 10 (20%), 14 (28%) and eight (16%) patients, respectively. There was no significant difference in the visual grade of air trapping among the nonsmokers, current smokers, and ex-smokers (P =.387) or among the nonsmokers, mild smokers, and heavy smokers (P =.231). There was also no significant difference in the semiquantitative ratio of air trapping among nonsmokers, current smokers, and ex-smokers (P =.859) or among nonsmokers, mild smokers, and heavy smokers (P =.897).

Various degrees of air trapping, including the mosaic or extensive types, can be observed in subjects with normal pulmonary function and have no correlation with the subject's current smoking status or cigarette consumption.