Rationale and Objectives
To evaluate the correlations of tracheal volume and collapsibility on inspiratory and end-expiratory computed tomography (CT) with lung volume and with lung function in smokers.
Materials and Methods
The institutional review board approved this study at each institution. 85 smokers (mean age 68, range 45–87 years; 40 females and 45 males) underwent pulmonary function tests and chest CT at full inspiration and end-expiration. On both scans, intrathoracic tracheal volume and lung volume were measured. Collapsibility of the trachea and the lung was expressed as expiratory/inspiratory (E/I) ratios of these volumes. Correlations of the tracheal measurements with the lung measurements and with lung function were evaluated by the linear regression analysis.
Results
Tracheal volume showed moderate or strong, positive correlations with lung volume on both inspiratory ( r = 0.661, P < .0001) and end-expiratory ( r = 0.749, P < .0001) scans. The E/I ratio of tracheal volume showed a strong, positive correlation with the E/I ratio of lung volume ( r = 0.711, P < .0001). A weak, negative correlation was found between the E/I ratio of tracheal volume and the ratio of forced expiratory volume in the first second to forced vital capacity ( r = −0.436, P < .0001). Also, a weak, positive correlation was observed between the E/I ratio of tracheal volume and the ratio of residual volume to total lung capacity ( r = 0.253, P = .02).
Conclusions
Tracheal volume and collapsibility, measured by inspiratory and end-expiratory CT scans, is related to lung volume and collapsibility. The highly collapsed trachea on end-expiratory CT does not indicate more severe airflow limitation or air-trapping in smokers.
The meaning behind the highly collapsed trachea, observed on expiratory scans of computed tomography (CT), is still controversial. Although it has been acknowledged that the central airways markedly decrease in size on dynamic or end-expiratory CT scans in patients with tracheomalacia (TM) or tracheobronchomalacia (TBM) , some studies have demonstrated that the highly collapsed trachea can also be observed on these expiratory scans in subjects with normal lung function . In patients with chronic obstructive pulmonary disease (COPD), published information on the size of the trachea is more limited. Although some investigations have demonstrated the presence of the highly collapsed central airways in COPD using dynamic expiratory or end-expiratory scans , it still remains unclear whether or not the highly collapsed trachea indicates reduced lung function.
In 2003, Ederle and colleagues demonstrated that, in both normal subjects and subjects with obstructive pulmonary disease, cross-sectional area of the trachea correlated with mean lung density (MLD) and cross-sectional area of the lung on both inspiratory and end-expiratory CT scans . They also showed that the changes in tracheal cross-sectional area between inspiratory and expiratory scans significantly correlated to the changes in MLD. Further, more recent studies have demonstrated significant correlations between MLD and lung volume (LV) . Based on these observations, it can be predicted that the observations of Ederle and colleagues would be reproduced using volumetric measurements, such as LV and tracheal volume.
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Materials and methods
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Subjects
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Table 1
Clinical Characteristics of the 85 Subjects
Mean ± SD (Range) Age (y) 67.8 ± 8.0 (45−87) Smoking index (pack-years) 48.4 ± 32.9 (2−180) FEV 1 /FVC 0.56 ± 0.14 (0.17−0.83) FEV 1 (%predicted) 60.8 ± 23.6 (14−126) RV/TLC 0.50 ± 0.10 (0.32−0.73) TLC (L) 6.45 ± 1.34 (4.10−10.20) RV (L) 3.24 ± 0.97 (1.50−7.00)
FEV 1, forced expiratory volume in 1 second; FVC, forced vital capacity; RV, residual volume; SD, standard deviation; TLC, total lung capacity.
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Pulmonary Function Tests
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Thin-section CT
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Measurements of the Trachea
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Measurements of the Lung Volume
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Reproducibility of Tracheal Volume Measurements
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Statistical Analysis
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Results
Reproducibility of Tracheal Volume Measurements
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Table 2
Reproducibility of Tracheal Volume Measurements
Mean Difference ± SD Limit of Agreement Tracheal volume (mL) Intraobserver error 0.05 ± 0.13 −0.20 to 0.30 Interobserver error 0.04 ± 0.15 −0.22 to 0.29
SD, standard deviation.
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Tracheal Measurements
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Table 3
Measurements of the Trachea and Lung Volume on Chest CT
Inspiration Expiration E/I ratio Cross-sectional area (mm 2 ) Upper trachea 273.3 ± 63.7 236.4 ± 60.2 0.87 ± 0.12 Lower trachea 273.1 ± 66.7 234.4 ± 62.1 0.86 ± 0.11 Tracheal volume (mL) 24.6 ± 6.8 19.5 ± 6.1 0.79 ± 0.13 Lung volume (l) 5.44 ± 1.35 3.83 ± 1.17 0.71 ± 0.14
CT, computed tomography; E/I, expiratory/inspiratory.
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CT-based Lung Volume
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Correlations between Tracheal Volume and CT-based Lung Volume
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Table 4
Correlations of Tracheal Volume with CT-based Lung Volume and with Lung Function
CT-based Lung Volume Lung Function Inspiration Expiration E/I Ratio FEV 1 /FVC FEV 1
(% predicted) RV/TLC Tracheal volume Inspiration 0.661 ∗ 0.472 ∗ −0.126 −0.221 ¶ −0.110 −0.182 Expiration 0.677 ∗ 0.749 ∗ 0.293 ‡ −0.445 ∗ −0.179 −0.020 E/I ratio 0.135 0.551 ∗ 0.711 ∗ −0.436 ∗ −0.333 † 0.253 ¶
CT, computed tomography; E/I, expiratory/inspiratory; FEV 1 , forced expiratory volume in 1 second; FVC, forced vital capacity; RV/TLC, ratio of residual volume to total lung capacity.
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Correlations between Tracheal Volume and Lung Function
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Discussion
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