Disproportionate contribution of right middle lobe to emphysema and gas trapping on computed tomography

Abstract

Rationale: Given that the diagnosis of chronic obstructive pulmonary disease (COPD) relies on demonstrating airflow limitation by spirometry, which is known to be poorly sensitive to early disease, and to regional differences in emphysema, we sought to evaluate individual lobar contributions to global spirometric measures.

Methods: Subjects with COPD were compared with smokers without airflow obstruction, and non-smokers. Emphysema (% low attenuation area, LAAinsp<-950 HU, at end-inspiration) and gas trapping (%LAAexp<-856 HU at end-expiration) on CT were quantified using density mask analyses for the whole lung and for individual lobes, and distribution across lobes and strength of correlation with spirometry were compared.

Results: The right middle lobe had the highest %LAAinsp<-950 HU in smokers and controls, and the highest %LAAexp<-856 HU in all three groups. While RML contributed to emphysema and gas trapping disproportionately to its relatively small size, it also showed the least correlation with spirometry. There was no change in correlation of whole lung CT metrics with spirometry when the middle lobe was excluded from analyses. Similarly, RML had the highest %LAAexp<-856 HU while having the least correlation with spirometry.

Conclusions: Because of the right middle lobe's disproportionate contribution to CT-based emphysema measurements, and low contribution to spirometry, longitudinal studies of emphysema progression may benefit from independent analysis of the middle lobe in whole lung quantitative CT assessments. Our findings may also have implications for heterogeneity assessments and target lobe selection for lung volume reduction.

Clinical trial registration: ClinicalTrials.gov NCT00608764.

Conflict of interest statement

Competing Interests: SPB and JCS have no conflicts of interest to declare. EAH is a founder and shareholder of VIDA Diagnostics Inc, a company that is commercializing lung image analysis software developed in part at the University of Iowa. JDN is a paid consultant with VIDA Diagnostics Inc and holds stock options in VIDA Diagnostics Inc, and has research grant funds from Siemens Health. APC is a paid consultant with VIDA Diagnostics Inc. This does not alter the authors’ adherence to PLOS ONE policies on sharing data and materials.

Figures

Figure 1. Correlation (Pearson’s correlation r) between the FEV1/FVC ratio with emphysema-like changes (%LAA<−950 HU at end inspiration) and gas trapping (%LAA<−856 HU at end expiration), for the whole lung and separately for the right middle lobe.

FEV1 = Forced Expiratory Volume in the first second. FVC = Forced Vital Capacity. LAA = Low Attenuation Areas. HU = Hounsfield Units. p<0.05 for all correlations. Right middle lobe showed significantly less correlation for FEV1/FVC (p<0.01).

Figure 2. Example of a CT scan obtained from a GOLD 0 smoker with FEV1 %predicted of 106 and an FEV1/FVC ratio of 0.78.

As seen in the left panel, the lungs were segmented, via VIDA’s Apollo Software, automatically with automated delineation of the individual lung lobes (Right Upper Lobe: Pink; Right Middle Lobe: Purple; Right Lower Lobe: Yellow; Left Upper Lobe Green, Left Lower Lobe: Blue). In the right panel, the distribution of the clustered regions of emphysema-like lung are shown as spheres demonstrating the cluster size expressed as sphere diameter and the centroid of the clustered region. Emphysema-like lung was defined at voxels below −950 HU. Spheres are color coded similarly to the coding in the left panel to distinguish lobar locations. The lobar distribution of percent emphysema-like lung is tabulated in the upper right portion of the right panel. Note that the right middle lobe shows significantly greater presence of emphysema-like lung as compared with the other lobes. However, the PFTs are more reflective of the percent emphysema in all lobes other than the right middle lobe.