Lung Volumes and Emphysema in Smokers with Interstitial Lung Abnormalities
The relationship between exposure to tobacco smoke and chronic obstructive pulmonary disease (COPD) is well described. Two manifestations of COPD include emphysematous destruction of the lung parenchyma and elevated measures of total lung capacity. However, there is increasing awareness that smoking may also result inareas of increased lung density — termed interstitial lung abnormalities — on high-resolution computed tomography (HRCT). The extent to which interstitial lung abnormalities may be associated with a lesser amount of emphysema and lower measures of total lung capacity than anticipated on the basis of known smoking exposure is unclear.
We determined the relationship between radiographic interstitiallung abnormalities and HRCT measures of total lung capacity and emphysema in a cohort of non-Hispanic white and black smokers who had been recruited for the COPDGene Study on the basis of a self-reported history of more than 10 pack-years of smoking. Since we oversampled participants on the basis of COPD status, we evaluated whether the associations between interstitial lung abnormalities and bothtotal lung capacity and emphysema were modified by COPD status.
Emphysema and COPD
Interstitial lung abnormalities were associated with a lower percentage of emphysema (at −950 and −910 Hounsfield units) in adjusted models. Participants with interstitial lung abnormalities had a 47% decrease in their odds of having COPD, the strength of this association was influenced by GOLD stage (P for theanalysis of variance between GOLD stages 2 through 4 and interstitial lung abnormalities <0.001).
Our analysis of HRCT scans from this large cohort shows that interstitial lung abnormalities are present in approximately 8% of smokers. The findings also show that interstitial lung abnormalities are associated with both reduced total lung capacity and a lesser amount of emphysema insmokers, and the magnitude of these reductions is greatest among those with COPD. We found that smokers with interstitial lung abnormalities have reduced total lung capacity (the extent of which varies according to the subtype of interstitial lung abnormality) and are at an increased risk for a restrictive lung deficit. Although reductions in total lung capacity are expected in established clinicalinterstitial lung disease, our data provide a quantitative estimate of the degree to which interstitial lung abnormalities are associated with reductions in total lung capacity.
A major finding of our analyses is the inverse association between interstitial lung abnormalities and the severity of COPD or of emphysema (particularly among participants with COPD). We considered the possibility thatinterstitial lung abnormalities would result in an erroneous underestimation of the amount of emphysema by increasing the overall lung density defined by Hounsfield-unit thresholds. Several lines of evidence suggest that a density shift in the distribution of Hounsfield units is unlikely to explain our findings. First, the associations we found between emphysema and interstitial lung abnormalitieswere not most prominent in the lower lobes, where more interstitial abnormalities are expected (see the Supplementary Appendix). Second, the reductions in emphysema noted in participants with interstitial lung abnormalities were paired with the physiological consequences of reduced emphysema (e.g., additional reductions in total lung capacity). Third, we noted inverse associations between thepresence of interstitial lung abnormalities and clinically diagnosable COPD, a variable that is independent of the measurement of emphysema with the use of HRCT.
VOLÚMENES Y ENFISEMA PULMONAR EN FUMADORES CON ANORMALIDADES INTERSTICIALES DEL PULMÓN
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