Quantitative Chest CT-defined Structure and Diffusion Capacity in Post-deployment US Veterans

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Abstract RATIONALE: Isolated diffusing abnormalities have been noted in Veterans deployed to Southwest Asia and Afghanistan who have environmental exposures which may reflect early structural pulmonary parenchymal and airway abnormalities that can be detected by CT imaging. METHODS: Veterans with land-based deployments, independent of medical treatment, were invited to participate in VA Cooperative Study #595, “Service and Health Among Deployed Veterans.” A subset of participants at 4 VA Medical Centers (Boston, Houston, Minneapolis, and Seattle) completed an additional assessment (a mean 14.7 years post-deployment) that included an inspiratory and expiratory high-resolution CT scan and measurement of diffusing capacity for carbon monoxide (DLCO). Scans were analyzed for %-emphysema (<-950 Hounsfield units (HU), inspiratory scan); %-gas-trapping (<-856 HU, expiratory scan); %-functional small airways disease (fSAD) defined as regions with gas trapping, but not due to emphysema; and Pi10, the square root of the airway wall area of a hypothetical 10-mm diameter airway, extrapolated by measuring distal to lobar airways. Multivariable linear regression models were used to assess associations between CT measures and diffusing capacity after adjustment for body mass index and study site. RESULTS: There were 117 participants (N=17, 14.5% women) with a mean (SD) age =47.0 (10.5) years and mean BMI=31.1 (6.3) who were deployed for a mean 13.0 (7.2) months, and included 15 (12.8%) former smokers with <10 pack-years (median 1.0)). Mean DLCO was 29.1 (6.5) mL/min/mmHg, DLCO % predicted 99.3% (14.4), with 6 (5.1%) participants with a DLCO below the lower limit of normal. Overall, the percentage of emphysema, gas trapping, and fSAD were below established pathological reference values: %-emphysema: median (25%ile-75%ile) =2.3 (0.8-4.4), 95%ile 9.3; %-gas trapping=3.0 (1.1 – 9.4), 95%ile 26.0; %-fSAD 2.5 (1.1-6.7) 95%ile, 22.9; Pi10=2.8 (2.7-2.9) mm. CT parameters were not associated with DLCO % predicted (Table). Sensitivity analyses that included models adjusting for smokers, pack-years and history of COVID infection were performed, and there was no association between CT measures and DLCO was seen. CONCLUSION: Among previously deployed Veterans who were randomly selected independent of symptoms or medical treatment the DLCO % predicted was generally preserved. There were no associations between the early CT changes of emphysema, gas trapping or airway thickness with diffusion capacity. This suggests that diffusing capacity may not be an early marker of mild lung abnormalities in deployed veterans.

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