Asbestos-induced pleural fibrosis and impaired exercise physiology

Our plain-English summary of Asbestos-induced pleural fibrosis and impaired exercise physiology, by J.F. Shih, J.S. Wilson, A. Broderick, et al (Chest, 1994).

The study

• Authors: J.F. Shih, J.S. Wilson, A. Broderick, J.L. Watt, J.R. Galvin, J.A. Merchant, D.A. Schwartz
• Journal: Chest
• Published: 1994;105(5):1370-6
• Cited by: 17 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

To further assess the clinical significance of asbestos-induced pleural fibrosis, we performed cardiopulmonary exercise testing in 90 subjects who were exposed to asbestos. Of the 82 subjects without an abnormal resperate exercise, 35 had normal pleura, 33 had circumscribed pleural plaques, and 14 had diffuse pleural thickening. Interstitial fibrosis (International Labor Organization [ILO]. > or = 1/10) was present in 14 of 35 subjects with normal pleura, 13 of 33 subjects with circumscribed pleural plaques, and 2 of 14 subjects with diffuse pleural thickening. Although pleural fibrosis did not appear to be related to impaired respiratory function with exercise in our entire cohort, this finding was confounded by a higher proportion of interstitial fibrosis in subjects with normal pleura. In fact, among study subjects without asbestosis, significant decreases in gas exchange (higher VD/VT and increased alveolar-arterial oxygen pressure difference) were observed at maximal exercise among subjects with pleural fibrosis. Interestingly, neither a higher respiratory rate nor a lower VT/FVC ratio was observed among those with pleural fibrosis, suggesting that the mechanical effects of pleural fibrosis on the chest wall do not explain the increased VD/VT. Using multivariate analyses to control for potential confounders, regression models showed that pleural plaques (p = 0.04) and diffuse pleural thickening (p = 0.03) were independently associated with significant increases in dead space ventilation (VD/VT) with maximal exercise. These findings indicate that asbestos-induced pleural fibrosis is independently associated with decrements in gas exchange with maximal exercise and suggest that interstitial lung disease, not detected on the routine chest x-ray film, may be responsible for this abnormal response to exercise.

In plain English

This University of Iowa study tested whether asbestos-related scarring of the lining around the lungs (pleural fibrosis) affects breathing during hard exercise. Ninety participants occupationally exposed to asbestos, with at least 1 year of high exposure and at least 20 years since first exposure, did a cycle exercise test. Eight with an abnormal heart response were excluded, leaving 82 men: 35 with normal pleura, 33 with circumscribed pleural plaques and 14 with diffuse pleural thickening. Across all 82, neither type of pleural fibrosis appeared related to impaired breathing or gas exchange. However, asbestosis on X-ray was more common in the normal-pleura group. Among men without asbestosis, those with diffuse pleural thickening had poorer gas exchange at maximal exercise than those with normal pleura. After adjustment for age, height, weight, smoking and X-ray scarring, the dead space ratio (a measure of wasted breathing) at maximal exercise averaged 4.8 percentage points higher with plaques and 8.1 higher with diffuse thickening than with normal pleura. The authors concluded that pleural fibrosis is independently associated with poorer gas exchange at maximal exercise. They suggested that lung scarring not seen on routine X-ray may be responsible.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Clinical study comparing cardiopulmonary exercise test results among asbestos-exposed workers grouped by type of pleural fibrosis on chest X-ray, with multivariate linear regression
• Who was studied: 90 occupationally asbestos-exposed subjects, identified through the Sheet Metal Worker's 1986 screening program or the University of Iowa Occupational Medicine Clinic. After 8 were excluded for abnormal cardiac responses to exercise, 82 men with a mean age of 62 years were analysed: 35 with normal pleura, 33 with circumscribed pleural plaques and 14 with diffuse pleural thickening.
• Exposure: Occupational asbestos exposure for at least 1 year in a high-exposure setting, with at least 20 years between first exposure and entry into the study. Fiber type is not reported.
• Main results:
  • Asbestosis (ILO profusion 1/0 or greater) was present in 14 of 35 (40.0%) subjects with normal pleura, 13 of 33 (39.4%) with circumscribed pleural plaques, and 2 of 14 (14.3%) with diffuse pleural thickening.
  • In all 82 subjects, neither circumscribed plaques nor diffuse pleural thickening appeared to be related to impaired respiratory function or gas exchange with exercise. The authors attributed this to confounding by the higher share of asbestosis among those with normal pleura.
  • Among subjects without asbestosis, those with diffuse pleural thickening (n=12) had a higher dead space ratio at maximal exercise than those with normal pleura (n=21): 25.6 plus or minus 8.9% versus 17.6 plus or minus 10.8%. They also had a higher alveolar-arterial oxygen difference: 21.3 plus or minus 10.5 versus 12.7 plus or minus 8.7 mm Hg. Both differences had p less than 0.05. Subjects with plaques fell in between.
  • Neither a higher respiratory rate nor a lower VT/FVC ratio accompanied the poorer gas exchange in those with pleural fibrosis. The authors said this suggests that decreased chest wall compliance alone does not explain the increased dead space ratio.
  • Among subjects with asbestosis, similar differences in gas exchange by pleural fibrosis were not observed.
  • Regression models were adjusted for age, height, weight, smoking history, pack-years and ILO profusion category. Compared with normal pleura, the dead space ratio at maximal exercise was on average 4.8 percentage points higher with circumscribed plaques (coefficient 0.048, p=0.04). It was 8.1 percentage points higher with diffuse pleural thickening (coefficient 0.081, p=0.03). The article's text describes these as percent increases.
• The authors' conclusion: The authors concluded that asbestos-induced pleural fibrosis is independently associated with decrements in gas exchange with maximal exercise. They suggested that interstitial lung disease not detected on the routine chest X-ray may be responsible for this abnormal response.
• Limitations the authors state:
  • The authors noted that asbestosis was less common in the diffuse pleural thickening group than usually observed. They explained this by the subjects having been selected on the basis of specific radiographic abnormalities the authors were interested in studying.

Topics

Medical terms in this study, explained in our Lexicon: Fibrosis, Pleura, Pleural Plaques, Diffuse Pleural Thickening, Interstitial Fibrosis, Interstitial Lung Disease, Chest X-Ray.

Related studies

• Exercise impairment in asbestos-induced pleural fibrosis. Is it really just interstitial fibrosis? — E.T. Mannix, W.J. Martin (1994)
• Asbestos-induced pleural fibrosis and impaired lung function — D.A. Schwartz, L.J. Fuortes, et al. (1990)
• Determinants of restrictive lung function in asbestos-induced pleural fibrosis — D.A. Schwartz, J.R. Galvin, et al. (1990)
• Abnormal wasted ventilation fraction and normal pulmonary hemodynamics during exercise in patients with exertional dyspnea — Z. Mohsenifar, H.V. Brown, et al. (1982)
• Exercise alveolar-arterial oxygen pressure difference in interstitial lung disease — C. Risk, G.R. Epler, et al. (1984)
• Lung function impairment as a guide to exercise limitation in work-related lung disorders — J.E. Cotes, J. Zejda, et al. (1988)

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