Resting diffusing capacity and severity of radiographic disease predict gas exchange abnormalities with exercise in former US coal miners

Background

The US Department of Labor (DOL) does not fund diffusing capacity (D_LCO) or metabolic measurements from cardiopulmonary exercise testing (CPET) for coal miners' disability evaluations. Although exercise arterial blood gas testing is covered, many miners are unable to perform maximal tests, and sampling at peak exercise can be challenging. We explored the relationship between resting D_LCO, radiographic disease severity, and CPET abnormalities in former US coal miners.

Methods

We analyzed data from miners evaluated between 2005 and 2015. Multivariable linear and logistic regression analyses were used to examine relationships between percent predicted (pp) forced expiratory volume in 1 s (FEV₁pp), D_LCOpp, VO₂maxpp, A-a oxygen gradient (A-a)pp, dead space fraction (Vd/Vt), disabling oxygen tension (PO₂), and radiographic findings of pneumoconiosis.

Results

Data from 2015 male coal miners was analyzed. Mean tenure was 28 years (SD 8.6). Thirty-twopercent had an abnormal A-a gradient (>150 pp), 20% had elevated Vd/Vt (>0.33), and 34% a VO₂max < 60 pp. D_LCOpp strongly predicted a disabling PO₂, with an odds ratio (OR) of 2.33 [2.09−2.60], compared to 1.18 [1.08−1.29] for FEV₁. Each increase in subcategory of small opacity (simple) pneumoconiosis increased the odds of a disabling PO₂ by 42% [1.29−1.57], controlling for age, body mass index, pack-years of tobacco smoke exposure, and years of coal mine employment.

Conclusions

D_LCO is the best resting pulmonary function test predictor of CPET abnormalities. Radiographic severity of pneumoconiosis was also associated with CPET abnormalities. These findings support funding D_LCO testing for impairment and suggest the term “small opacity” should replace “simple” pneumoconiosis to reflect significant associations with impairment.