Impairment of pulmonary mixing in simple and complicated silicosis

Two simple indices of pulmonary mixing (helium mixing time in a closed circuit, and mixing ventilation) were measured in 274 patients (mostly miners) with silicosis, aged 50 years or less (mean 42.4 years); the results were analysed in respect to silicosis category and type, presence of co-existent chronic bronchitis and ventilatory performance, and were compared to those of a group of normal subjects to assess statistical significance. The average results for the whole group were 5.81 min for mixing time (tmix) and 62.3 1 for mixing ventilation (Vmix), representing 1.8 times and 2.4 times the normal, respectively; the results were higher for patients with co-existent chronic bronchitis within each radiological category, except the last ("C" opacities), but the differences were significant only for patients with small opacities and with "suspect pneumoconiosis". Among patients with small rounded opacities the co-existence of healed tuberculosis (apical fibrous nodules) or of other X-ray changes did not influence the pulmonary mixing; no difference was found between the three types of small opacities. A slight tendency towards lower values in patients with early conglomeration ("A" opacities) allows the suggestion of conglomeration starting in "slow" lung spaces. The best results were those of patients with normal ventilatory function followed by those with a restrictive and then by those with an obstructive ventilatory impairment. The most severe mixing disturbance was found among patients with chronic bronchitis and obstructive defect with "suspect pneumoconiosis", who are not covered by occupational disablement compensation.