Abstract

Background
Continuing workplace exposure to causative agent is associated with a poor prognosis in occupational asthma. Two separate phenotypes have been previously described based on the presence of sputum eosinophilia (>2.2%). Eosinophilic subjects have significantly increased fractional exhaled nitric oxide levels (FENO) and show higher non-specific bronchial reactivity and bronchodilator reversibility, lower pre-bronchodilator FEV1, FEV1/FVC and worse St George Respiratory Questionnaire (SGRQ) scores.

Aims
To determine whether the rate of FEV1 decline (aFEV1) is different in a group of workers with occupational asthma who have normal FENO compared to those with raised FENO measurements . (Logan® 2000; +/- 9.6 ppb at flow of 200 L/min).

We postulated that the rate of FEV1 decline during continued exposure would be less in those with normal FENO measurements at presentation compared to those with raised FENO.

Methods
51 consecutive workers with occupational asthma who had FENO measurements and sputum induction studies whilst exposed to the causative agent were followed up with regular FEV1 measurements until complete removal from exposure (Minimum follow up period of = 1 year; mean 5.4 yrs, SD 2.92). All workers were advised to avoid continuing exposure at diagnosis. The aFEV1 in the normal and raised FENO groups was calculated by linear regression analysis using all measurements made over the follow-up period.

Results
In the normal FENO group (n=38) which was predominant, only 5/32 (15.6%) had DFEV1 > -60 mL/year compared to the high FENO group (5/7, 71.4% had D FEV1 > -60 mL/year).

Mean DFEV1 in the normal FENO group was -6.86 mL/year (95% CI, -0.041 to 0.028); only 7 out of 13 workers with raised FENO at presentation remained exposed for >1yr before complete removal from exposure, making DFEV1 assessment and statistical analysis unreliable in this group.

Conclusions
Our previous work has showed DFEV1 of 100.9 mL/yr (SEM 17.7) in 90 workers with OA (not phenotyped by FENO) caused by low molecular weight agents with continuing exposure who were then followed up over a mean of 2.9 yrs.

In this study, workers with normal FENO at presentation (with ongoing exposure) shows a FEV1 decline of 6.9 mL/year over a longer period, suggesting a better prognosis for this group despite continuing exposure to the causative agent. However, higher dropout rates in workers with rapid FEV1 decline in general needs to be considered.

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