Impact of chronic airflow obstruction in a working population

Study design

Subjects were recruited in 31 health prevention centres that are accessible to all workers. The sample was constructed according to national statistics on age and sex distribution in the general population of the considered age range (subjects aged ≥40 yrs). Subjects filled in a standardised auto-questionnaire, and a technician who was not aware of the answers to the questionnaire measured forced expiratory volume in one second (FEV₁) and forced vital capacity (FVC). All subjects received an information note before their participation.

Questionnaire and spirometry

The data obtained from each individual included the usual demographic and anthropometric description, information on risk factors, clinical symptoms, associated chronic diseases and presumptive diagnosis, and previous assessment of lung function.

The questionnaire was derived from the European Community Respiratory Health Survey questionnaire 15. Details of its content have already been published. Dyspnoea was measured using the modified UK Medical Research Council 0–4 scale, and quality of life was assessed using a generic five-dimension questionnaire, the European quality of life five-dimension (EuroQol-5D/EQ-5D) questionnaire 16. Declared missed working days during the previous year were recorded.

Pre-bronchodilator spirometry was performed by trained technicians using spirometers that were calibrated daily. Three measures were performed, with the best technically adequate measurement being chosen for analysis. Subjects were excluded from the analyses when only one spirometric measurement was adequate or when the variation between the two highest FEV₁ was >200 mL. Predicted values were calculated using European Respiratory Society equations 17, 18. Airflow obstruction was defined by an FEV₁/FVC ratio of <0.70.

Statistical analysis

The present study was powered to assess the prevalence of chronic airflow obstruction in the chosen population. Considering the expected prevalence of airflow obstruction of ∼5%, an allowed risk of error of 0.5% of this percentage and possible analysis in two strata, 4,500 individuals had to be recruited for the present survey. Results are expressed as percentages or mean±sd.

Based on questionnaire data, analyses were performed in three populations: the whole population, nonasthmatics, and subjects with no declared respiratory or cardiac disease.

In each of these populations, percentages were compared by two-way and multi-way frequency analysis, and means by ANOVA and unpaired t-tests. Relationships between continuous variables were assessed using linear regression. A difference was considered significant at a p-value of <0.05. Bonferroni corrections were performed for multiple comparisons.

Finally, multivariate logistic regression analyses were performed in order to identify independent predictors of quality of life and work loss. For these analyses, the dependent variables were: 1) the scores of each EQ-5D domain, and 2) the presence or absence of ≥1 missed working day, respectively. This last analysis was restricted to the active fraction of the population, i.e. subjects aged <65 yrs.

Characteristics of the studied population

A total of 5,008 subjects were included in the present survey during an 8-month recruitment period; this corresponded to a 93.5% response rate among subjects initially asked to participate. Adequate questionnaire and spirometry data were available for 4,764 (95.1%) subjects (table 1⇓). Their mean age was 60 yrs; 81% of subjects were aged <70 yrs and only 2% were aged >80 yrs. Known chronic respiratory disease was reported by 9.1% of subjects for asthma, 2.6% for COPD and 5.8% for chronic bronchitis. Questions regarding symptoms revealed that chronic bronchitis was genuinely present in 3.9% of subjects. The prevalence of chronic airflow obstruction in nonasthmatics was 7.5%. In the majority (59%) of these patients, the severity of airflow obstruction was mild (FEV₁ of ≥80% of the predicted value) and chronic bronchitis was absent. The severity of airflow obstruction was moderate (FEV₁ of 50–80% pred) in 36.1%, and severe or very severe (FEV₁ of <50% pred) in 4.8%. Among nonasthmatic subjects in whom airflow obstruction was found, only 6.1% had been previously diagnosed as having a chronic respiratory disease.

Impact of chronic airflow obstruction: dyspnoea and quality of life

In the whole population and in subjects with no known heart or lung disease, dyspnoea grade correlated significantly (p<0.0001) but weakly (r = 0.28) with the EQ-5D visual analogue scale score. There were also significant relationships between dyspnoea grade and EQ-5D subscores (mobility, activity, anxiety, autonomy and pain; all p<0.001; data from the whole population are shown in table 2⇓).

Altogether, 7.2% of subjects with grade 0 dyspnoea showed airflow obstruction versus 10.7% of those with dyspnoea of grade ≥1, 14.8% of those with dyspnoea of grade ≥2 and 21.8% of those with grade 4 dyspnoea.

In subjects without known chronic respiratory or heart disease, but who were diagnosed as having airflow obstruction during the examination, the severity of dyspnoea increased with that of airflow obstruction (fig. 1⇓).

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Fig. 1—

UK Medical Research Council dyspnoea grades (□: 0; ░: 1; ▒: 2; ▪: 3) by forced expiratory volume in one second (FEV₁) in subjects with no known heart or lung disease but in whom chronic airflow obstruction (FEV₁/forced vital capacity of <0.70) was demonstrated. No subjects reported grade 4 dyspnoea. % pred: % predicted.

In the whole population, patients who declared having asthma, COPD or chronic heart failure exhibited lower aggregated EQ-5D scores than controls, with all domains of the questionnaire being affected (data not shown). In patients with no chronic respiratory or cardiac disease, EQ-5D visual analogue scale scores decreased with FEV₁ (% pred; mean score 70.5±14.8 versus 72.8±14.8 in subjects with and without airflow obstruction; p = 0.004), whereas, in all EQ-5D domains, airflow obstruction was associated with lower proportions of patients declaring no limitation than among subjects with no airflow obstruction (87 versus 93% for mobility, 97 versus 99% for autonomy, 90 versus 93% for activity, 38 versus 46% for pain and 49 versus 55% for anxiety; all p<0.05). Multivariate analysis showed that age and FEV₁ were independent predictors of all EQ-5D domains except anxiety, which was mainly influenced by sex (table 3⇓).

Missed working days

The number of missed working days was significantly associated with: 1) health status, as measured by EQ-5D visual analogue scale and all EQ-5D domains except autonomy; and 2) dyspnoea grade (data not shown; p<0.01 for all analyses).

Most subjects reported no missed working days during the year preceding the examination (fig. 2⇓). However, in all groups (whole population, nonasthmatics and subjects with no declared respiratory or cardiac disease), a decreased FEV₁/FVC ratio was associated with an increase in the number of missed working days (6.71±39.09 versus 1.45±14.33 days in subjects with no respiratory or heart disease; p = 0.0015). In the whole population and in nonasthmatics, the number of missed working days was higher in subjects reporting symptoms of chronic bronchitis than in other subjects (5.56±15.45 versus 1.75±16.54 days; p = 0.05), and patients with an FEV₁ of 50–80% pred missed more working days than those with an FEV₁ of ≥100% pred (4.62±32.80 versus 1.54±13.90 days; p = 0.04). These differences were nonsignificant in nonasthmatics declaring no comorbid respiratory or cardiac disease. Other differences between FEV₁ categories were also nonsignificant. On multivariate analysis, chronic bronchitis was the only independent predictor of missed working days (odds ratio 6.0; 95% confidence interval 2.8–12.7; p<0.0001; table 3⇑), but interacted strongly with airflow obstruction (p<0.001 in test for interaction).

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Fig. 2—

Distribution of number of missed working days reported during the previous year in the population with no known lung or heart disease, by presence (▓) or absence (□) of a) chronic bronchitis and b) airflow obstruction.

Limitations and strengths of the study

To the best of the present authors’ knowledge, the present study is one of the largest spirometrically based population studies assessing the impact of chronic airflow obstruction using a validated quality-of-life questionnaire.

Airflow obstruction was defined according to pre-bronchodilator spirometry results, which is not in accordance with current guidelines. However, this was inevitable since French health prevention centres are not permitted to deliver any kind of medication. In addition, the use of pre-bronchodilator results is in line with several recent epidemiological studies, such as those of De Marco and co-workers 15, 19. In their most recent article, these authors conclude that the use of pre-bronchodilator values results in exposure to a risk of overestimation of the prevalence of COPD. They also show that this risk is minimised by exclusion of asthmatic subjects. For this reason, all analyses were performed in the whole population, in nonasthmatics alone and in subjects with no known heart or respiratory disease. Regarding FEV₁/FVC ratio, the use of the fixed 0.70 cut-off rather than the lower limit of normal for the diagnosis of airflow obstruction may result in overestimation of the prevalence of the disease in the elderly (and underestimation in younger adults, although the latter did not form part of the present study population). However, for practical reasons, the fixed cut-off is the recommended means of diagnosing chronic airflow obstruction in current guidelines. In addition, elderly subjects and those aged <40 yrs represented a small part of the present population, limiting the overall risk of error.

The mode of recruitment was certainly a source of bias since: 1) the main purpose of health examination centres is prevention; and 2) visits to these centres are offered mainly to the working population. Thus subjects of the studied population are less likely to exhibit symptoms or be known to have any disease than the general population (healthy worker effect). Accordingly, the proportion (6.1%) of patients with a known diagnosis of COPD was lower than in other studies 4, 5, 20. Thus the prevalence and impact of chronic airflow obstruction may be underestimated, which may help to explain the low number of subjects with an FEV₁ of <50% pred and the low proportion of subjects with airflow obstruction who reported symptoms of chronic bronchitis, severe dyspnoea or a previously known diagnosis of respiratory disease. In addition, the results presented herein cannot be extrapolated to the nonworking population. Despite these considerations, the overall prevalence data were very similar to those from other studies in developed countries regarding asthma, chronic bronchitis and airflow obstruction 4, 5, 21–23.

Another limitation is the declarative nature of the data on work loss. There is no means of assessing how this may have affected the present results. It can only be assumed that, since missed working days are important events in the life of workers, they are likely to be remembered accurately enough and not significantly overestimated.

Impact of airflow obstruction

Despite its mild severity, patients with chronic airflow obstruction exhibited poorer quality of life and increased dyspnoea and number of missed working days. This observation remained true even in patients with no known heart or lung illness.

Dyspnoea

In the present patients, impairment of quality of life was influenced by dyspnoea, although this symptom was much less severe than in patients with possible COPD identified in the Confronting COPD survey 6. For instance, among persistent smokers with a Global Initiative for Chronic Obstructive Lung Disease airflow obstruction stage of >1 (in whom dyspnoea was the most prevalent), only 8.3% reported a dyspnoea grade of ≥3, whereas the corresponding figure in the Confronting COPD survey was 42.6%. Thus even mild dyspnoea can be associated with impaired quality of life.

Quality of life

Among subjects with chronic airflow obstruction identified in the general population, a diagnosis of COPD is more likely when quality of life is more impaired 24. In addition, lung function is an independent predictor of all EQ-5D domains except anxiety. However, despite the under-representation of severe cases and marked underdiagnosis of COPD in the present study, quality of life was impaired by airflow obstruction. In the USA, the National Health and Nutrition Examination Survey (NHANES) also showed that undiagnosed airflow obstruction in the general population is associated with impaired health and functional status. In that study, health status was not assessed using a dedicated questionnaire but rather with questions about general health status, walking 402 m, lifting or carrying something as heavy as 4.54 kg and needing help with personal care 25. In the Confronting COPD survey 6, patients with airflow obstruction also reported activity restrictions related to their respiratory condition. Depending upon the item under consideration, 30–70% of subjects with or at risk of COPD reported limitations in sport and recreation, physical exertion, social life, sleep, household chores and sex life. In line with these findings, data from the Spanish EPIDEPOC study found significant impairment of quality of life measured with the 12-item short-form health survey questionnaire in stable COPD subjects followed in primary care, even in mild disease stages 26.

Impact on work

COPD is not only a disease of the elderly; its prevalence in subjects aged ≤65 yrs is not negligible and has a noticeable impact on resource utilisation and healthcare costs 27. In these subjects, ability to work is an important determinant of life satisfaction 28. In the Confronting COPD survey 6, 35.7% of patients reported that “their condition prevented them from working, limited their ability to work or had caused them working time loss in the past year”. This percentage increased to 45.3 in subjects aged <65 yrs, who were less likely to form part of the 54.8% of the population who were retired 7. The proportion of retired subjects was much lower (1.3%) in the present population; this is related to the mode of recruitment since health examination centres target only working subjects. The much lower proportion of subjects reporting work loss in the present population is probably related to several factors: slightly younger age (60 versus 63 yrs in the Confronting COPD survey 6), different mode of recruitment (systematic prevention visits by subjects who presume they are healthy versus general population) and variable studied; the present study used only the number of missed working days to assess disease consequences on work, whereas Rennard et al. 7 also included reported “limitations in the ability to work” in their assessment, introducing a possibly more subjective component, the contribution of which it is difficult to assess. Finally, there was no means of determining whether or not airflow obstruction was less severe in the present population than in the Confronting COPD survey 6, since lung function was not assessed in that study.

In NHANES III, even patients with spirometrically mild COPD were less likely to be in the labour force than controls. More precisely, mild, moderate and severe COPD were associated with a 3.4, 3.9 and 14.4% reduction in the labour force participation rate relative to those without COPD 29.

Finally, multivariate analysis did not identify lung function as an independent predictor of missed working days, whereas chronic bronchitis was so identified. Thus, when the COPD fraction of a population has mostly mild or mild-to-moderate disease, work loss seems to relate more to chronic bronchitis than to airflow obstruction. However, this result should be interpreted with caution since the power of the analysis was relatively low, due to the small number of subjects reporting any missed working day (n = 69) among the population restricted to those aged <65 yrs with no cardiac or respiratory comorbid illness (n = 1,310). This also prevented reliable multivariate analysis with the number of missed working days divided into subcategories.

In conclusion, the present study on a large sample from health prevention centres found a significant impact of mild-to-moderate airflow obstruction on dyspnoea, quality of life and missed working days despite a very low proportion of subjects spontaneously reporting symptoms or a known respiratory disease. Such findings suggest that systematic screening for chronic obstructive pulmonary disease may be justified in the general population since identified cases suffer from a significant although underestimated impact of the disease, which might be limited by early implementation of preventive and therapeutic measures.