The impact of smoking on airflow limitation in subjects with history of asthma and inactive tuberculosis

Abstract

Background: Although smoking is the most important and modifiable cause of chronic obstructive pulmonary disease (COPD), other risk factors including asthma and tuberculosis (TB) are also associated. It is common for COPD patients to have more than one of these risk factors. The aims of this study were to determine the prevalence of airflow limitation (FEV1/FVC<0.7) according to the risk factors and to investigate their impact and interaction in airflow limitation.

Methods: From the Korean National Health and Nutrition Examination Survey between 2008 and 2012, we analyzed participants over 40 years of age by spirometry, chest radiograph and questionnaire about asthma and smoking history.

Results: Of 12,631 participants, 1,548 (12.3%) had airflow limitation. The prevalence of airflow limitation in smokers (≥10 pack-year), asthmatics, and those with inactive TB was 23.9%, 32.1%, and 33.6%. The prevalence increased with the number of risk factors: 86.1% had airflow limitation if they had all three risk factors. Impacts of inactive TB and asthma on airflow limitation were equivalent to 47 and 69 pack-years of smoking, respectively. Airflow limitation resulted from lower levels of smoking in those with inactive TB and asthma. A potential interaction between smoking and inactive tuberculosis in the development of airflow limitation was identified (p = 0.054).

Conclusions: Asthma and inactive TB lesions increase susceptibility to smoking in the development of airflow limitation. People with these risk factors should be seen as a major target population for anti-smoking campaigns to prevent COPD.

Fig 1. Flow diagram of study participants.

KNHANES = Korean National Health and Nutrition Examination Survey.

Fig 2. The prevalence of airflow limitation according to participant characteristics.

Smoking is defined as ≥ 10 pack-years. Airflow limitation is defined as FEV₁/FVC < 0.7. *Participants with smoking history less than 10 pack-years and without inactive TB and asthma.

Fig 3. Development of airflow limitation according to smoking history and underlying disease.

(A) Plots of FEV₁/FVC according to smoking intensity and its 50th percentile prediction line show lower levels of smoking are required for airflow limitation in participants with asthma or inactive TB than in those without. (B, C) FEV₁ was lower in participants with asthma or inactive TB than in those without. The gray region represents the 95% confidence interval of FEV₁. FVC = forced vital capacity, FEV₁ = forced expiratory volume in one second.

Fig 4. The impact of inactive TB on airflow limitation according to smoking history.

The gray region represents the 95% confidence interval of the odds ratios. PY = pack-years.