Chronic Airflow Limitation, Emphysema, and Impaired Diffusing Capacity in Relation to Smoking Habits in a Swedish Middle-aged Population

Abstract

Rationale: Chronic obstructive pulmonary disease (COPD) includes respiratory symptoms and chronic airflow limitation (CAL). In some cases, emphysema and impaired diffusing capacity of the lung for carbon monoxide (Dl_CO) are present, but characteristics and symptoms vary with smoking exposure. Objective: To study the prevalence of CAL, emphysema, and impaired Dl_CO in relation to smoking and respiratory symptoms in a middle-aged population. Methods: We investigated 28,746 randomly invited individuals (52% women) aged 50-64 years across six Swedish sites. We performed spirometry, Dl_CO testing, and high-resolution computed tomography and asked for smoking habits and respiratory symptoms. CAL was defined as post-bronchodilator forced expiratory volume in 1 second divided by forced vital capacity (FEV₁/FVC) < 0.7. Results: The overall prevalence was 8.8% for CAL, 5.7% for impaired Dl_CO (Dl_CO < LLN), and 8.8% for emphysema, with a higher prevalence in current smokers than in ex-smokers and never-smokers. The proportion of never-smokers among those with CAL, emphysema, and impaired Dl_CO was 32%, 19%, and 31%, respectively. Regardless of smoking habits, the prevalence of respiratory symptoms was higher among people with CAL and impaired Dl_CO than those with normal lung function. Asthma prevalence in never-smokers with CAL was 14%. In this group, asthma was associated with lower FEV₁ and more respiratory symptoms. Conclusions: In this large population-based study of middle-aged people, CAL and impaired Dl_CO were associated with common respiratory symptoms. Self-reported asthma was not associated with CAL in never-smokers. Our findings suggest that CAL in never-smokers signifies a separate clinical phenotype that may be monitored and, possibly, treated differently from smoking-related COPD.

Keywords: chronic obstructive pulmonary disease; emphysema; impaired DlCO; respiratory symptoms; smoking.

Figure 1.

Flowchart of inclusions and exclusions of study participants. Dl_CO = diffusing capacity of the lung for carbon monoxide; SCAPIS = Swedish Cardiopulmonary Bioimage Study.

Figure 2.

Prevalence of CAL with 95% confidence intervals by smoking status, according to GOLD, GLI < LLN, and SCAPIS < LLN. CAL = chronic airflow limitation; GLI = Global Lung Function Initiative; GOLD = Global Initiative for Obstructive Lung Disease; LLN = lower limit of normal; SCAPIS = Swedish Cardiopulmonary Bioimage Study.

Figure 3.

Venn diagram illustrating the cooccurrence of chronic airflow limitation (CAL), emphysema, and impaired diffusing capacity of the lung for carbon monoxide (Dl_CO) in (A) current smokers (n = 1,439), (B) ex-smokers (n = 2,000), and (C) never-smokers (n = 1,706), respectively. Note that the circles are proportional to the prevalence of CAL, emphysema, and impaired Dl_CO within, but not between, smoking status groups.

Figure 4.

Prevalence ratios for respiratory symptoms (breathlessness [upper panel], chronic bronchitis [middle panel], and wheeze [lower panel]) by CAL and smoking status (left panel), by emphysema and smoking status (middle panel), and by impaired Dl_CO and smoking status (right panel). Red color intensity is proportional to magnitude of prevalence ratio. The ratios are adjusted for sex, age, and site. CAL = chronic airflow limitation; Dl_CO = diffusing capacity of the lung for carbon monoxide.