Smoking duration alone provides stronger risk estimates of chronic obstructive pulmonary disease than pack-years

Abstract

Background: Cigarette smoking is the strongest risk factor for COPD. Smoking burden is frequently measured in pack-years, but the relative contribution of cigarettes smoked per day versus duration towards the development of structural lung disease, airflow obstruction and functional outcomes is not known.

Methods: We analysed cross-sectional data from a large multicentre cohort (COPDGene) of current and former smokers. Primary outcome was airflow obstruction (FEV1/FVC); secondary outcomes included five additional measures of disease: FEV1, CT emphysema, CT gas trapping, functional capacity (6 min walk distance, 6MWD) and respiratory morbidity (St George's Respiratory Questionnaire, SGRQ). Generalised linear models were estimated to compare the relative contribution of each smoking variable with the outcomes, after adjustment for age, race, sex, body mass index, CT scanner, centre, age of smoking onset and current smoking status. We also estimated adjusted means of each outcome by categories of pack-years and combined groups of categorised smoking duration and cigarettes/day, and estimated linear trends of adjusted means for each outcome by categorised cigarettes/day, smoking duration and pack-years.

Results: 10 187 subjects were included. For FEV1/FVC, standardised beta coefficient for smoking duration was greater than for cigarettes/day and pack-years (P<0.001). After categorisation, there was a linear increase in adjusted means FEV1/FVC with increase in pack-years (regression coefficient β=-0.023±SE0.003; P=0.003) and duration over all ranges of smoking cigarettes/day (β=-0.041±0.004; P<0.001) but a relatively flat slope for cigarettes/day across all ranges of smoking duration (β=-0.009±0.0.009; P=0.34). Strength of association of duration was similarly greater than pack-years for emphysema, gas trapping, FEV1, 6MWD and SGRQ.

Conclusion: Smoking duration alone provides stronger risk estimates of COPD than the composite index of pack-years.

Trial registration number: Post-results; NCT00608764.

Keywords: COPD epidemiology; emphysema; tobacco and the lung.

Conflict of interest statement

Competing interests: None declared.

© Article author(s) (or their employer(s) unless otherwise stated in the text of the article) 2018. All rights reserved. No commercial use is permitted unless otherwise expressly granted.

Figures

Figure 1

Panel A shows linear slopes for adjusted means of FEV1/FVC over categorized Cigarettes per day. Panel B shows linear slopes for adjusted means of FEV1/FVC over categorized Duration. Color-coded data points represent estimated adjusted means of FEV1/FVC by categorized Duration (A) or Cigarettes/day (B). All categorization is based on 10 unit increments. Panel C shows linear slopes for adjusted means of CT Emphysema over categorized Cigarettes per day. Panel D shows linear slopes for adjusted means of CT Emphysema over categorized Smoking Duration. Color-coded data points represent estimated adjusted means of CT emphysema by categorized Duration (C) or Cigarettes/day (D). All categorization is based on 10 unit increments. Least square means of FEV1/FVC and CT emphysema are adjusted for age, race, sex, body-mass-index, scanner type, center, age of smoking onset and current smoking status. FEV1/FVC = Ratio of forced expiratory volume in the first second to the forced vital capacity. CT = Computed Tomography.

Figure 2

shows summary linear slopes for lung function (A. FEV1/FVC and B. FEV1), structural lung disease (C. CT emphysema and D. CT gas trapping), and quality of life (E. SGRQ) and exercise capacity (F. 6MWD) over categorized Cigarettes per day, categorized Smoking Duration and over categorized Pack-years. All categorization is based on 10 unit increments. All outcome least square means adjusted for age, race, sex, body-mass-index, scanner type, center, age of smoking onset and current smoking status. FEV1 = forced expiratory volume in the first second. FVC = forced vital capacity. CT = computed tomography. SGRQ = St George’s Respiratory Questionnaire. 6MWD = six minute walk distance.

Figure 3

Three dimensional surface plots demonstrate the relationships between linear slopes for the adjusted means of CT emphysema over categorized Cigarettes/day and over categorized Smoking Duration (years). The adjusted means CT emphysema slopes for Cigarettes/day are relatively flat across all duration categories whereas adjusted means CT emphysema increases linearly with increasing Smoking Duration across all categories of cigarettes/day. All categorization is based on 10 unit increments. Color panel on the right shows adjusted means of CT emphysema adjusted for age, race, sex, body-mass-index, CT scanner type, center, age of smoking onset and current smoking status. CT = Computed Tomography. Please note that surface plots were drawn for 23 of the 25 combinations of smoking cigarettes/day and smoking duration, and the two combinations with insufficient participants were treated as zero to smoothen the surface plots.