CHARACTERIZATION OF PULMONARY FUNCTION DECLINE FOR SUSCEPTIBLE AND NON-SUSCEPTIBLE SMOKERS

Purpose: The existence of long-time smokers with both very-good and very-poor lung function suggests that some people are more susceptible to deleterious effects from smoking than others. We sought to characterize heterogeneity in smoking susceptibility by testing candidate models of pulmonary function decline as explanations for spirometric measurements in the Lung Health Study (LHS) and the National Health and Nutrition Examination Surveys (NHANES). 

Method: We used a maximum likelihood framework to evaluate linear models of pulmonary function decline for smokers in the broad U.S. population. Candidate models described distributions in the rate of decline of FEV₁ and FVC, two key measures of lung function. Models were evaluated according to their likelihood with respect to NHANES III and LHS, two landmark studies that include high-quality pulmonary function measurements, whose candidate participants were drawn from the general U.S. population between 1986 and 1994. Censoring, explicitly described in the LHS selection criteria and implied by the safety criteria for NHANES spirometry measurement, were simulated for each candidate model and accounted for in model likelihood functions. 

Result: Serial measurements in LHS show that pulmonary function measurements for individuals are highly variable (SD­ of 200ml and 250ml for FEV­₁ and FVC, respectively). We find that variability in the rate of decline in LHS is of a similar magnitude to the rate of decline itself (Mean 47ml/year and 50ml/year, SD 47ml/year and 56ml/year for FEV₁ and FVC, respectively). In contrast, variability in the rate of decline for NHANES smokers is significantly lower than the mean observed rate of decline (Mean 36ml/year and 31ml/year, SD 11ml/year and 9ml/year for FEV­₁ and FVC, respectively). Because individual-level measurements of pulmonary function are highly variable relative to the rate of decline of pulmonary function, several model classes fit the combined LHS and NHANES data well.

Conclusion: We provide quantitative support for the hypothesis of susceptible and non-susceptible smokers.  Many smokers whose spirometry qualifies for a COPD diagnosis are declining at a normal rate. Smokers with increasingly low pulmonary function are increasingly likely to have experienced rapid lung function declines. Focusing smoking cessation interventions on susceptible smokers would increase these costly interventions’ impact.