The mediating role of cytokine IL-6 on the relationship of FEV(1) upon 6-minute walk distance in chronic obstructive pulmonary disease

OBJECTIVES: To explore the mediating role of protein interleukin-6 (IL-6) on the relationship between forced expiratory volume in 1 second (FEV(1)) and 6-minute walk distance (6MWD) and, further, to determine whether status variables (such as age, sex, and body mass index [BMI]) operate as moderators of this mediation relationship. DESIGN: Moderated mediation model. SETTING: An inpatient pulmonary rehabilitation center in Italy. PARTICIPANTS: All 153 patients involved in the screening of a randomized controlled clinical trial (ClinicalTrials.gov identifier: NCT01253941) were included in this study. All patients were Global initiative for chronic Obstructive Lung Disease (GOLD) stages I–IV and were aged 70.1±9.1 years. MEASUREMENTS: At run-in phase of the protocol, clinical and functional screening included BMI, fasting plasma levels of protein (IL-6), spirometry, and standardized 6-minute walking test, measured at the start of the respiratory rehabilitation program. METHODS: The size of the indirect effect of the initial variable (FEV(1)) upon the outcome variable (6MWD) through the intervening variable (IL-6) was computed and tested for statistical significance. Moderated mediation analyses were subsequently conducted with age, sex, and BMI. RESULTS: FEV(1) averaged 53.4%±21.2%, and 6MWD 66.4%±41.3% of predicted. Median protein IL-6 was 6.68 pg/mL (interquartile range: 5.96). A bootstrapped mediation test supported the predicted indirect pathway (P=0.003). The indirect effect through IL-6 log units accounted for 17% of the total effect between FEV(1) and 6MWD. Age functioned as a significant moderator of the mediational pattern. For individuals aged <70 years, the standardized indirect effect was significant (0.122, 95% confidence interval [CI]: 0.044–0.254, P=0.004), and for individuals >70 years it was not significant (0.04, 95% CI: –0.010 to 0.142, P=0.10). CONCLUSION: This moderated mediation result based on concurrent data suggests, but does not prove, a causal role of systemic inflammatory syndrome on progression from functional impairment to “frailty” status and substantial disability in aging chronic obstructive pulmonary disease.