Single breath nitrogen test as predictor of lung function decline and COPD over an 8-year follow-up

Background

The single breath nitrogen (SBN₂) test was proposed for early detection of “small airways disease” in the seventies. Few longitudinal studies have subsequently evaluated the relationships between SBN₂ test measurements and lung function decline or COPD incidence.

Aim

This study evaluates whether SBN₂ test abnormalities may be significant predictors of lung function decline and COPD incidence over an 8-year follow-up.

Study Design and Methods

In this longitudinal study, 907 adults (20+ years old; 56% males) from the prospective Po River Delta epidemiological study underwent SBN₂ test at baseline and spirometry testing at both baseline and follow-up 8-year apart. Multinomial and multiple regression models were used to assess associations of SBN₂ indexes and rates of FEV₁ decline or risk of COPD incidence over time, after adjusting for sex, height and baseline age, FEV₁ and smoking status. COPD was defined according to either GOLD or ATS-ERS criteria.

Results

Among SBN₂ indexes, only the slope of alveolar plateau (N₂-slope) was significantly associated with rates of FEV₁ decline (7.93 mL/year for a one-unit change in N₂-slope, p<0.0001), and with an increased risk of developing COPD as defined by GOLD (RR 1.81, 95%CI 1.29-2.52, mild; RR 2.78, 95%CI 1.70-4.53, moderate or severe obstruction) and ATS-ERS criteria (RR 1.62, 95%CI 1.14-2.29, mild; RR 3.40, 95%CI 1.72-6.73, moderate or severe obstruction).

Conclusion

In this population-based study, N₂-slope from SBN₂ test is a significant predictor of lung function decline and COPD incidence over an 8-year follow-up, confirming the role of the “small airways disease” in the natural history of COPD.