The single breath nitrogen test and mortality--A 38 years follow up

Spirometry data predict mortality, but are less sensitive to detect dysfunction in small airways as compared to the slope of phase III (the N2 slope) of the single breath nitrogen test. The association between the N2 slope and mortality has been studied with conflicting results. In the present study the prognostic importance of the N2 slope was tested taking spirometry variables into account.

A systematic general population sample of 595 middle-aged men had a baseline investigation with lung function tests including spirometry and the N2 slope. Age, smoking, and anthropometry variables were registered. The cohort was followed up regarding survival for 38 years.

The sample was subdivided by tertiles of the N2 slope. A proportional hazards regression analysis was performed for each group of covariates: anthropometric, smoking variables, and spirometry variables, after accounting for age. Covariates with significant impact on mortality and the highest chi-square levels were smoking habit score and forced expired volume in 1 s corrected for height. These variables, in addition to age and the N2 tertiles were entered into a final proportional hazards regression analysis. In this multivariate model, mortality was significantly related to age (p < .0001), smoking habit score (p < .0001) and the N2 tertiles (p = .0004), but not to FEV1 when N2 slope was allowed for in the model.

Dysfunction in small airways as measured by the N2 slope is significantly associated with overall mortality in middle-aged men, and outrivals spirometry as a predictor in multivariate analysis.