Pulmonary function and survival in idiopathic vs secondary usual interstitial pneumonia

The usual interstitial pneumonia (UIP) pattern of lung injury may occur in the setting of connective tissue disease (CTD), but it is most commonly found in the absence of a known cause, in the clinical context of idiopathic pulmonary fibrosis (IPF). Our objective was to observe and compare longitudinal changes in pulmonary function and survival between patients with biopsy-proven UIP found in the clinical context of either CTD or IPF.

We used longitudinal data analytic models to compare groups (IPF [n = 321] and CTD-UIP [n = 56]) on % predicted FVC (FVC %) or % predicted diffusing capacity of the lung for carbon monoxide (Dlco %), and we used both unadjusted and multivariable techniques to compare survival between these groups.

There were no significant differences between groups in longitudinal changes in FVC % or Dlco % up to diagnosis, or from diagnosis to 10 years beyond (over which time, the mean decrease in FVC % per year [95% CI] was 4.1 [3.4, 4.9] for IPF and 3.5 [1.8, 5.1] for CTD-UIP, P = .49 for difference; and the mean decrease in Dlco % per year was 4.7 [4.0, 5.3] for IPF and 4.3 [3.0, 5.6] for CTD-UIP, P = .60 for difference). Despite the lack of differences in pulmonary function, subjects with IPF had worse survival in unadjusted (log-rank P = .003) and certain multivariable analyses.

Despite no significant differences in changes in pulmonary function over time, patients with CTD-UIP (at least those with certain classifiable CTDs) live longer than patients with IPF--an observation that we suspect is due to an increased rate of mortal acute exacerbations in patients with IPF.