Per cent low attenuation volume and fractal dimension of low attenuation clusters on CT predict different long-term outcomes in COPD.

Abstract

Fractal dimension (<i>D</i>) characterises the size distribution of low attenuation clusters on CT and assesses the spatial heterogeneity of emphysema that per cent low attenuation volume (%LAV) cannot detect. This study tested the hypothesis that %LAV and <i>D</i> have different roles in predicting decline in FEV₁, exacerbation and mortality in patients with COPD. Chest inspiratory CT scans in the baseline and longitudinal follow-up records for FEV₁, exacerbation and mortality prospectively collected over 10 years in the Hokkaido COPD Cohort Study were examined (n=96). The associations between CT measures and long-term outcomes were replicated in the Kyoto University cohort (n=130). In the Hokkaido COPD cohort, higher %LAV, but not <i>D</i>, was associated with a greater decline in FEV₁ and 10-year mortality, whereas lower <i>D</i>, but not %LAV, was associated with shorter time to first exacerbation. Multivariable analysis for the Kyoto University cohort confirmed that lower <i>D</i> at baseline was independently associated with shorter time to first exacerbation and that higher LAV% was independently associated with increased mortality after adjusting for age, height, weight, FEV₁ and smoking status. These well-established cohorts clarify the different prognostic roles of %LAV and <i>D</i>, whereby lower <i>D</i> is associated with a higher risk of exacerbation and higher %LAV is associated with a rapid decline in lung function and long-term mortality. Combination of %LAV and fractal <i>D</i> may identify COPD subgroups at high risk of a poor clinical outcome more sensitively.