Which spirometric indices best predict subsequent death from chronic obstructive pulmonary disease?

Abstract

BACKGROUND Previous epidemiological studies have related mortality from chronic obstructive pulmonary disease (COPD) to forced expiratory volumes (FEV₁or FEV_0.75) and it is unknown whether other spirometric indices might have greater predictive power. METHODS A case-control study of fatal COPD was conducted within a cohort of London civil servants who performed forced expiratory spirograms in 1967–9 and were followed up for mortality over 20 years. The spirograms of 143 men who died of COPD (ICD8 491, 492 or 519.8) were compared with those of 143 controls individually matched for age, height, and smoking habit who survived longer than their matched case. Flow rates in different parts of the spirogram were compared within case-control pairs and analysed as predictors of fatal COPD by conditional logistic regression. RESULTS Within pair case:control ratios of FEV₁, mid expiratory flow rates (e.g. FEF_50–75) and FEF_75–85 were highly intercorrelated (r>0.7) but correlations with FEF_85–95 were weaker (r1/FVC ratio were stronger predictors of death from COPD within the first 10 years than of later deaths (15–19 years). After adjustment for FEV₁, mid expiratory flow rates independently predicted fatal COPD but end expiratory flow rates did not. The FEV₁ adjusted mortality ratios associated with a 10% decrement in each index were 2.24 (95% CI 1.54 to 3.76) for FEF_50–75, 1.20 (95% CI 1.00 to 1.42) for FEF_75–85, and 1.10 (95% CI 0.96 to 1.26) for FEF_85–95. CONCLUSION This study confirms FEV₁ and mid expiratory flow rates as powerful predictors of mortality from COPD, and suggests that measurement of end expiratory flow rates would add little extra predictive information.