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

BACKGROUND Previous epidemiological studies have related mortality from chronic obstructive pulmonary disease (COPD) to forced expiratory volumes (FEV(1) or 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 FEV(1), 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 (r<0.5). All indices except the FEV(1)/FVC ratio were stronger predictors of death from COPD within the first 10 years than of later deaths (15-19 years). After adjustment for FEV(1), mid expiratory flow rates independently predicted fatal COPD but end expiratory flow rates did not. The FEV(1) 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 FEV(1) 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.