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 (FEV1 or FEV0.75) 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. FEF50-75) and FEF75-85 were highly intercorrelated (r>0.7) but correlations with FEF85-95 were weaker (r<0.5). All indices except the FEV1/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 FEF50-75, 1.20 (95% CI 1.00 to 1.42) for FEF75-85, and 1.10 (95% CI 0.96 to 1.26) for FEF85-95. 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.