The Efficiency Index (EFFi), based on volumetric capnography, may allow for simple diagnosis and grading of COPD

BACKGROUND: Spirometry, the main tool for diagnosis and follow-up of COPD, incompletely describes the disease. Based on volumetric capnography (VCap), an index was developed for the diagnosis and grading of COPD, aimed as a complement or alternative to spirometry. METHODS: Nine non-smokers, 10 smokers/former smokers without COPD and 54 smokers/former smokers with COPD were included in the study. Multiple breath washout of N(2) and VCap were studied with Exhalyzer D during tidal breathing. VCap was based on signals for flow rate and CO(2) and was recorded during one breath preceding N(2) washout. Efficiency Index (EFFi) is the quotient between exhaled CO(2) volume and the hypothetical CO(2) volume exhaled from a completely homogeneous lung over a volume interval equal to 15% of predicted total lung capacity. RESULTS: EFFi increased with increased Global initiative for chronic Obstructive Lung Disease (GOLD) stage and the majority of subjects in GOLD 2 and all subjects in GOLD 3 and 4 could be diagnosed as having COPD using the lower 95% confidence interval of the healthy group. EFFi also correlated with N(2) washout (r=−0.73; p<0.001), forced expiratory volume in 1 second (r=0.70; p<0.001) and diffusion capacity for carbon oxide (r=0.69; p<0.001). CONCLUSION: EFFi measures efficiency of tidal CO(2) elimination that is limited by inhomogeneity of peripheral lung function. EFFi allows diagnosis and grading of COPD and, together with FEV(1), may explain limitation of physical performance. EFFi offers a simple, effortless and cost-effective complement to spirometry and might serve as an alternative in certain situations.