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

Abstract

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 N₂ and VCap were studied with Exhalyzer D during tidal breathing. VCap was based on signals for flow rate and CO₂ and was recorded during one breath preceding N₂ washout. Efficiency Index (EFFi) is the quotient between exhaled CO₂ volume and the hypothetical CO₂ 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 N₂ 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 CO₂ elimination that is limited by inhomogeneity of peripheral lung function. EFFi allows diagnosis and grading of COPD and, together with FEV₁, 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.

Keywords:

• chronic obstructive pulmonary disease
• volumetric capnography
• carbon dioxide
• single breath

Acknowledgments

We would like to thank the staff at the Lung Research Unit at Region Skåne Hospital for clinical support. This work was supported by independent research grants from the Swedish Heart and Lung Foundation, Evy and Gunnar Sandberg’s Foundation, Crafoord Foundation and Royal Physiographic Society in Lund.

Author contributions

All authors participated in designing the study and critically revised the manuscript. LJ tested the patients, performed primary analysis, co-wrote the manuscript and performed all statistical calculations. ET, JA and LB helped with collecting data. BJ proposed and performed the calculations and analysis of EFFi and co-wrote the manuscript. All authors approved the final version of the article and are accountable for the accuracy and integrity of the work.

Disclosure

The authors report no conflicts of interest in this work.