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

Linnea Jarenbäck, Ellen Tufvesson, Jaro Ankerst, Leif Bjermer, Björn Jonson, Linnea Jarenbäck, Ellen Tufvesson, Jaro Ankerst, Leif Bjermer, Björn Jonson

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: carbon dioxide; chronic obstructive pulmonary disease; single breath; volumetric capnography.

Conflict of interest statement

Disclosure The authors report no conflicts of interest in this work.

Figures

Figure 1
Figure 1
Single breath test curve for CO2 for calculations of EFFi. Notes: Analyzed volume starts when the CO2 fraction reaches 0.002 and continues to 15% of predicted TLC. For breaths not covering 15% of TLC, the alveolar plateau was extrapolated. EFFi was calculated as the quotient between the volume of CO2 exhaled, area A, and the hypothetical volume of CO2 exhaled from a completely homogenous lung, area (A+B). Abbreviations: EFFi, Efficiency Index; TLC, total lung capacity.
Figure 2
Figure 2
SBT CO2 curves from the subject with median value of EFFi in each group. Note: The analyzed volume is normalized with respect to TLCp, and CO2 is normalized to the value at 15% TLCp. Abbreviations: EFFi, Efficiency Index; GOLD, Global initiative for chronic Obstructive Lung Disease; SBT CO2, single breath test for CO2; TLCp, predicted value for total lung capacity.
Figure 3
Figure 3
Grading COPD with small airway parameters. Notes: Grading of COPD severity based on (A) EFFi, (B) DL,CO %p and (C) Sacin. 95% confidence interval based on the healthy group is depicted for each parameter. Abbreviations: %p, % of predicted value; EFFi, Efficiency Index; GOLD, Global initiative for chronic Obstructive Lung Disease; DL,CO, diffusion capacity of carbon monoxide over the lung; Sacin, ventilation heterogeneity in the acinar airways.
Figure 4
Figure 4
Correlation between EFFi and lung function parameters. Note: Correlation between EFFi and (A) FEV1 %p, (B) FEV1/FVC ratio, (C) DL,CO and (D) Sacin. Abbreviations: %p, % of predicted value; EFFi, Efficiency Index; DL,CO, diffusion capacity of carbon monoxide over the lung; FEV1, forced expiratory volume in 1 second; FVC, forced vital capacity; Sacin, ventilation heterogeneity in the acinar airways.

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