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

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

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

Grading COPD with small airway parameters. Notes: Grading of COPD severity based on (A) EFFi, (B) D_L,CO %p and (C) S_acin. 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; D_L,CO, diffusion capacity of carbon monoxide over the lung; S_acin, ventilation heterogeneity in the acinar airways.

Figure 4

Correlation between EFFi and lung function parameters. Note: Correlation between EFFi and (A) FEV₁ %p, (B) FEV₁/FVC ratio, (C) D_L,CO and (D) S_acin. Abbreviations: %p, % of predicted value; EFFi, Efficiency Index; D_L,CO, diffusion capacity of carbon monoxide over the lung; FEV₁, forced expiratory volume in 1 second; FVC, forced vital capacity; S_acin, ventilation heterogeneity in the acinar airways.