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

Figures & data

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.

Table 1 Patient demographics and characteristics

	Never smokers (n=9)	Smoking controls (n=10)	GOLD 1 (n=8)	GOLD 2 (n=22)	GOLD 3 (n=14)	GOLD 4 (n=10)	p-value
Gender (male/female)	6/3	7/3	5/3	12/10	9/5	2/8	ns
Smoking status (former/current)	0/0	7/3	6/2	15/7	12/2	10/0	<0.001
Age (years)	64 (62–69)	69 (67–70)	70 (68–71)	68 (63–70)	69 (61–72)	64 (62–69)	ns
Height (cm)	172 (166–173)	175 (164–178)	174 (165–189)	173 (166–178)	174 (171–178)	169 (156–173)	ns
Weight (kg)	66 (62–78)	84 (65–91)	84 (70–93)	74 (70–83)	75 (68–84)	69 (52–74)	0.058
FEV1 (%p)	96 (89–110)	95 (90–100)	92 (84–96)	64 (56–70)	40 (34–42)	26 (20–28)	<0.001
FEV1 (L)	2.47 (2.40–2.97)	3.02 (2.13–3.35)	3.11 (2.29–3.47)	1.86 (1.53–2.15)	1.06 (0.90–1.41)	0.61 (0.53–0.76)	<0.001
FEV1/FVC	0.78 (0.74–0.81)	0.73 (0.73–0.78)	0.65 (0.60–0.67)	0.55 (0.49–0.61)	0.39 (0.35–0.42)	0.38 (0.33–0.64)	<0.001
DL,CO (%p)	95 (85–99)	81 (71–96)	86 (70–104)	72 (56–79)	48 (28–56)	21 (15–25)	<0.001
Sacin (L^−1)	0.15 (0.08–0.21)	0.17 (0.05–0.25)	0.17 (0.15–0.22)	0.27 (0.21–0.43)	0.39 (0.35–0.54)	0.44 (0.35–0.64)	<0.001

Note: Data are presented as n/n and median (interquartile range).

Abbreviations: %p, % of predicted value; GOLD, Global initiative for chronic Obstructive Lung Disease; D_L,CO, diffusion capacity of carbon monoxide over the lung; FEV₁, forced expiratory volume in 1 second; FVC, forced vital capacity; ns, not significant; S_acin, ventilation heterogeneity in the acinar airways.

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.