Differences in the Measurement of Functional Residual Capacity Between Body Plethysmographs of Two Manufacturers

Figures & data

Table 1 Baseline Characteristics

	Jaeger n = 1988	Ganshorn n = 156	p value
GOLD grades 1/2/3/4	187/847/766/188 (9.4/42.6/38.5/9.5%)	13/73/52/18 (8.3/46.8/33.3/11.5%)	0.485
Age [years]	65.1 ± 8.4	64.1 ± 9.2	0.184
Height [cm]	171 ± 9	171 ± 10	0.840
BMI [kg/m^2]	26.6 ± 5.2	26.5 ± 4.9	0.791
Sex [male/female]	1231 (61.9%)/757 (38.1%)	75 (48.1%)/81 (51.9%)	<0.001

Table 2 Results from Multivariate Regression Analyses of the Dependent Variables Given in the Top Row and the Predictors Given Below

Predictors	RV/TLC [%]	FRC %Predicted	RV %Predicted	TLC %Predicted	FRC [l]	RV [l]	TLC [l]
FEV1 %pred	−0.2 [−0.2; −0.2]	−1.4 [−1.5; −1.3]	−1.5 [−1.7; −1.4]	−0.6 [−0.6; −0.5]	−0.05 [−0.05; −0.04]	−0.04 [−0.04; −0.03]	−0.04 [−0.04; −0.03]
FVC %pred	−0.2 [−0.3; −0.2]	0.5 [0.4; 0.6]	−0.1 [−0.2; 0.1]	0.5 [0.5; 0.6]	0.02 [0.02; 0.02]	0.00 [0.00; 0.00]	0.03 [0.03; 0.04]
Age [years]	0.3 [0.2; 0.3]	−0.1 [−0.3; 0.0]	−1.0 [−1.2; −0.8]	−0.2 [−0.3; −0.2]	0.00 [0.00; 0.01]	0.01 [0.01; 0.02]	−0.02 [−0.02; −0.01]
Height [cm]	−0.1 [−0.2; −0.1]	0.3 [0.1; 0.5]	0.2 [−0.1; 0.4]	−0.2 [−0.3; −0.1]	0.04 [0.04; 0.05]	0.03 [0.02; 0.04]	0.07 [0.07; 0.08]
BMI [kg/m^2]	−0.2 [−0.2; −0.1]	−1.4 [−1.7; −1.2]	−1.3 [−1.6; −1]	−0.4 [−0.6; −0.3]	−0.05 [−0.05; −0.04]	−0.03 [−0.04; −0.02]	−0.03 [−0.03; −0.02]
Sex [female vs male]	3.1 [2.3; 3.8]	15.2 [11.9; 18.5]	14.2 [9.4; 18.9]	7.8 [5.9; 9.7]	−0.43 [−0.53; −0.32]	−0.24 [−0.35; −0.13]	−0.81 [−0.93; −0.70]
Plethysmograph [Ganshorn vs Jaeger]	−6.9 [−7.9; −5.8]	−20.9 [−25.4; −16.4]	−34.0 [−40.5; −27.6]	−10.2 [−12.7; −7.7]	−0.67 [−0.82; −0.53]	−0.77 [−0.92; −0.63]	−0.61 [−0.77; −0.46]

Notes: Non-standardized regression coefficients and their 95% confidence intervals are given. % predicted refers to GLI reference values. Please note that the differences between plethysmographs regarding the absolute volumes are of similar magnitude and that their confidence intervals overlap, indicating that the difference regarding FRC is that which determined those regarding RV and TLC as derived parameters.

Abbreviations: FRC, functional residual capacity; RV, residual volume; TLC, total lung capacity.

Figure 1 Difference in FRC between body plethysmographs. The horizontal axis shows FEV₁ % predicted (GLI), the vertical axis FRC in litres measured by either the Vyaire (blue circles, n=1988) or the Ganshorn (red diamonds, n=156) plethysmograph. Additionally, LOESS fit lines are shown for both groups in the respective colors, indicating a difference of more than 0.5 litres between FRC values at a given value of FEV₁ % predicted. When residual terms after adjustment for FEV₁, FVC, sex, age, height and BMI were used, essentially the same picture was obtained, with the difference that the dependence on FEV₁ disappeared. This demonstrated that the difference shown in the graph was not due to residual confounding but a robust result even in unadjusted data.