Impaired Carbon Monoxide Diffusing Capacity is the strongest lung function predictor of decline in 12 minute-walking distance in COPD; a 5-year follow-up study

Figures & data

Table 1. Demographic characteristics and comprehensive indices of pulmonary function for all COPD patients evaluated at baseline visit

Variables	All subjects n = 84	Participants n = 34	Non-participants n = 50	p value
Sex (f/m)	63/21	22/12	41/9	0.07
Age (y)	64 ± 7	63 ± 8	65 ± 7	0.20
BMI (kg/m^2)	23.6 ± 4.3	23.3 ± 3.4	23.4 ± 4.1	0.87
Smoking History (pack-years)	40 ± 9	39 ± 9	40 ± 9	0.66
Current smoker (n (%))	19 (23%)	9 (26%)	10 (20%)	0.73
Completed pulmonary rehabilitation program (n (%))	51 (61%)	22 (65%)	29 (58%)	0.54
FEV1 (L)	1.1 ± 0.4	1.2 ± 0.4	1.0 ± 0.3	0.002
FEV1 (% of predicted)	39.2 ± 12.9	38.3 ± 12.5	36.6 ± 12.1	0.14
DLco (mmol/min/kPa)	3.4 ± 1.3	3.8 ± 1.2	3.0 ± 1.0	0.004
DLco (% of predicted)	47.0 ± 16.1	50.6 ± 15.7	44.7 ± 16.1	0.10
12MWD (m)	850 ± 200	928 ± 193	802 ± 165	0.002

Values presented as mean ± SD. BMI = body mass index; FEV₁ = forced expiratory volume during 1 second; DLco = diffusing capacity of the lung for carbon monoxide; 12MWD = 12-minute walk distance. The p values for t-test for continues variables and chi²-test for categorical variables were used and p < 0.05 was considered as statistically significant.

Table 2. Indices of lung function parameters for COPD patients participating in both baseline and follow-up visits, n = 34

Variables	Values at baseline	Values at follow-up	p-value
TLC (L)	6.6 ± 1.3	7.1 ± 1.6	0.001
FRC (L)	5.0 ± 1.2	5.4 ± 1.4	0.002
IC (L)	1.7 ± 0.4	1.7 ± 0.6	0.99
IC/TLC (%)	25.9 ± 6.8	23.9 ± 7.3	0.10
VC (L)	2.9 ± 0.9	2.6 ± 0.8	<0.001
RV (L)	3.7 ± 0.9	4.5 ± 1.3	<0.001
RV/TLC (%)	56.2 ± 9.5	63.2 ± 8.1	<0.001
Raw (Pa*s/L)	668 ± 277	595 ± 197	0.12
FVC (L)	3.0 ± 1.0	2.5 ± 0.9	<0.001
FEV1 (L)	1.2 ± 0.4	1.1 ± 0.4	0.13
DLco (mmol/min/kPa)	3.8 ± 1.1	3.1 ± 1.4	<0.001
12MWD	928 ± 193	789 ± 273	<0.001

Values presented as mean ± SD. BMI = body mass index; TLC = total lung capacity, FRC = functional residual capacity, IC = inspiratory capacity, VC = vital capacity, RV = residual volume, Raw = airway resistance, FVC = forced vital capacity. p-values for paired t-test were used and p < 0.05 was considered as statistically significant.

Table 3. Baseline lung function values in lower, middle and upper tertiles of 12-minute walk distance at baseline

Variables	Lower tertile < 789m n = 28	Middle tertile 789m–921m n = 28	Lower tertile >921m n = 28	p value
TLC (L)	6.4 ± 1.3	6.4 ± 1.5	6.6 ± 1.5	0.54
FRC (L)	5.0 ± 1.3	4.6 ± 1.3	4.8 ± 1.4	0.43
IC (L)	1.5 ± 0.4	1.8 ± 0.6	2.0 ± 0.5	<0.001
IC/TLC (%)	23.4 ± 7.7	28.1 ± 7.4	30.3 ± 8.6	0.002
VC (L)	2.5 ± 0.6	2.8 ± 0.9	3.4 ± 0.9	<0.001
RV (L)	3.9 ± 1.1	3.6 ± 1.1	3.3 ± 1.2	0.036
RV/TLC (%)	60.3 ± 8.6	55.6 ± 9.4	48.7 ± 9.7	<0.001
Raw (Pa*s/L)	827 ± 439	545 ± 207	426 ± 162	<0.001
FVC (L)	2.3 ± 0.6	2.5 ± 0.7	3.1 ± 1.0	<0.001
FEV1 (L)	0.8 ± 0.2	1.1 ± 0.3	1.3 ± 0.4	<0.001
DLco (mmol/min/kPa)	2.9 ± 0.9	3.2 ± 0.9	3.9 ± 1.4	<0.001

p value is presented for trend along different tertiles. p < 0.05 was considered as ­statistically significant.

Table 4. Stepwise regression models of determinants of 12-minute walk distance

	12MWD at baseline	12MWD at follow-up	Change in 12MWD
FEV1 at baseline DLco at baseline R^2	0.53 0.12 0.37	– – –	– 0.51 0.26
DLco at follow-up RV at follow up R^2	– – –	0.60 –0.51 0.79	– – –
Change in DLco Change in FVC R^2	– – –	– – –	0.44 0.41 0.48

Coefficients in the table are standardized regression coefficients (ß) for respective lung function parameter with exception for R², which represents the coefficient of determination of the model (predictive value of the multiple linear regression model including only the lung function parameters presented in the table).

Figure 1. Explanatory value (expressed in r²-values from simple linear regression models) of each of the lung function parameters at baseline for the patients’ exercise capacity at baseline. *Indicates significant relation (p < 0.05).

Figure 2. Explanatory value (expressed in r²-values from simple linear regression models) of each of the lung function parameters at follow-up for the patients’ 12MWD at follow-up. *Indicates significant relation (p < 0.05).

Figure 3. Explanatory value (expressed in r²-values from simple linear regression models) of each of the lung function parameters at baseline for the changes in 12MWD during 5-year follow-up. *Indicates significant relation (p < 0.05).

Figure 4. (A) Scatter plot for changes in 12MWD (y-axis) as function of DLco (x-axis), including regression line. (B) Scatter plot for changes in 12MWD (y-axis) as function of FEV₁ (x-axis), including regression line.