Different COPD Disease Characteristics are Related to Different Outcomes in the 6-minute Walk Test

Figures & data

Table 1.  Characteristics of the 370 COPD patients

	Women (n 147)	Men (n 223)	Total (n 370)	p
Sex,%	40	60
Age, mean (SD)	62.4 (6.3)	64.0 (7.1)	63.3 (6.8)	0.03*
Smoking habits,%
Ex	52.4	57.4	55.4	0.34
Current	47.6	42.6	44.6
Depression (CES-D),%
No	77.6	82.3	80.4	0.32
Yes	22.4	17.4	19.5
Charlson Co-morbidity Index,%
<3	89.7	77.1	82	0.02*
>3	10.2	22.8	18
Physical activity per week,%
3 hrs or more	42.2	45.6	44.3	0.77
1–2 hrs	46.7	42.7	44.3
No activity	11.1	11.7	11.4
FEV1 in% predicted, mean (SD)	49.2 (13.3)	51.1 (14.1)	50.4 (13.8)	0.19
RV in% predicted	150.1 (44.4)	139.5 (42.4)	143.7 (43.4)	0.02*
PaO2, kPa, mean (SD)	9.2 (1.1)	9.4 (1.1)	9.3 (1.1)	0.03*
PaCO2, kPa, mean (SD)	5.3 (0.5)	5.3 (0.5)	5.3 (0.5)	0.97
Dyspnea (MRC scale),%
<3	85.8	85.5	86	0.81
>3	14.2	14.5	14
C-reactive Protein (ug/mL), mean (SD)	8.8 (12.7)	7.9 (11.1)	8.3 (11.8)	0.41
Fat Mass Index, kg/m2 (FMI), mean (SD)	9.9 (3.7)	7.3 (2.5)	8.3 (3.3)	<0.001*
Fat Free Mass Index, kg/m2 (FFMI), mean (SD)	14.8 (2.4)	18.5 (2.8)	17.0 (3.2)	<0.001*
Heart rate, mean (SD)
Before 6mwt	88 (14.6)	84 (14.6)	86 (14.7)	0.02
After 6mwt	114 (18.0)	107 (17.9)	110 (17.9)	<0.001*
SpO2, mean (SD)
Before 6mwt	94 (2.9)	94 (2.4)	94 (2.6)	0.49
After 6mwt	91 (6.2)	91 (5.1)	91 (5.6)	0.44
Self-experienced dyspnea (Borg scale), median
Before 6mwt	1	0.3	0.5	0.04*
After 6mwt	4	3	3	0.06
Desaturation <90% and 4% reduction,%	24	23	23	0.92

*p < 0.05

Table 2.  Bivariate associations between outcomes and categorical predictor variables

	n	Walking distance		Change in Borg score			O2 desaturation
		mean (SD)	p*	median	95% CI	p^†	n (%)	p**
Sex
Women	147	415 (95.9)	<0.01	2	1.6, 2.4	0.22	35 (24.0)	0.92
Men	223	444 (107.1)		2	1.6, 2,4		51 (23.0)
Smoking habits
Ex	205	431 (106.4)	0.62	2.5	2.1, 2.9	<0.001	62 (30.2)	<0.001
Current	165	434 (100.3)		2	1.6, 2.4		24 (14.6)
Dyspnea (MRC scale)
0	58	469 (109.4)	0.03	1	0.6, 1.5	<0.001	4 (6.9)	<0.001
1	120	471 (84.2)		2	1.6, 2.4		18 (15.0)
2	114	407 (92.6)		3	2.4, 3.6		38 (33.3)
3	32	377 (85.2)		3	1.8, 4.3		13 (40.6)
4	17	303 (130.8)		3	1.4, 4.7		7 (41.2)
Charlson Co-morbidity Index
1	220	451 (98.5)	0.67	2	1.6, 2.4	0.10	52 (23.6)	0.27
2	84	422 (99.8)		2.5	1.9, 3.1		24 (29.6)
3	42	388 (112.6)		3	2.2, 3.8		6 (14.3)
4	24	385 (108.2)		2	0.6. 3.5		4 (16.7)
Depression (CES-D)
No	243	444 (105.2)	<0.001	2	1.6, 2.4	0.01	61 (25.1)	0.97
Yes	59	391 (96.6)		3	2.2, 3.8		14 (23.7)
Physical activity pr week
3 hrs or more	151	470 (91.0)	<0.001	2	1.6, 2.4	0.34	34 (22.5)	0.94
1–2 hrs	151	417 (97.8)		2	1.5, 2.5		35 (23.2)
No activity	39	364 (105.3)		2	1.0, 3.1		8 (20.5)

*t-test and anova.

^†Kruskal-Wallis and Wilcoxon Mann-Whitney.

**chi squared.

Table 3.  Bivariate associations between the three outcomes and continuous predictor variables

	Walking distance		Change in Borg score		O2 desaturation
	correlation coeffisient	p*	correlation coeffisient	p^†	Mean no	yes	p**
Age	−0.30	<0.001	0.05	0.41	63.0	64	0.21
Fat Free Mass Index, kg/m2 (FFMI)	0.13	0.02	0.01	0.90	17.1	16.8	0.56
Fat Mass Index, kg/m2 (FMI)	−0.27	<0.001	0.07	0.21	8.1	16.9	0.055
FEV1 in% predicted (decrease)	−0.34	<0.001	0.34	<0.001	47.0	16.10	<0.001
RV in% predicted	−0.25	<0.001	0.22	<0.001	138.8	16.11	<0.001
PaO2, kPa	0.23	<0.001	–0.16	<0.01	9.5	16.12	<0.001
PaCO2, kPa	0.03	0.65	0.01	0.85	5.3	16.13	<0.001
C-reactive Protein (ug/mL)	−0.14	<0.01	0.06	0.26	7.8	16.14	0.23

*Pearson's r.

^†Spearman's rho.

**t-test.

Table 4.  Multivariate regression analyses with 6 minute walking distance, change in Borg score and oxygen desaturation

Parameters*	N	Walking distance**			Change in Borg score^†			O2 desaturation***
		Coefficient (R2 = 0.47)	95%CI	P	Coefficient (pseudo R2 = 0.16)	95%CI	P	Odds ratio (pseudo R2 = 0.36)	95%CI	P
Men	157	46.2	17.6, 74.8	0.001	−0.64	−1.34, 0.11	0.095	0.73	0.25, 2.08	0.55
Age per 10 yrs increase	240	−26.2	−43.6, −8.8	<0.01	0.12	−0.34, 0.58	0.62	1.02	0.50, 4.18	0.95
Fat Free Mass Index, kg/m2 (FFMI)		−2.8	−7.5, 1.8	0.21	0.14	0.02, 0.27	0.02	1.13	0.95, 1.36	0.17
Fat Mass Index, kg/m2 (FMI)	240	−5.3	−10.7, 0.1	0.056	0.15	0.02, 0.28	0.03	1.08	0.88, 1.33	0.46
Current smokers	108	−6.98	−30.7, 16.8	0.54	−0.72	−1.35, −0.09	0.03	0.38	0.13, 1.07	0.06
FEV1 in% predicted (pr 10% decrease)	240	−21.8	−32.3, −11.4	<0.001	0.13	−0.14, 0.41	0.34	2.58	1.59, 4.18	<0.001
RV in% predicted	240	−1.7	−4.6, 1.1	0.22	0.07	−0.01, 0.14	0.078	0.91	0.80, 1.03	0.12
PaO2, kPa	240	6.59	−3.9, 17.1	0.19	−0.13	−0.42, 0.14	0.34	0.32	0.20, 0.52	<0.001
PaCO2, kPa	240	19.5	−4.2, 43.2	0.09	−0.25	−0.88, 0.37	0.43	0.91	0.36, 2.36	0.85
MRC dyspnea 1	83	24.2	−6.9, 55.3	<0.001	0.09	−0.75, 0.93	0.07	0.77	0.17, 3.53	0.50
MRC dyspnea 2	80	−7.1	−41.8, 27.7		1.04	0.12, 1.96		1.39	0.30, 6.56
MRC dyspnea 3	23	−37.1	−82.8, 8.7		0.70	−0.51, 1.92		2.71	0.45, 16.17
MRC dyspnea 4	9	−85.0	−149.7, −20.4		0.09	−1.62, 1.79		1.56	0.17, 14.65
Charlson comorbidity index 2	48	−9.7	−36.9, 17.6	<0.01	1.07	0.34, 1.80	<0.01	1.33	0.48, 3.68	0.42
Charlson comorbidity index 3	30	−59.0	−93.0, −25.0		1.15	0.29, 2.03		0.39	0.10, 1.55
Charlson comorbidity index 4	12	−45.8	−97.9, 6.3		−0.42	−1.77, 0.93		0.86	0.12, 5.99
Depression (CES-D)	42	0.49	−28.3, 29.3	0.97	0.38	−0.38, 1.14	0.33	0.65	0.22, 1.94	0.44
C-reactive Protein (ug/mL)	240	−8.0	−19.7, 3.7	0.16	−0.25	−0.55, 0.04	0.09	1.05	0.69, 1.61	0.81
1–2 hrs physical activity pr week	104	−41.9	−65.7, −18.3	<0.001	0.08	−0.56, 0.71	0.67	0.82	0.33, 2.09	0.92
No physical activity pr week	27	−81.6	−117.9, −45.4		0.45	−0.53, 1.42		0.85	0.22, 3.35
Constant	240	640.6	394.6, 886.6		−0.13	−6.43, 6.17	0.97

*Reference categories (0 coefficients) are women, ex-smokers, 0 MRC dyspnea, 1 Charlson comorbidity index, no depression, over 3 hrs physical activity.

**Linear regression model.

^†quantile regression model.

***Logistic regression model.