One Year Change in 6-Minute Walk Test Outcomes is Associated with COPD Prognosis

Figures & data

Figure 1. Flow chart of the study design and the COPD patients who performed 6MWT at baseline and at follow-up 1 year later in the Bergen COPD Cohort Study.

Figure 2. Timeline of the measurements of risk factors and outcomes.

Table 1. Characteristics of 295 COPD patients in the Bergen COPD Cohort Study.

	All	Women	Men	p value
N	295	115	180
Age, yr (SD)	63.2 (6.4)	62.0 (5.7)	63.9 (6.7)	0.009
Ex or current smoker, n (%)				0.37
Ex	166 (56.3)	61 (53.0)	105 (58.3)
Current	129 (43.7)	54 (47.0)	75 (41.7)
FEV1 % predicted, mean (SD)	51 (13.4)	52 (12.7)	50 (13.8)	0.16
mMRC dyspnea scale,^a n (%)
0	46 (17.1)	17 (16.3)	29 (17.6)
1	103 (38.3)	40 (38.5)	63 (38.2)
2	87 (32.3)	34 (32.7)	53 (32.1)	0.65
3	25 (9.3)	8 (7.7)	17 (10.3)
4	8 (3.0)	5 (4.8)	3 (1.8)
Charlson Comorbidity Index, n (%)
1	177 (60.0)	76 (66.1)	101 (56.1)	0.066
2	66 (22.4)	26 (22.6)	40 (22.2)
3	52 (17.6)	13 (11.3)	39 (21.7)
Exacerbations 12 mo prior inclusion, n (%)	46 (15.6)	23 (20.0)	23 (12.8)	0.095
Blood gas (kpa), baseline	9.4 (1.1)	9.2 (1.0)	9.5 (1.1)	0.03
SpO2 at rest, baseline, mean (SD)	94.5 (2.5)	94.3 (2.7)	94.5 (2.4)	0.53
RV/TLC %, mean (SD)	44.0 (9.2)	46.3 (9.8)	42.5 (8.5)	<0.001
Participation pulmonary rehabilitation, n (%)				0.75
No	221 (74.9)	85 (73.9)	136 (75.6)
Yes	74 (25.1)	30 (26.1)	44 (24.4)
6MWD baseline, m (SD)	445.8 (92.0)	429.1 (88.6)	456.4 (92.9)	0.013
6MWD follow up, m (SD)	453.7 (100.7)	430.9 (99.4)	468.3 (99.2)	0.002
Desaturation baseline, n (%)	58 (19.7)	23 (20.0)	35 (19.4)	0.91
Desaturation follow up,^b n (%)	85 (30.0)	34 (29.6)	51 (28.7)	0.87
Change 6MWD, ≥30 m decrease, n (%)	69 (23.4)	30 (26.1)	39 (21.7)	0.38
Change desaturation,^c n (%)
No desaturation	198 (70.0)	78 (69.6)	120 (70.2)	0.93
Desaturation only at follow-up	37 (13.1)	14 (12.5)	23 (13.5)
Desaturation in both tests	48 (17.0)	20 (17.9)	28 (16.4)
All cause mortality, n (%)	84 (28.5)	25 (21.7)	59 (32.8)	0.04
Respiratory mortality, n (%)	54 (18.3)	15 (13.0)	39 (21.7)	0.06

FEV₁, forced expiratory volume in 1 second; mMRC, modified medical research council; 6MWD, 6-min walk distance; RV/TLC, residual volume/total lung capacity.

^a mMRC: 26 missing.

^b Desaturation: 12 missing.

^c Based on population of 283, those who desaturated only at baseline 6MWT is excluded.

Figure 3. Associations of change in desaturation status in 6MWT in 1 year and all-cause and respiratory mortality after 9 years in 295 COPD patients, Kaplan-Meier for crude analyses and Cox proportional hazard regression for adjusted analyses (graph created using the posthoc stcurve syntax in Stata which plots mortality by desaturation category while setting each covariate to its mean values). (a) Unadjusted all-cause mortality. (b) After adjustment for sex, age, 6MWD, mMRC dyspnea scale, Charlson comorbidity index, smoking status, exacerbations 12 month prior inclusion and participation in pulmonary rehabilitation. (c) Unadjusted respiratory mortality. (d) After adjustments for sex, age, 6MWD, mMRC dyspnea scale, Charlson comorbidity index, smoking status and exacerbations 12 month prior inclusion and participation in pulmonary rehabilitation.

Table 2. Associations of 1 year change in walking distance and desaturation status in two 6MWTs with all-cause mortality and respiratory mortality after 9-year follow-up obtained from the Norwegian Cause of Death Registry, among 295 COPD patients in the Bergen COPD Cohort Study, using cox proportional hazard regression analyses.

Mortality		Crude HR (95% CI)	p value	Adjusted HR (95% CI)	p value^a
One year change in 6MWD	Category^b	n = 295		n = 269
All cause	Change >30 m	1.6 (1.0–2.6)	0.04	1.4 (0.9–2.3)	0.16
Respiratory	Change >30 m	1.5 (0.8–2.7)	0.17	1.4 (0.7–2.6)	0.31
One year change in desaturation status	Category^c	n = 283		n = 258
All cause	Desaturation only at second 6MWT	1.6 (0.9–2.9)	0.14	2.0 (1.0–3.8)	0.04
	Desaturation in both 6MWT	2.2 (1.3–3.6)	0.002	2.7 (1.5–5.0)	0.001
Respiratory	Desaturation only at second 6MWT	1.9 (0.9–4.1)	0.09	2.3 (1.0–5.2)	0.04
	Desaturation in both 6MWT	2.7 (1.5–5.1)	0.001	3.6 (1.7–7.6)	0.001

6MWT, 6-minute walk test.

^a Adjusted for baseline values of sex, age, 6MWD, mMRC dyspnea scale, Charlson comorbidity index, smoking status, exacerbations 12 month prior inclusion, and participation in pulmonary rehabilitation.

^b Reference category is «other change».

^c Reference category is «no desaturation».

^d Missing mMRC values (n = 21 for change in 6MWD and n = 20 for desaturation status).

Table 3. Associations of desaturation status in two 6MWTs with all-cause mortality and respiratory mortality after 9-year follow-up obtained from the Norwegian Cause of Death Registry, among 295 COPD patients in the Bergen COPD Cohort Study, using cox proportional hazard regression analyses, with additional adjustment for baseline values of FEV₁% predicted and blood gas values.

Mortality		Adjusted HR (95% CI)	p value^a
One year change in desaturation status	Category^b	n = 236
All cause	Desaturation only at second 6MWT	1.6 (0.8–3.2)	0.19
	Desaturation in both 6MWT	1.9 (1.0–3.8)	0.06
Respiratory	Desaturation only at second 6MWT	2.2 (0.9–5.0)	0.068
	Desaturation in both 6MWT	2.8 (1.2–6.5)	0.017

^a Adjusted for baseline values of sex, age, 6MWD, MRC dyspnea scale, Charlson comorbidity index, smoking status, exacerbations 12 month prior inclusion, participation in pulmonary rehabilitation, FEV₁% predicted and blood gas.

^bReference category is «no desaturation».

^c Missing MRC values (n = 21 for change in 6MWD and n = 20 for desaturation status).

Figure 4. Estimated decline in FVC % predicted with 95% CI from year 1 through 5 according to desaturation status in two 6MWTs in year 0 and 1 in 295 COPD patients, from linear mixed effects models.

Table 4. One year change in desaturation status in two 6MWTs and association with decline in FVC % predicted over 5 years in 295 COPD patients from the Bergen COPD Cohort Study.[AQ3]

Outcome	No desaturation		Desaturation 6MWT2		Desaturation 6MWT1 + 6MWT2		Crude predicted mean difference (95% CI)^a	Crude predicted mean difference (95% CI)^b	p interaction^c	Adjusted^d predicted mean difference (95% CI)^a	Adjusted^d predicted mean difference (95% CI)^b	p interaction^c
	No. of patients	Observed mean value ± SE	No. of patients	Observed mean value ± SE	No. of patients	Observed mean value ± SE
FVC									0.03			0.04
1 yr	195	87% ± 1.1%	37	91% ± 2.8%	47	81% ± 2.3%	3.6% (−3.7 to 10.8)	−7.0% (−14.0 to −0.4)		4.2% (−2.9 to 11.3)	−4.3% (−11.0 to 2.5)
2 yr	190	85% ± 1.1%	36	87% ± 3.2%	44	78% ± 2.4%	2.3% (−4.7 to 9.4)	−7.4% (−13.8 to −0.9)		3.0% (−3.9 to 9.8)	−4.4% (−10.9 to 2.2)
3 yrs	185	83% ± 1.1%	36	86% ± 2.7%	44	77% ± 2.5%	1.1% (−6.2 to 8.4)	−7.7% (−14.3 to −1.1)		1.8% (−5.2 to 8.7)	−4.5% (−11.2 to 2.2)
5 yrs	151	86% ± 1.2%	27	83% ± 4.2%	31	78% ± 3.4%	−1.4% (−10.0 to 7.2)	−8.5% to (−16.4 to −0.6)		−0.7% (−9.0 to 7.6)	−4.8% (−12.7 to 3.2)
p trend		<0.001		<0.001		0.012

6MWT, 6-minute walk test.

^aNo desaturation vs desaturation 6MWT2 (new desaturators).

^bNo desaturation vs desaturation 6MWT1 + 6MWT2 (repeated desaturators).

^c Interaction between desaturation status (no desaturation vs new desaturators) in 6MWT and time.

^d Adjusted for baseline values of: sex, age, walking distance, mMRC dyspnea scale, Charlson comorbidity index, smoking status and participation in pulmonary rehabilitation.

Table 5. One year change in walking distance in 6MWT and association with decline in FEV₁% predicted over 5 years in 295 COPD patients from the Bergen COPD Cohort Study.

Outcome	Change <30 m decrease 6MWD		Change >30 m decrease 6MWD		Crude predicted mean difference (95% CI)^a	p interaction^b	Adjusted^c predicted mean difference (95% CI)^d	p interaction^d
	No. of patients	Observed mean value ± SE	No. of patients	Observed mean value ± SE
FEV1						0.06		0.06
1 yr	222	51% ± 0.9%	69	50% ± 1.8%	−1.6% (−6.4 to 3.1)		−1.6% (−6.0 to 2.8)
2 yr	217	50% ± 1.0%	65	47% ± 1.9	−2.2% (−7.0 to 2.6)		−2.2% (−6.6 to 2.2)
3 yrs	213	49% ± 1.0%	64	46% ± 1.9	−2.8% (−7.8 to 2.1)		−2.8% (−7.3 to 1.8)
5 yrs	172	48% ± 1.2%	50	46% ± 2.3	−2.8% (−9.6 to 1.6)		−4.0% (−9.2 to 1.3)
p trend		<0.001		<0.001

6MWD, 6-minute walk distance.

^b Interaction between 30 m decrease in 6MWD and time.

^c Adjusted for baseline values of: sex, age, walking distance, mMRC dyspnea scale, Charlson comorbidity index and smoking status and participation in pulmonary rehabilitation.

^d One year change >30 m in 6MWD vs one change <30 m in 6DWD.