The relationship between steps of 6MWT and COPD severity: a cross-sectional study

Figures & data

Table 1 Demographic characteristics, pulmonary function and HRQoL questionnaire scores of 70 COPD patients

Characteristics	Total
	(n=70)
Age (years)	63.0 (54.0–68.0)
BMI (kg/m^2)	22.8 (20.9–24.8)
DLCO (% predicted)	76.7 (55.6–91.5)
TLC (% predicted)	86.3±11.1
RV (% predicted)	92.2 (80.0–109.3)
RV/TLC (%)	42.1±9.3
R5 (kPa/L/s)	0.4 (0.3–0.5)
R20 (kPa/L/s)	0.3±0.1
X5 (kPa/L/s)	−0.1 (−0.2 to −0.1)
mMRC grade	0.0 (0.0–1.0)
SGRQ score	17.5 (9.0–36.0)
CCQ score	1.6 (1.1–2.3)
CAT score	13.5 (11.0–18.3)
D6MWT (m)	533.0 (495.8–562.5)
S6MWT (step)	739 (701–756)

Note: Results were expressed in the form of mean ± SD or median (IQR 25–75) as appropriate.

Abbreviations: BMI, body mass index; CAT, Chronic Obstructive Pulmonary Disease Assessment Test; CCQ, Clinical Chronic Obstructive Pulmonary Questionnaire; D6MWT, distance of 6-minute walk test; DL_CO, diffusing capacity of the lungs for carbon monoxide; HRQoL, health-related quality of life; mMRC, modified Medical Research Council dyspnea scale; R₂₀, reactance at 20 Hz; R₅, reactance at 5 Hz; RV, residual volume; S6MWT, steps of 6MWT; SGRQ, St George’s Respiratory Questionnaire; TLC, total lung capacity; X₅, reactance at 5 Hz.

Table 2 Comparison between pre- and post-6MWT pulmonary function

	Pre-6MWT	Post-6MWT	P-value
IC (% predicted)	79.6±19.1	81.1 (69.3–97.0)	0.031*
FVC (% predicted)	88.3±19.3	88.0±18.4	0.694
FEV1 (% predicted)	68.2±23.5	68.3±23.4	0.917
FEV1/FVC (%)	56.8 (42.4–65.2)	57.6 (46.0–66.3)	<0.001*
SpO2 (%)	96.0 (95.0–97.0)	97.0 (96.0–98.0)	0.020*
Pulse (n/min)	78.0 (70.8–88.3)	98.9±15.2	<0.001*

* Notes: Statistical significance. Results were expressed in the form of mean ± SD or median (IQR 25–75) as appropriate.

Abbreviations: 6MWT, 6-minute walk test; IC, inspiratory capacity.

Table 3 Relationship between D6MWT or S6MWT and other parameters

Functional parameters	D6MWT (m)		S6MWT (step)
	ρ	P-value	ρ	P-value
Age (years)	−0.485	<0.001*	−0.229	0.057
Pre-6MWT IC (% predicted)	0.145	0.230	0.338	0.004*
Post-6MWT IC (% predicted)	0.160	0.189	0.359	0.002*
Pre-6MWT FVC (% predicted)	0.305	0.010*	0.363	0.002*
Post-6MWT FVC (% predicted)	0.267	0.025*	0.296	0.013*
Pre-6MWT FEV1 (% predicted)	0.330	0.005*	0.426	<0.001*
Post-6MWT FEV1 (% predicted)	0.310	0.009*	0.388	0.001*
Pre-6MWT FEV1/FVC (%)	0.373	0.001*	0.406	<0.001*
Post-6MWT FEV1/FVC (%)	0.293	0.014*	0.373	0.001*
Pre-6MWT SpO2 (%)	0.182	0.132	0.289	0.015*
Post-6MWT SpO2 (%)	0.245	0.041*	0.201	0.095
DLCO (% predicted)	0.411	<0.001*	0.548	<0.001*
RV/TLC (%)	−0.326	0.006*	−0.286	0.016*
R20 (kPa/L/s)	0.244	0.042*	0.171	0.157
X5 (kPa/L/s)	0.264	0.027*	0.336	0.004*
mMRC grade	−0.439	<0.001*	−0.451	<0.001*
SGRQ score	−0.332	0.005*	−0.344	0.004*
CCQ score	−0.435	<0.001*	−0.370	0.002*
CAT score	−0.393	0.001*	−0.431	<0.001*

* Note: Statistical significance.

Abbreviations: CAT, Chronic Obstructive Pulmonary Disease Assessment Test; CCQ, Clinical Chronic Obstructive Pulmonary Questionnaire; D6MWT, distance of 6-minute walk test; DL_CO, diffusing capacity of the lungs for carbon monoxide; IC, inspiratory capacity; mMRC, modified Medical Research Council dyspnea scale; RV, residual volume; R₂₀, reactance at 20 Hz; S6MWT, steps of 6MWT; SGRQ, St George’s Respiratory Questionnaire; TLC, total lung capacity; ρ, correlation coefficient; X₅, reactance at 5 Hz.

Figure 1 Correlation between D6MWT or S6MWT and IC in COPD.

Notes: *Statistical significance. Neither pre-6MWT IC (A) nor post-6MWT IC (C) is significantly correlated with distance of 6MWT. Both pre-6MWT IC (B) and post-6MWT IC (D) are significantly correlated with steps of 6MWT.
Abbreviations: 6MWT, 6-minute walk test; D6MWT, distance of 6MWT; IC, inspiratory capacity; S6MWT, steps of 6MWT.

Table 4 Stepwise multiple linear regression analysis for D6MWT

Variables	Coefficient (95% CI)	Standardized β	P-value	VIF
Intercept	741.063 (651.726–830.400)
mMRC grade	−17.986 (−34.754 to −1.218)	−0.288	0.036*	2.245
Age (years)	−2.328 (−3.516 to −1.140)	−0.357	<0.001*	1.036
CAT score	−4.069 (−7.602 to −0.536)	−0.306	0.025*	2.198
Adjusted R^2=0.445

* Notes: Statistical significance. Variables: age, FVC, FEV₁, DL_CO, RV/TLC, R₂₀, X₅, mMRC grade, SGRQ score, CAT score and CCQ score.

Abbreviations: β, regression coefficient; CAT, Chronic Obstructive Pulmonary Disease Assessment Test; CCQ, Clinical Chronic Obstructive Pulmonary Questionnaire; D6MWT, distance of 6-minute walk test; DL_CO, diffusing capacity of the lungs for carbon monoxide; mMRC, modified Medical Research Council dyspnea scale; R₂₀, reactance at 20 Hz; RV, residual volume; SGRQ, St George’s Respiratory Questionnaire; TLC, total lung capacity; VIF, variance inflation factor; X₅, reactance at 5 Hz.

Table 5 Stepwise multiple linear regression analysis for S6MWT

Variable	Coefficient (95% CI)	Standardized β	P-value	VIF
Intercept	699.972 (634.661–765.283)
DLCO (% predicted)	1.126 (0.576–1.677)	0.429	<0.001*	1.306
CAT score	−3.571 (−5.812 to −1.329)	−0.334	0.002*	1.306
Adjusted R^2=0.417

* Notes: Statistical significance; r: correlation coefficient. Variables: IC, FVC, FEV₁, SpO₂, DL_CO, RV/TLC, X₅, mMRC grade, SGRQ score, CAT score and CCQ score.

Abbreviations: β, regression coefficient; CAT, Chronic Obstructive Pulmonary Disease Assessment Test; CCQ, Clinical Chronic Obstructive Pulmonary Questionnaire; DL_CO, diffusing capacity of the lungs for carbon monoxide; IC, inspiratory capacity; mMRC, modified Medical Research Council dyspnea scale; RV, residual volume; S6MWT, steps of 6-minute walk test; SGRQ, St George’s Respiratory Questionnaire; TLC, total lung capacity; VIF, variance inflation factor.