Usefulness of the 6-minute walk test as a screening test for pulmonary arterial enlargement in COPD

Figures & data

Figure 1 Patient flow diagram.

Abbreviations: PA, pulmonary artery; A, aorta.

Figure 2 Measurement of the PA and A diameters at the PA bifurcation.

Notes: (A) PA:A ≤1, (B) PA:A >1. The κ-values for intraobserver and interobserver agreements for detecting PA:A >1 were 0.87 (95% confidence interval 0.74–0.99) and 0.75 (95% confidence interval 0.58–0.91), respectively.
Abbreviations: PA, pulmonary artery; A, aorta.

Table 1 General characteristics of the patients with PA:A ≤1 and PA:A >1

Variable	PA:A ≤1 (n=35)	PA:A >1 (n=29)	P-value
Age (years)	71 (66.5–76.5)	78 (70–79)	0.06b
Male sex (%)	28 (80)	22 (75.9)	0.76a
Body mass index (kg/m^2)	21.1 (19.5–23.4)	22.4 (20.4–24.7)	0.33b
Smoking history (pack-years)	40 (26.8–60)	40 (35–50)	0.49b
PE	0	1 (3.4%)	0.45a
SAS	1 (2.8%)	0	1a
GOLD (0/1/2/3/4)	14/3/13/4/1	7/2/8/11/1	0.14a
mMRC (0/1/2/3/4)	1/7/22/5/0	0/2/21/5/1	0.36a
COPD Assessment Test (points)	15 (8–22)	14 (11.5–22)	0.62b
BODE index (points)	2 (1–3)	4 (2–5)	<0.001b
FEV1 (% of predicted value)	71.6 (60.5–80.8)	52.6 (39.6–72.1)	0.013b
FVC (% of predicted value)	82.3 (50.3–93.6)	75.8 (42.7–86)	0.04b
FEV1:FVC ratio	68 (61–73.3)	53.8 (48.8–69.4)	0.023b
DLCO (% of predicted value)	72.5 (55.5–82.9)	44.6 (37.7–49.6)	0.005b
RV/TLC (%)	43.7 (37.4–48.6)	48.8 (39.8–53.8)	0.12b
LAA (%)	6.8 (2.8–14.7)	25.4 (11.3–33.4)	<0.001b
Admission for exacerbation	1 (2.9)	7 (24.1)	0.019a
6MWD (m)	450 (400–510.5)	325 (238–446)	<0.001b
Baseline SpO2 (%)	97 (95–97.5)	95 (93–96)	0.001b
Lowest SpO2 (%)	92 (91–94)	86 (84–88)	<0.001b
Baseline PR (bpm)	74 (69.5–81.5)	77 (70–88)	0.28b
Highest PR (bpm)	108 (98–115.5)	112 (108–120)	0.014b
Baseline modified Borg Scale	0 (0–0)	0 (0–0.5)	0.1b
Highest modified Borg Scale	2 (0–5)	5 (2–5)	0.04b
Diameter of aorta (cm)	3.7 (3.4–3.9)	3.5 (3.3–3.7)	0.2b
Diameter of pulmonary artery (cm)	2.9 (2.7–3.3)	3.7 (3.5–3.9)	0.002b

Notes: Data presented as counts (%) or median (interquartile range). P-values calculated by

a Pearson’s χ² test (categorical) and

b Mann–Whitney U-test (continuous).

Abbreviations: PA, pulmonary artery; A, aorta; PE, pulmonary embolism; SAS, sleep apnea syndrome; GOLD, Global initiative for chronic Obstructive Lung Disease; mMRC, modified Medical Research Council; BODE, body mass index, obstruction (airflow), dyspnea, and exercise; FEV₁, forced expiratory volume in 1 second; FVC, forced vital capacity; D_LCO, diffusion capacity of lung carbon monoxide; RV, residual volume; TLC, total lung capacity; LAA, low-attenuation area; 6MWD, 6-minute walking distance; SpO₂, peripheral oxygen saturation; PR, pulse rate.

Table 2 Linear relationships between PA:A ratio, lung function, and index of 6MWT

Variable	r	r^2	P-valuea
Lowest SpO2 (%)	−0.68	0.46	<0.001
DLCO (% of predicted value)	−0.61	0.37	<0.001
6MWD (m)	−0.43	0.18	<0.001
BODE index	0.41	0.17	<0.001
Baseline SpO2 (%)	−0.36	0.13	0.003
LAA (%)	0.36	0.13	0.004
FVC (% of predicted value)	−0.34	0.12	0.006
FEV1 (% of predicted value)	−0.29	0.08	0.019
Highest PR (bpm)	0.26	0.07	0.035

Note:

a P-value calculated by Spearman’s rank-correlation coefficient.

Abbreviations: PA, pulmonary artery; A, aorta; 6MWT, 6-minute walking time; SpO₂, peripheral oxygen saturation; D_LCO, diffusion capacity of lung carbon monoxide; 6MWD, 6MW distance; BODE, body mass index, obstruction (airflow), dyspnea, and exercise; LAA, low-attenuation area; FVC, forced vital capacity; FEV₁, forced expiratory volume in 1 second; PR, pulse rate.

Table 3 Cutoff points and ROC-curve parameters for the prediction of PA:A ratio >1

Variable	Best cutoff	AUC (95% CI)	Sensitivity	Specificity
Lowest SpO2 (%)	89	0.94 (0.88–1)	86.2	93.1
DLCO (%)	51	0.87 (0.78–0.96)	79.3	85.7
6MWD (m)	388	0.75 (0.62–0.87)	69	80
BODE index (points)	4	0.74 (0.61–0.87)	58.6	85.7

Abbreviations: ROC, receiver-operating characteristic; PA, pulmonary artery; A, aorta; AUC, area under the curve; SpO₂, peripheral oxygen saturation; D_LCO, diffusion capacity of lung carbon monoxide; 6MWD, 6-minute walking distance; BODE, body mass index, obstruction (airflow), dyspnea, and exercise; CI, confidence interval.

Table 4 The performance data of 6MWT and lung function for PA:A ratio >1

Variable	Sensitivity (95% CI)	Specificity (95% CI)	PPV (95% CI)	NPV (95% CI)	+LR (95% CI)	−LR (95% CI)
SpO2 <90%	93.1 (77.2–99.2)	94.3 (80.8–99.3)	93.1 (77.2–99.2)	94.3 (80.8–99.3)	16.2 (4.2–62.8)	0.07 (0.02–0.28)
SpO2 ≤88%	79.3 (60.3–92)	97.1 (85.1–99.9)	95.8 (78.9–99.9)	85 (70.2–94.3)	27.7 (4–193.3)	0.21 (0.1–0.44)
ΔSpO2 ≥4%	96.6 (82–99.9)	45.7 (28.8–63.4)	59.6 (44.3–73.6)	94.1 (71.3–99.9)	1.8 (1.3–2.4)	0.08 (0.01–0.54)
Baseline SpO2 ≤95%	58.6 (38.9–76.5)	71.4 (53.7–85.4)	63 (42.4–80.6)	67.6 (50.2–82)	2.1 (1.1–3.8)	0.58 (0.36–0.94)
6MWD <350 m	58.6 (38.9–76.5)	82.9 (66.4–93.4)	73.9 (51.6–89.8)	70.7 (54.5–83.9)	3.4 (1.6–7.5)	0.5 (0.32–0.8)
SpO2 <90% and 6MWD <350 m	51.7 (32.5–70.6)	97.1 (85.1–99.9)	93.8 (69.8–99.8)	70.8 (55.9–83)	18.1 (2.5–128.9)	0.5 (0.34–0.73)
SpO2 ≤88% and 6MWD <350 m	51.7 (32.5–70.6)	100 (85.5–100)	100 (69.8–100)	71.4 (56.7–83.4)	–	0.48 (0.33–0.7)
ΔSpO2 ≥4% and 6MWD <350 m	55.2 (35.7–73.6)	91.4 (76.9–98.2)	84.2 (60.4–96.6)	71.1 (55.7–83.6)	6.4 (2.1–19.9)	0.49 (0.32–0.74)
FEV1 <80%	79.3 (60.3–92)	31.4 (16.9–49.3)	48.9 (34.1–63.9)	64.7 (38.3–85.8)	1.2 (0.9–1.5)	0.65 (0.28–1.56)
FEV1 <50%	44.8 (26.4–64.3)	85.7 (69.7–95.2)	72.2 (46.5–90.3)	65.2 (49.8–78.6)	3.1 (1.3–7.8)	0.64 (0.45–0.92)
DLCO <50%	75.9 (56.5–89.7)	85.7 (69.7–95.2)	81.5 (61.9–93.7)	81.1 (64.8–92)	5.3 (2.3–12.3)	0.28 (0.15–0.54)

Abbreviations: 6MWT, 6-minute walking test; PA, pulmonary artery; A, aorta; PPV, positive predictive value; NPV, negative PV; LR, likelihood ratio; SpO₂, peripheral oxygen saturation; 6MWD, 6MW distance; FEV₁, forced expiratory volume in 1 second; D_LCO, diffusion capacity of lung carbon monoxide.

Figure 3 Receiver-operating characteristic curve with lowest SpO₂ during 6MWT identifying PA:A ratio >1.

Abbreviations: SpO₂, peripheral oxygen saturation; 6MWT, 6-minute walking test; PA, pulmonary artery; A, aorta; AUC, area under the curve.