Masking the 6-Minute Walking Test in the COVID-19 Era

The pandemic due to the coronavirus disease (COVID-19) has changed every aspect of life. Different measures have been implemented in pulmonary function test laboratories to ensure patient and staff safety (1–6); however, there are technical aspects that need to be clarified. One of these is whether face mask use affects the distance a subject can walk during the 6-minute walking test (6-MWT) as well as the oxygen saturation (Sp_O2), heart rate (HR), dyspnea, and fatigue. The aim of this study was to investigate whether two types of face mask modified the walking distance during the 6-MWT among survivors of COVID-19.

Methods

Subjects over 18 years of age who were hospitalized for pneumonia due to a polymerase chain reaction–confirmed diagnosis of COVID-19 who were at least 30 days from discharge were invited to participate; those who could not walk were excluded from the study. It was decided at random (according to a table of random numbers) whether the subject was entered to the surgical or N95 face mask group and whether the first walk was done with or without a face mask. The following two types of face masks were used: the surgical type, which is a pleated spunbond/melt-blown/spunbond nonwoven fabric made from 100% polypropylene (Mds Medical Dress Supplier) and the folding N95 particle face mask, which is made of polypropylene and 3M polyester (model 9010, 3M).

All participants performed two 6-MWTs according to American Thoracic Society/European Respiratory Society standards, with a recovery time of 30 minutes between the two 6-MWTs (7).

The National Institute of Respiratory Diseases “Ismael Cosío Villegas” in México City science and bioethics committee approved the study (C16–20), and participants signed the informed consent form. Anthropometric data are presented as means and standard deviations or as number and percentage; Student’s t tests, Mann-Whitney U tests, and χ² tests were used to compare groups. Analysis of variance was used to analyze the results of the 6-MWT with or without the face mask, and Spearman’s correlation coefficient (r_sp) and concordance correlation coefficient (CCC) were used to search for associations. Multivariable linear regression analysis was performed to investigate whether the difference walked in meters between both 6-MWTs (with and without face mask [dependent variable]) was predicted by some variables selected by their potential influence in the outcome, such as using or not using the face mask, the type of face mask, the presence of desaturation, the degree of dyspnea and fatigue (10-grade Borg Scale) (8), overweight or obese status, and tobacco use (independent variables). The Enright and colleagues’ reference equation, adjusted for body mass index, was used to calculate the percentage predicted for the 6-MWT (9).

Results

We included 77 individuals aged 44 (±12) years, with 49 (64%) men. Table 1 shows their general characteristics; 41 (53%) subjects were assigned to the N95 face mask group. Without wearing the face mask, 43 (56%) subjects had a decrease in Sp_O2 of >4%, with 29 (67%) of them experiencing a desaturation event of an Sp_O2 ≤88% during the 6-MWT. No differences were observed in the meters walked, Sp_O2, HR, dyspnea, or fatigue, between tests with or without the face mask (surgical or N95) (Table 2).

Table 1.

General characteristics of individuals

	Surgical (n = 36)	N95 (n = 41)
Age, y	45.9 ± 11.7	43.3 ± 12.3
Sex, M, n (%)	23 (64)	26 (63)
Weight, kg	76.2 ± 11.6	78.5 ± 13.5
Height, cm	164.1 ± 6.8	164 ± 7.7
BMI, kg·m2	28.2 ± 3.6	29.1 ± 4.4
Overweight or obese, n (%)	31 (86.1)	37 (90)
High blood pressure, n (%)	4 (11.1)	5 (12.2)
Heart disease, n (%)	1 (2.8)	1 (2.4)
Diabetes, n (%)	4 (11.1)	6 (14.6)
Gastritis or GERD, n (%)	1 (2.8)	3 (7.3)
Tobacco smoker, n (%)	2 (5.6)	7 (17.1)
Asthma, n (%)	1 (2.8)	1 (2.4)
COPD, n (%)	1 (2.8)	0

Definition of abbreviations: BMI = body mass index; COPD = chronic obstructive pulmonary disease; GERD = gastroesophageal reflux disease; SD = standard deviation.

Data are expressed in mean ± SD (minimum to maximum) unless otherwise specified. All parameters, P > 0.05. Anthropometric comparisons were made between the surgical and N95 face mask groups (Student’s t test or Mann-Whitney U test). The χ² test was used for categorical variables

Table 2.

6-MWT differences with or without a surgical or N95 face mask

	Surgical Facemask (n = 36)		N95 Facemask (n = 41)
	With Face Mask	Without Face Mask	With Face Mask	Without Face Mask
6-MWD, m	516.2 ± 77.8	517.6 ± 90.6	535.5 ± 78.9	537 ± 73.6
6-MWD, % of predicted	86.7 ± 18.8	86.5 ± 16.5	88.6 ± 17	88.5 ± 17.2
Basal SpO2, %	92.4 ± 1.6	93 ± 1.9	93 ± 1.6	93 ± 1.8
Lowest SpO2, %	88.2 ± 3.2	88.3 ± 3.5	87.8 ± 3.9	87.5 ± 4.1
Final SpO2, %	90.3 ± 3.5	90.5 ± 3.7	89.4 ± 4.3	89.4 ± 4.3
1-min SpO2, %	92.6 ± 2.8	93.1 ± 2.1	91.8 ± 3.3	92.1 ± 2.9
3-min SpO2, %	93.3 ± 1.8	93.5 ± 1.6	94 ± 1.5	93.6 ± 1.7
Basal HR, bpm	83 ± 12	83 ± 12	82 ± 13	82 ± 14
Highest HR, bpm	122 ± 15	123 ± 16	124 ± 16	123 ± 13
Final HR, bpm	114 ± 19	116 ± 18	121 ± 17	118 ± 17
1-min HR, bpm	98.1 ± 17.6	95.7 ± 16.7	101.1 ± 17.6	100.8 ± 18.5
3-min HR, bpm	91.5 ± 14.2	91.6 ± 15.5	92.6 ± 14.2	92.4 ± 14.7
Dyspnea basal score	0.2 ± 0.7	0.3 ± 0.7	0.3 ± 0.4	0.3 ± 0.6
Dyspnea final score	1.2 ± 1.2	1.0 ± 1.1	1.8 ± 1.5	1.2 ± 1.4
Dyspnea 1-min score	1 ± 1.1	0.9 ± 1	1.3 ± 1.2	1 ± 1.2
Dyspnea 3-min score	0.6 ± 0.8	0.5 ± 0.8	0.7 ± 0.7	0.5 ± 0.7
Fatigue basal score	0.6 ± 1.0	0.6 ± 1.3	0.6 ± 1.0	0.6 ± 1.0
Fatigue final score	1.8 ± 2.2	1.8 ± 1.2	2.4 ± 2.0	2.3 ± 2.0
Fatigue 1-min score	1.6 ± 1.9	1.7 ± 2.1	2.1 ± 1.5	1.8 ± 1.7
Fatigue 3-min score	1.1 ± 1.8	1.1 ± 1.9	1.3 ± 1.2	1.3 ± 1.4

Definition of abbreviations: 6-MWD = 6-minute walking distance; 6-MWT = 6-minute walking test; HR = heart rate; Sp_O2 = oxygen saturation measured by pulse-oximeter.

Data are presented in mean ± SD. The 1-minute and 3-minute scores correspond with measurements at 1 minute and 3 minutes after the 6-MWT. The dyspnea and fatigue scores were measured with the Borg Scale (8). The 6-MWD% predicted is the percentage predicted of the 6-MWD according to Enright and Sherrill (9). All parameters P > 0.05 between with and without the face mask according to the type of face mask (analysis of variance).

Figure 1 shows the association (r _sp = 0.90, P < 0.001) and agreement (CCC = 0.94) between the 6-MWT with and without the face mask. The mean difference between the two walks was −0.65 (±28) m, as shown in Figure 1B; the 95% limit of agreement was −55 to 54 m. In 12 (16%) subjects, the difference between walks was >30 m, which was considered the minimum clinically significant limit (MCSL) for changes in the 6-MWT (7). The multivariable analysis showed that this difference was independent of the type of face mask used (P = 0.74), presence of desaturation (P = 0.63), degree of dyspnea before (P = 0.75) and after (P = 0.5) the 6-MWT, obesity (P = 0.89), and tobacco use (P = 0.93).

Figure 1.

(A) Spearman correlation (r _sp = 0.9, P < 0.001) and (B) concordance correlation coefficient (CCC) between the meters walked during the 6-minute walking test (6-MWT) with and without face mask (CCC = 0.94). The small dashed lines represent the minimal clinically significant difference (±30 m) proposed in the evaluation of patients with chronic lung diseases, the dashed and dotted lines represent the 95% limits of agreement, and the large dashed lines represent the average of difference between the walking meters during the 6-MWT with and without the face mask. Open circles = surgical face mask; open triangles = N95 face mask. 6-MWD = 6-minute walking distance; w/wo = with/without.

When the analysis was conducted according to the type of face mask used during the walk, distances walked with or without face mask were very similar, as follows: with the surgical type face mask, r _sp = 0.91 (P < 0.001), CCC = 0.94, mean of the differences = −1.36 (±28.7) and 95% confidence interval = −57.7 to 55 m, whereas with the N95 face mask, r _sp = 0.93 (P < 0.001), CCC = 0.94, mean of the differences = −0.02 (±27.2), and 95% confidence interval = −53.4 to 53.3 m.

Discussion

The main finding of this study is that the mean difference in the 6-MWD when using either a surgical or N95 face mask compared with not wearing a face mask was −0.65 m, with a broad 95% limit of agreement; 84% of the subjects had agreement within MCSL (±30 m).

The 6-MWT aims to measure the distance that a subject can walk during 6 minutes in a 30-m corridor (7, 10–12). It is indicated in the diagnosis, prognosis, and monitoring of individuals with chronic lung diseases (13–15). Although its importance is based on the analysis of the effects of the treatment on the meters walked in 6 minutes, the 6-MWT also allows for measuring the functional status through other parameters, such as the Sp_O2, HR, dyspnea, fatigue, and blood pressure (7, 13).

Recently, some authors have suggested the usefulness of the 6-MWT in the initial diagnosis of COVID-19 (16), for early discrimination of mild from severe cases; however, there are concerns about the likely transmission of COVID-19 via the air to healthcare personnel and individuals with other chronic lung diseases who attend pulmonary function test laboratories (17, 18). Therefore, new preventive measures have been proposed, one of which is the use of face mask to reduce the risk of infection (16).

Traditionally, the 6-MWT is performed without a face mask, and the use of a face piece that covers the mouth and nostrils can increase the CO₂ concentrations to 3.0% (±0.5%), especially in subjects performing low-intensity exercise (19, 20). In this sense, using an N95 respirator may be associated with increased breathing effort, sensation of suffocation, and altered results during the 6-MWT. As far as we know, there is only one study that has evaluated the use of a surgical face mask during the 6-MWT (21); these authors found no difference in the meters walked (P = 0.99) in healthy subjects, and the only significant difference was in the degree of dyspnea (P < 0.001). Our study found that in 84% of the participants, the difference in meters walked was within the MCSL (±30 m) (13, 22–24), and no differences were obtained in the degree of dyspnea (25).

This study has limitations. Although this is a cohort of individuals who recovered from COVID-19, the number of participants in each group could be higher.

Conclusions

Surgical or N95-type face masks can be used during the 6-MWT, especially among those recovering from COVID-19, and the results regarding the meters walked as well as other variables, such as Sp_O2, HR, and degree of dyspnea, are similar to those obtained without using a face mask.