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In 102 healthy Caucasians, 20–50 years old, we investigated the effect of anthropometrics on the 6-min walk test (6MWT), in order to provide reference values for walk distance (6MWD), oxygen saturation (SpO2), pulse rate (PR), respiratory rate (RR), breathlessness perception (VAS) and for the walking distance and body weight product (DW).
The mean 6MWD and DW values were 593±57 and 638±44m (P<0.01) and 35,030±5306 and 48,882±6555kgm (P<0.01), respectively for women and for men. While walking, SpO2 remained unaltered and subjects reached 67±10% of their maximal predicted heart rate and a RR mean value of 19±4bpm. VAS ratings were significantly higher in females as compared to males (24±15 vs. 18±5mm, P<0.05), however, when corrected for PR change while walking, they were not different. The equation by stepwise multiple regression analysis included height, age and gender for the 6MWD and accounted for 42% of the total variance.
This study confirms the relevant effect of anthropometrics on walking capacity and suggests that when rating dyspnea, the change in heart rate during walking should be considered.
In the last decade, the 6-min walk test (6MWT) has been increasingly used to assess functional exercise performance across various patient populations.
The effects of oxitropium bromide on exercise performance in patients with stable chronic obstructive pulmonary disease. A comparison of three different exercise tests.
Six minute corridor walk test as an outcome measure for the assessment of treatment in randomized, blinded intervention trials of chronic heart failure: a systematic review.
The increasing acceptance of this test is due its simplicity and that it does not require sophisticated equipment and can be easily performed by even the most severely debilitated patients. Moreover, the 6MWT better reflects activities of daily living than other walk tests.
published reference values for the 6MWD for subjects 20–40 years old.
Accordingly, the aim of the present study was to investigate in healthy subjects, 20–50 years old, the effect of demographics and anthropometrics on the 6MWT, performed according to the standardized approach provided by the ATS guidelines.
Furthermore, we aimed to provide reference values in such subjects for the 6MWD and other primary variables measured in the 6MWT, such as oxygen saturation (SpO2), pulse rate (PR), respiratory rate (RR), and breathlessness perception. In this study, we also considered a derived variable of the 6MWT, the product of walking distance and body weight (DW), which is considered to mimic the work of walking.
We studied a sample of 102 healthy subjects (54 females) from all decades of age between 20 and 50 years (mean±standard deviation (sd) age: 34±9 years). They were volunteers recruited from the University campus and from the surrounding community. All subjects were free from injury and had no history of hospitalization or chronic disease influencing their exercise capacity. Moreover, they were not involved in any competitive sport and they were lifetime non-smokers. All subjects gave informed consent to participate in the study.
Lung function test
Lung function was measured by a flow-sensing spirometer and a body plethysmograph connected to a computer for data analysis (Vmax 22 and 6200, Sensor Medics, Yorba Linda, USA). Baseline total lung capacity (TLC), forced expiratory volume in 1s (FEV1), vital capacity (VC), and FEV1/VC ratio were recorded. Carbon monoxide transfer capacity (TLCO) was measured by the single breath method and considered valid only if the inspiratory volume was at least 90% of VC. At least three measurements were made for each lung function variable to ensure reproducibility. Predicted values of lung volumes and expiratory flows as well as TLCO were obtained from regression equations by Quanjer et al.
All subjects received the same instructions before the walk and were encouraged by the investigator who repeated set phrases every 30s during the walk. Each subject underwent the 6MWT in an undisturbed 30-m indoor, level, hospital corridor. The second 6MWT was performed in the same manner as the first, following a rest of at least 60min. The 6MWD covered during the test was recorded in meters. Results from the second walk were only used for analysis to allow for any learning effect.
Before and immediately after the 6MWT, all subjects rated the magnitude of their perceived breathlessness on an interval scale, which was a 100-mm horizontal visual analogue scale (VAS).
The VAS consisted of a horizontal ruler without any mark on the subject's side with the words “not at all breathless” and “extremely breathless” on the left and right end, respectively. The subject had to indicate his breathlessness perception at the moment of the assessment, by moving a marker along the ruler. Breathlessness perception ratings were expressed in mm from 0 to 100 and corresponded to the distance of the marker from the left end of the VAS. In each subject, resting and after-walk RR (bpm) were also recorded before and immediately after the 6MWT.
The SpO2 (%) and PR (bpm) were continuously monitored from 5min before the walk until test completion, as well as 5min after completion, or until recovery of the baseline value by using a lightweight (0.3kg) portable pulse oximeter (Healthdyne, Model 920M, Marietta, GA, USA). This device was carried by each subject with a shoulder strap and a finger probe and was applied to the non-dominant hand. The oxygen saturation and heart rate readings were recorded in the oximeter memory every 10s. For each subject, the resting SpO2 values (resting SpO2, %), such as the average of the SpO2 readings taken before the walk, and the mean saturation recorded during the walk (walking SpO2, %) were noted. The resting PR values (resting PR, bpm) which were the average of the PR readings taken before the walk, the mean PR recorded during the walk (walking PR, bpm) and the maximum PR sustained for more than 10s (maximum PR, bpm) during the walk were also noted. The predicted maximum PR was calculated by the following equation: 210–0.66×age.
All pulmonary function test results are expressed as percent of predicted value or as absolute value. Data are reported as mean±sd and differences in numerical data between groups were determined by the unpaired t-test. Furthermore, relationships were estimated by the Pearson correlation coefficient (r). Stepwise multiple regression analysis was used to determine the best predictor variables for the 6MWT dependent variables. Percentage of total variance in the dependent variable, accounted for by the predictor variables, is expressed as the adjusted square of the multiple correlation coefficient (r2). A P-value of less than 0.05 was taken as significant.
Results
Lung function values of healthy subjects are summarized in Table 1. The DW was calculated as the product of the walk distance (in m)×body weight (in kg).
Table 1Lung function values of the 102 study subjects.
The mean 6MWD was 614±56m, ranging from 459 to 738m, and the mean DW was 41,548±9112kgm, ranging from 26,100 to 66,976kgm. Oxygen saturation remained unaltered throughout the walk. Mean resting and walking SpO2 values were, respectively, 98±0.9% and 97±1.3%. The mean change in SpO2 was 1.0±1.1%.
PR and RR were significantly affected by the walk. Mean resting, walking and maximum PR values were, respectively, 78±11, 108±19 (P<0.001 vs. resting values), and 126±19bpm (P<0.001 vs. resting values). Moreover, during the walk, subjects reached 67±10% of their maximal predicted heart rate. Resting and after-walk RR were, respectively, 14±3 and 19±4bpm (P<0.001). In all subjects, the mean value of ratings on the VAS after the walk was 21±15mm.
Gender significantly affected the 6MWD (Table 2). On average, the 6MWD and the DW values were 45m and 13,582kgm greater in male subjects, when compared to female subjects. Mean resting, walking and maximum PR values were significantly higher in female subjects, as compared to male subjects. Moreover, ratings on the VAS were significantly higher in females as compared to males, however, when corrected for the maximum PR percentage change of resting PR (VAS/PRchange), there was no difference between females and males.
Table 2Personal details and 6-MWT measures of the study subjects.
Women (n=54)
Men (n=48)
Age, years
33±9
36±8
Height, cm
164±7
176±7**
Weight, kg
59±8
77±9**
BMI, kg/m2
22±3
25±2**
6MWD, m
593±57
638±44**
DW, kgm
35,030±5306
48,882±6555**
PR base, bpm
81±10
75±10**
PR mean, bpm
115±18
101±17**
PR maximal, bpm
132±18
119±18**
VAS, mm
24±15
18±15*
VAS/PRchange
0.65±0.41
0.53±0.48
Values are expressed as mean±sd; **P< 0.01, *P< 0.05.
In all subjects, the 6MWD was inversely and directly related, respectively, to age (r=−0.42; P<0.001) and height (r=0.46; P<0.001) (Fig. 1). After-walk VAS was directly related to after-walk RR (rs=0.27, P<0.01) and to PR max (rs= 0.24, P<0.05).
Figure 1Relationship between 6MWD and age (upper panel) and height (lower panel) in 102 healthy subjects 20–50 years old. The linear regression line is superimposed, surrounded with the 95% confidence interval lines for the regression line. r=Pearson correlation coefficient.
The regression equation generated by stepwise multiple regression analysis for the 6MWD included height, age and gender. This model accounted for 42% of the total variance for the 6MWD (Table 3). Moreover, multiple regression analysis for the ratings on the VAS after walking could not account for more than 10% of the total variance, the main predictor variable being RR after walking.
Table 3Predicting model for walk distance (6MWD, m).
The present study provides reference values for the primary variables measured in the 6MWT in healthy subjects between 20 and 50 years of age. The reference values may be useful to interpret 6MWT performance in patients with significant respiratory or cardiac dysfunction at a relatively young age, such as those with cystic fibrosis, obesity, interstitial lung disease, or dilatative cardiomyopathy. Moreover, the reference values provided by this study apply to the 6MWT protocol proposed by the American Thoracic Society guidelines.
In these guidelines, some recommendations and suggestions concerning technical aspects and quality assurance of the 6MWT are present. Specifically, the recommended length of the corridor is 30m so as to avoid too many turns during the test, one practice walk should be considered to allow for the learning effect and encouragement should be given regularly to ensure maximal motivation and performance. In this regard, previous studies did not take into account some of these methodological aspects of the 6MWT. For example, in the study by Gibbons and colleagues,
As expected, the present study confirmed that several demographic and anthropometric factors can influence the 6MWT performance in healthy subjects. In our model, gender, height and age can reliably predict the 6MWD. Intuitively, a taller height is associated with a longer stride, which probably results in a longer distance walked by taller subjects. Moreover, the shorter distance walked by the older participants in our study may be explained by changes in skeletal muscles, as compared to the younger subjects. A gradual reduction of skeletal muscle mass and strength has been demonstrated in the elderly,
however our findings suggest that the decline in skeletal muscle function could be earlier. Our model can explain only 40% of the variance in the 6MWD, which is in agreement with previous reports.
Potential sources of variance other than age, gender or height should be considered. One potential source of variance may be the different attitudes, beliefs as well as the mood of the participants. Indeed, it has been demonstrated that psychological status is related to exercise capacity both in healthy subjects
Another potential source of variance may be the different peripheral muscle conditioning of the participants, since we included in this study both habitually active and sedentary healthy subjects.
The DW is a simplification of the equation proposed by Cavagna and Margaria
as a measure for assessing the functional capacity during a 6MWT. In COPD patients, DW was superior to the 6MWD, when correlated with oxyhemoglobin saturation changes during walk and with anaerobic threshold and maximal oxygen uptake.
In addition, in a large group of COPD patients, the DW could discriminate better than the 6MWD, patients with low work capacity from those with high work capacity as assessed by bike ergometry.
Therefore, DW is considered to be a measure that reflects the work of walking better than the 6MWD and is suggested to evaluate exercise tolerance, if a cardiopulmonary exercise test is unavailable.
As expected, in our study, DW was higher in males than in females, since males were heavier and covered a longer 6MWD than females.
In this study, we found that both resting and walking PR values were significantly higher in females as compared to males. Previous studies showed gender related differences in heart rate and reported an increase in resting heart rate in women.
The epidemiology of resting heart rate in a national sample of men and women: associations with hypertension, coronary heart disease, blood pressure, and other cardiovascular risk factors.
reported higher heart rates in women following submaximal exercise when compared to those of men.Baroreflex heart rate regulation may be different between women and men
Lastly, the present study confirms the common appreciation that in healthy subjects the 6MWT is a submaximal test, since the maximal heart rate achieved was an average 68%.
In this study, we found that the walking-induced breathlessness was weakly related to after-walking RR and maximum PR, but was not reliably predicted by any demographic and anthropometric variables of the subjects. We also observed a slight gender difference in breathlessness perception, which lacked statistical significance when corrected for heart rate change while walking. Our results mainly support the view that the underlying mechanisms of exertion dyspnea are quite elusive, and that subjectivity is inherently involved in the symptom. However, they also suggest that in healthy subjects, walk-induced dyspnea could be linked to the perception of both walk-induced tachycardia and tachypnea.
In summary, this study provides reference values for the 6MWT variables for Caucasian subjects 20–50 years old. Our results confirm the relevant effect of anthropometric characteristics on walking capacity and suggest that, when rating dyspnea, the change in heart rate while walking should be considered.
Acknowledgment
We gratefully acknowledge the help of Prof. Francesco Donato from the Department of Experimental and Applied Medicine, University of Brescia, Italy, in analysing the data.
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Comparison of short-term functional outcomes following unilateral and bilateral lung volume reduction surgery.
The effects of oxitropium bromide on exercise performance in patients with stable chronic obstructive pulmonary disease. A comparison of three different exercise tests.
Six minute corridor walk test as an outcome measure for the assessment of treatment in randomized, blinded intervention trials of chronic heart failure: a systematic review.
The epidemiology of resting heart rate in a national sample of men and women: associations with hypertension, coronary heart disease, blood pressure, and other cardiovascular risk factors.