The Historical And Clinical Significance Of The 6 Minute Walk Test

1. The Historical and Clinical Significance of the 6-
Minute Walk Test

By Dustin Grinnell


Introduction
The six-minute walk test (6MWT) is a simple, objective, and reproducible measurement of
functional capacity. Instructed to walk as far as they can in six minutes the primary outcome
measurement for a patient is a six-minute walk distance (6MWD).
The test is most commonly used to assess the functional status of patients with cardiovascular
and/or respiratory diseases. It is used commonly in interventions, or before and after certain
treatments. It is often used in rehabilitation programs, to measure physical fitness, or in some
cases as predictive tool of morbidity or mortality.
The purpose of this paper is to provide a qualitative review of the literature surrounding the
6MWT. It will first explore the history and evolution of the test, and then provide an overview of
its current applications in the clinic. Finally it will examine its significance in the clinic defined
by its benefits, limitations, and predictive or prognostic value.

History
The time-distance walk was first introduced by Balke in the 1960’s as a straightforward
and objective measure of functional capacity. The test simply required a patient to walk as far as
they possibly could in a set period of time with the primary outcome measure being the total
distanced walked in the time allotted [1].

2. As a simple measure of physical fitness Cooper developed the specific time of 12 minutes
and was responsible for the introduction of the 12-minute walk test [2]. In 1976 C.R. Macgavin
and colleagues began using the 12-minute walk test as a measure of functional capacity in
patients with chronic bronchitis [3].
In 1982 Butland and colleagues evaluated the possibility of reducing the time patients
would be required to walk. The health statuses of patients under study were often so poor
patients would frequently lack the motivation for even 12 minutes of physical activity. In some
cases patients were simply limited by their symptoms. Their study compared two, six, and twelve
minute walk tests and determined that 6-minute results correlated strongly with the 12-minute.
This ultimately led to the halving of 12-minute time requirement and the introduction of 6MWT
as the standard assessment of functional capacity [4].
The 6MWT was used widely in many test centers and laboratories on patient populations
with chronic respiratory disease and respiratory failure. In 1985 it was first used in patients with
heart disease by Guyatt et al. [5] and Lipkin et al., who also began using it as an exercise test
around the same time [6].
As the test became used more widely slight variations on aspects of the test began to
evolve. From investigator to investigator testing conditions began to change and thus protocols
began to differ. First, the shapes and distances of the walking courses began to vary. Some
investigators used a straight (out and back) track, others a continuous track: circular, oval, or
rectangular. Those who used a straight track (often a hallway or corridor) varied the distance
between start and end points, either 20, 30, or 50 meters. Moreover, while most used a hard-
surfaced hallway or corridor, there were some who performed the 6MWT on a treadmill. Other
differences developed over time including: the use of verbal encouragement, the use of a

3. learning or practice session to familiarize patients with the procedure, the decision to walk with
or behind a patient if they needed assistance or help with supplemental oxygen, and the use and
timing of administration of medications like bronchodilators.
Also, depending on the research question and/or testing population different investigators
began to include various secondary measurements to complement the 6MWD result. These
include: blood pressure, respiratory rate, heart rate, fatigue, blood saturation (using pulse
oximetry), and/or dyspnea (using the Borg Dyspnea Index Scale).
To reduce the variation from protocol to protocol and standardize the 6MWT guidelines
were published by the American Thoracic Society in 2002 [7]. Based on a great deal of
experimental evidence this publication made clear how the 6MWT should be run.
Even with the introduction of a standardized procedure and its thrust to reduce variation
Butland showed that outcomes from the 6MWT would naturally vary with anthropometric
variables, namely: age, gender, height and weight [8]. Taking these anthropometric variables into
account soon reference equations were published, which enabled investigators to make 6MWD
predictions (which could theoretically be compared to the actual distanced walked [9, 10]).
Finally, worth noting, are tests apart from the 6MWT, which assess aerobic and
functional capacity. These include stair climbing, the shuttle walk test [11, 12] the cardiac stress
test [13] and the cardiopulmonary exercise test [14].

Clinical Applications
The 6MWT is primarily a test for measuring functional capacity in patients with
cardiovascular or respiratory diseases. When complications are cardiovascular chronic heart

4. failure is often the main disease under study [15, 16, 17]. For respiratory, patient diseases are
commonly: chronic lung disease [18] or chronic obstructive pulmonary disease [19].
The 6MWT has been shown to be very responsive [20] so it is used often to measure
changes in (functional) status over time. It is therefore used before and after interventions, or
treatments, for patients with the abovementioned cardiorespiratory conditions, or in pulmonary
rehabilitation programs in patients undergoing lung volume reduction [21] or lung
transplantation [22].
The 6MWT is often used at the beginning of rehabilitation programs to determine a
patient’s exercise capacity, and at the end to assess the degree of improvement and/or the success
of a program. Because of the test’s sensitivity future exercise guidelines can be created [23]
based on the outcome of a patient’s walking results.
Recently it has been suggested that the 6MWT could allow patients to measure their own
changes in functional capacity, i.e. be used as a self-administered exercise test [24].
Because the elderly are often limited in functionality and exercise capacity the 6MWT is
a commonly used as a performance-based measure of functional capacity in this population [25]
and as a predictor of disability [26], morbidity and mortality [27-33].

Clinical Significance
Clinical Benefits:
Easy Administration
The 6MWT is both simple for the evaluators in terms of set-up, and for the patients who
ultimately perform it. In March of 2002 The American Thoracic Society published a valuable
reference for investigators interested in using the 6MWT, complete with specific guidelines and

5. a step-by-step protocol. Compared to cardiopulmonary testing which is traditionally expensive
and time-consuming the 6MWT is both inexpensive and relatively quick to perform. In addition,
the test can be carried out in a non-laboratory setting, requires very little equipment and can be
performed without the presence of a physician.

Safe: Submaximal and Self-paced
The submaximal nature and the self-paced aspect of the 6MWT makes it’s a very safe
evaluation. Because the test is relatively low-intensity it allows the assessment of many
individuals who would be otherwise limited by their symptoms during an evaluation of their
functional status or exercise capacity. This is true in the various disease states in which the
6MWT is indicated for and especially true in elderly persons where physical limitations can
prevent more “strenuous” evaluations of functional capacity [34]. In two very large studies
thousands of patients were subjected to the test without any complications or complaints of
limiting symptoms [35, 36]. And finally, should complications arise, because the test is self-
paced, patients can stop the test at any time they feel necessary.

Reliable, Valid, and Reproducible
Despite variation in testing protocols and natural variation in characteristics of patients
the 6MWT has been proven to be a reliable, valid [37] and reproducible [38] test of functional
capacity. Furthermore, interpretation of the results is relatively straightforward and has been
enhanced with the introduction of reference equations to calculate predicted distances. And
finally, because it is a “walking” test the task is representative of an activity performed in normal
daily living. It is therefore somewhat of an indicator of a patient’s quality of life.

6. Clinical Limitations:
Reference Data and Equations
On the whole there is little reference data outside certain age and gender populations and
although the current reference equations are useful, according to the American Thoracic
Society’s 6MWT guidelines optimal reference equations have yet to be created [7, 9]. Therefore
different reference equations are used to predict 6MWDs and consequently there is variation in
the prediction values. This fact makes it difficult to compare predicted distances by the reference
equations from study to study and until more comprehensive and better reliable equations are
published predicting patient six-minute walk distances is somewhat limited [10].
In addition, because of Troosters and colleagues results even if the ATS guidelines are
strictly adhered to there will always be inherent variability in the 6MWD as a result of
differences from patient to patient the most common sources being: age, gender, height and
weight [8].

Use of Alternative Protocols
As discussed earlier over time aspects of the 6MWT began to diverge among
investigators. Although the American Thoracic Society’s guidelines have reduced the variation
in the 6MWD outcome, it is difficult to compare 6MWD’s from study to study due to the
widespread use of alternative protocols and slightly different test parameters.
In the past there was little agreement on three parameters: the walking course shape (use
of straight vs. continuous), and with the straight shape: the length that should be used, as well as
the track surface used (hallway vs. treadmill).

7. Weiss et al. determined that the layout of the track did have an effect on the 6MWD.
Participants who were tested on a continuous track (circular or oval) walked 92.2-feet longer
than those tested on a straight (down and back) course [39]. It is now commonly agreed that the
test should be performed on a straight track. With regards to the distance between the start and
end points, it was found that from 50 to 164 ft there was no difference between walking
distances; it is for this reason that the ATS recommends using 100 ft, or 30 m [7].
Although the use of a treadmill provides many advantages over a hallway or corridor
many studies have shown that mean walking distances are 14% shorter with the use of a
treadmill compared to distance of patients walking in a 100 ft straight corridor [40-42]. For this
reason it is not recommended that the test be performed on a treadmill.
When a patient has difficulty walking some investigators have aided patients by walking
behind or next to them, or if they have supplemental oxygen helped them push their tank. It is
recommended that neither should be done [7] as it appears doing either of three introduces great
variation into the total distance walked. Compared to patients who walked alone, patients who
are aided in the abovementioned ways have been proved to walk greater distances [43].
Many studies have shown that 6MWDs can vary based on the use of encouraging words.
The common finding is that the use of verbal encouragement results in significant improvement
in the primary outcome measure [44]. In an effort to eliminate this source of variation it is
commonly agreed on that the use of standardized encouragement should be used [7, 45].

The Learning and Ceiling Effect
It is well understood that patients should be properly introduced to the test’s protocol and
conditions in order to reduce apprehension or fear a patient may have prior to beginning. On top

8. of formally familiarizing patients some studies have tried to control this variable with a practice
walk or a learning session [46].
However, many studies have shown that doing leads to increased 6MWDs, because of the
so called “learning effect” [4, 47]. Although it has been shown that there is no difference
between walk tests done 30 minutes or 24 hours apart [48], Pinna et al. showed that the learning
effect causes significant improvements in walking distances over the first five walks [49]. It is
well accepted now that the use of one or two practice tests sufficiently familiarizes the patient
with the 6MWT while introducing the least variation in the distances they walk [50].
It is worth noting that a 6MWT result can be too high to detect further improvement due
to the so called “ceiling effect,” evidenced by Frost et al. [51].

Patient Health Condition
Although the 6MWT is a submaximal test of functional capacity Reybrouk points out that
there are those with very serious preexisting health problems which may exclude them from even
the most low-intensity of exercises, including the 6MWT [52].

Predictive/Prognostic Value:
Despite its many applications, benefits, and countless refinements since its introduction
to the clinic, the 6MWT has been controversial in regards to its predictive or prognostic value.
This section will explore the usefulness of the 6MWT and its value in this regard.

9. Distanced Walked (6MWD)
In general an improvement or decline in distanced walked is clinically important. An
increase in distanced walked could indicate an overall improvement of functional capacity, the
efficacy of an intervention, or rehabilitation program, an improvement in exercise capacity, or be
a predictor of morbidity or mortality [53, 54]. How much of an improvement is clinically
significant? Redelmeier and colleagues showed that mean improvements of 70-170 meters are
significant and indicate a change in functional status. The minimum clinically important
difference was found to be 54 meters [18].
In many studies the test has been used as a one-time measure of functional capacity/status
in adults with chronic heart failure or chronic lung disease because the 6MWD has shown to
correlate strongly with functional status questionnaires given to patients [55].

Use of Normative Data
Comparing patient data to normative data can be useful in the interpretation of results.
There are many studies which provide these data. One study showed a median 6MWD to be
approximately 580 meters for 117 healthy men and 500 meters for 173 healthy women [56]. In a
study by Gibbons et al. with a mean age of 45.1 years the mean 6MWD was 698 m [10]. In an
elderly population with a mean age of 65 years a study by Troosters et al. years showed a mean
6MWD of 613 m [57]. In another population of elderly patients with a mean age of 74.1 years
the mean 6MWD was 505 m for men and 467 m for women [58]. Most likely these results vary
due to the differences in the protocols each investigator used. Even with the variation, the data
can serve as useful benchmarks upon which to compare one’s own results.

10. Use of Reference Equations
Provided current reference equations are used, (by Enright and Sherill – based off of
6MWT performances of 117 healthy men and 173 healthy women aged 40 to 80 years [9])
Enright has pointed out that one can interpret 6MWT results by comparing the one-time distance
walked and the distance predicted by the reference equations, measured as: the percentage of
predicted value [59]. This is useful in understanding where a patient’s performance stands when
considering the variables which are responsible for the most 6MWD variation.

Morbidity and Mortality
Years of study has shown that physical fitness or aerobic capacity is a strong predictor of
morbidity and mortality [60, 61]. Based on many studies, it is widely agreed upon that the
6MWT is also a strong predictor of both [27-33].

Predictor of Morbidity
In a study by Bautmans et al. there was a significant change in the total distance walked
depending on a patient’s health status. Specifically, as patient health problems increased the total
distance they walked in the 6 minutes decreased. Because much of the variability was explained
away with their statistical methods the authors suggest that this test may be a useful indicator of
cardiovascular pathology [62].
Harada et al. showed that the 6MWT can be used as a simple measure of mobility in the
elderly. Results showed patients’ walk distances correlated strongly with their activity level and
were greater for more active patients compared to those who were less active [63]. They suggest

11. that 6MWT can be used as a performance-based measure of functional limitation and thus a
predictor of mobility.
It has been shown that the 6MWT can be used as an overall predictor of disability [64]. In
a study by Bean et al. it was shown that lower-extremity impairment reduced performance on the
6MWT. Using leg power and strength impairments as measurements their study of 45
community-dwelling elders with limited-mobility proved the 6MWT to be a strong predictor of
performance in the 6MWT [65] and a good predictor of disability.

Predictor of Mortality
Many studies have shown a link between 6MWT performance and mortality. Studies
showing this link have showed strong associations with certain 6MWDs and death. Although not
every study reaches the same prediction distances they all show a high risk for “low” 6MWDs.
In a study by Bittner et al. mortality rate was 10.23% in patients who walked <350m and 2.99%
in participants who walked more than 450m on 6MWT [66]. A study of patients with mild-to
moderate heart failure showed mortality rates for those patients that walked <300 meters to be
significantly higher than patients whose distance was greater than 300 [67]. In a study by
Miyamoto et al. patients with primary pulmonary hypertension patients walking <332 m had a
significantly lower survival rate than those who exceeded this distance [56].
In patients with mild-to-moderate heart failure the 6MWT was shown to be a reliable
predictor of cardiac death. Arslan and collogues showed mortality rates for patients who walked
<300 meters to be significantly higher than those whose walked distances greater than 300 (79%
vs. 7%) [16].

12. Maximal Exercise Testing and the 6MWT: Equal Predictors of Exercise Capacity
In exercise testing peak VO2 has been shown to be a strong indicator of some conditions,
including the severity of chronic heart failure [68]. There are several studies which show the
6MWT correlates strongly with VO2 max in patients with COPD and heart failure [5, 55, 69, 70].
Therefore when maximal testing is not possible it is reasonable to suggest that the 6MWT, a
submaximal test of functional capacity, can be a worthy substitute with equal predictive value for
this condition.

Conclusion
Overall the aim of this paper was provide a comprehensive review of the literature
surrounding the commonly used measure of functional capacity, the 6MWT. The review’s focus
was threefold: to first describe the history of the 6MWT and chronicle its advance, second to
identify its applications in the clinic, and third to determine its clinical significance by examining
the work that’s demonstrated its benefits, limitations and predictive or prognostic value. Since its
introduction the 6MWT has undergone many changes and modifications. The protocol has and
still is applied in various forms depending on the investigator or research question. Despite this
variation it is unmistakable that the test’s use in the clinic has increased because of the many
benefits it provides. Despite its current limitations the test remains quite significant and will most
likely continue to grow an objective research tool. It is likely that in the not so distance future
updated reference equations will be published and all investigators will use the ATS guidelines
both of which will reduce the test’s limitations and enhance its significance. Also as more
populations are studied and the test is used in different research questions the significance of
6MWT the scope of its predictive value will grow even more.

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