The Historical And Clinical Significance Of The 6 Minute Walk Test
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The Historical And Clinical Significance Of The 6 Minute Walk Test

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The Historical And Clinical Significance Of The 6 Minute Walk Test The Historical And Clinical Significance Of The 6 Minute Walk Test Document Transcript

  • 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].
  • 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
  • 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 View slide
  • 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 View slide
  • 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.
  • 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).
  • 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
  • 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.
  • 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.
  • 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
  • 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].
  • 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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  • compared with short physical performance battery. J Gerontol Med Sci 2000;55A:M221- M231. [65] Bean JF, Kiely DK, Leveille SG, Herman S, Huynh C, Fielding R, Frontera W. The 6- minute walk test in mobility-limited elders: what is being measured? Journal of gerontology: Medical Sciences 2002, Vol. 57A, No. 11, M751-M756. [66] Bittner V, Weiner DH, Yusuf S, et al. Prediction of mortality and morbidity with the 6- minute walk test in patients with left ventricular dysfunction. JAMA 1993;270:1702- 1707. [67] Rostagno C, Olivo G, Comeglio M, Boddi V, Banchelli M, Galanti G, Gensini GF. Prognostic value of 6-minute walk corridor test in patients with mild to moderate heart failure: comparison with other methods of functional evaluation. European Journal of Heart Failure 2003;5:247-52. [68] Lainchbury JG, Rischards AM. Exercise testing in the assessment of chronic congestive heart failure. Heart 2002;88(5):538-43. [69] Faggiano P, D’ Aloia A, Gualeni A, Lavatelli A, Giordano A. Assessment of oxygen uptake during the 6-minute walking test in patients with heart failure: preliminary experience with a portable device. Am Heart J 1997;134(21):203-6. [70] Mak VHF, Bugler JR, Roberts CM, et al. Effect of arterial oxygen desaturation on six minute walk distance, perceived effort, and perceived breathlessness in patients with airflow limitation. Thorax 1993; 48:33-38.