Cardiology

The 6 Minute Walk Test

General description of procedure, equipment, technique

The 6-min walk test (6 MWT) is a submaximal exercise test that entails measurement of distance walked over a span of 6 minutes. The 6-minute walk distance (6 MWD) provides a measure for integrated global response of multiple cardiopulmonary and musculoskeletal systems involved in exercise.

The 6 MWT provides information regarding functional capacity, response to therapy and prognosis across a broad range of chronic cardiopulmonary conditions. Main strengths of the 6 MWT stem from its simplicity in concept and performance, low cost, ease of standardization, and acceptance by test subjects, including those who are deconditioned, elderly, or frail.

This chapter only provides details of the 6 MWT as it pertains to cardiovascular diseases. There is extensive literature published on the utility of the 6 MWT in pulmonary disorders/rehabilitation and rheumatologic conditions including fibromyalgia and systemic sclerosis.

Indications and patient selection

The main indication for performing a 6 MWT in a patient with cardiopulmonary disease is to measure response to medical or surgical intervention. It is also used to measure functional status and evaluate prognosis in various medical and surgical conditions. Some authors have compared it to a “biomarker” that can be used as a surrogate endpoint.

Common conditions where 6 MWT may provide useful information on response to therapy include:

  1. Pulmonary arterial hypertension (PAH)

  2. Heart failure (HF)

  3. Cardiac rehabilitation/coronary artery disease (CAD)

Conditions where 6 MWT has been shown to be a significant predictor of morbidity and mortality include

  1. PAH

  2. HF

  3. Peripheral arterial disease (PAD)

Conditions where 6 MWT has been helpful for one time measurement of functional status include:

  1. PAH

  2. HF

  3. CAD/cardiac rehabilitation

  4. PAD

The 6 MWT should not be considered to be a replacement for cardiopulmonary exercise stress testing. Rather, the information it provides is complementary to it.

Contraindications

The only absolute contraindication is patient refusal to do the test for any reason. Any person at an increased risk for arrhythmia, cardiovascular collapse, or respiratory distress with walking at a usual pace may be at an increased risk for an adverse event during the 6 MWT.

We think that the ATS guidelines are very conservative in their recommendation that anyone who has had a myocardial infarction or unstable angina in the prior month should be excluded from the test. AHA/ACC guidelines on exercise testing (last revised in 2002) recommend that patients who present with myocardial infarction may undergo either a submaximal exercise test 4 to 7 days after myocardial infarction or a symptom-limited test early (14 to 21 days) or late (3 to 6 weeks) after discharge.

Although these guidelines don’t provide recommendations specific to the 6 MWT, it is logical to extrapolate their recommendations to the 6 MWT, especially in postmyocardial infarction patients with regards to safety considerations. Each person determines his or her own pace of walking, and the chance of an adverse event is very low. Physicians/supervisors should determine if it is safe to proceed with the test after evaluating individual characteristics of each case.

Details of how the procedure is performed

In 2002, the American Thoracic Society (ATS) published guidelines on how to perform the 6 MWT. This guideline stressed the need for a standardized protocol to perform the 6 MWT to minimize variation in results.

Details in this text on how to perform the 6 MWT are adapted from the ATS guidelines, but other standardized protocols are available. For a commonly used reference, please refer to work by Guyatt et al.

Although the 6 MWT is very safe, emergency equipment should be at hand. The test should be conducted by personnel qualified to use this equipment. Physicians are not required to be present in most situations, but should be readily available. The physician ordering the test may decide if a physician should supervise the test.

According to ATS guidelines, a 30 m distance course is recommended. Turnaround points should be identified. Three-meter interval measurements are marked with colored tape on the floor. Shorter corridor lengths may increase the 6 MWD due to more frequent turns involved.

Required equipment

1. Stop watch or timer

2. Two small cones to mark the lap boundaries

3. Measurement scale for floor measurement

4. Mechanical lap counter

5. Resuscitation equipment

The 6 MWT is performed on a walking track in our facility. The track has been marked at 3-m intervals so that accurate measurement of the walking distance can be performed. Chairs are available at 30-m intervals in case the patients become so symptomatic that they have to stop and sit.

Patient preparation

1. Comfortable clothing should be worn

2. Environment temperature should be ambient

3. Shoes should be comfortable and any walking aids that the patient ordinarily uses should be used

4. Light meals are acceptable before morning and afternoon tests

Technique

1. Don’t perform a warm up before the test.

2. The patient should rest comfortably for 10 minutes prior to the test. During this time blood pressure and heart rate should be measured and potential contraindications assessed.

3. Pulse oximetry is optional. If it is used, it should be ensured that readings are stable prior to starting the 6 MWT, and signal is optimized.

4. Before the test starts, the patient should stand up and rate his/her dyspnea and fatigue. The Borg scale may be used for this.

5. Set the lap counter to zero and timer to 6 minutes. Assemble all necessary equipment and move to starting point.

6. The supervisor may walk a lap to demonstrate performance of the test to the patient. During the test the supervisor should never walk with or in front of the patient as the patient may try to match the supervisor’s pace. The supervisor may walk behind the patient to support the patient in case of staggering or to prevent falling. The patient is allowed to rest during the test if he/she gets fatigued.

7. Use standardized phrases and an even tone for encouragement at completion of each minute of the test. For an example of a standardized script, the reader is referred to the 2002 ATS guidelines. At our institution, we provide standardized encouragement every 30 seconds using phrases “keep up the good work” or “you are doing fine” as done in the SOLVD trial.

8. Resting during the test is allowed, but don’t stop the clock. If the patient cannot go any further, the test should be stopped and distance covered recorded.

9. Stop the test if patient develops chest pain, intolerable dyspnea, staggering, diaphoresis, intolerable cramps, and/or ashen appearance. Test supervisors should be trained to provide appropriate care at this point.

10. At the conclusion of the test, ask the patient to rate his/her dyspnea and fatigue levels. Record the reason for stopping the test.

Additional details about technique

1. If the patient is on supplemental oxygen, use the same oxygen level that he or she normally uses with exercise.

2. The 2002 ATS guidelines do not recommend routine measurement of pulse oximetry during the test. In untreated patients with pulmonary hypertension, oxygen desaturation >10% during 6 MWT has been associated with mortality. With the widespread availability of light-weight portable oximeters, monitoring of oxygen saturation during walking can be reliably performed.

3. Quality control and standardization are important to reduce variability in measurement. Persons supervising the test should be trained using a standard protocol, and should conduct supervised 6 MWTs before they are able to perform them independently. A practice test generally should not be needed. If a practice test is done, there should be a wait time of about an hour between the two tests, and the longer 6 MWD should be reported.

4. Several studies have shown that walking distance tends to increase with repeated test administration due to familiarization or learning effect. Estimates for this learning effect have ranged from 4.5% to 33% of initial distance walked. Because the distance walked tends to plateau after 3 walks, 1 to 2 practice walks have been suggested before determining an individual’s functional capacity. The initial learning effect has been shown to be maintained over at least 2 months of follow-up.

Interpretation of results

Interpretation of results is guided by whether the test is used for prognostication, assessment of functional status or therapeutic monitoring.

A single value of 6 MWD can be reported as an absolute value or as a percent predicted for age and gender. For prognostication, 6 MWD can be compared to published values for the particular disease category.

Changes in 6 MWD over time can be reported as absolute difference, percentage change or change in percentage predicted.

When evaluating changes in 6 MWD with treatment, some investigators have advocated the concept of minimal clinically important difference (MCID) (i.e., the observed difference in walking distance that is perceived as important to the patient or physician). However, no broadly applicable data on what constitutes a reasonable MCID exist, and MCID may vary by disease category and severity, and other characteristics of the patient population.

A 30-m increment over a baseline 6 MWD of 250 m may be more important than the same increment over a baseline 6 MWD of 450 m. Expressing percentage change in 6 MWD may be more helpful in this setting. In our clinical practice, we have also found that expressing 6 MWD as percentage of normal predicted values is helpful. For example, telling patients that their 6 MWD at the end of cardiac rehabilitation was “90% predicted for a healthy adult” of their age and gender may be very encouraging for some of them.

The role of the 6 MWT in various cardiovascular conditions as a prognostic marker and for predicting response to therapy is profiled below.

Heart failure

In a systematic review of the use of the 6 MWT in heart failure trials, there was poor correlation between NYHA class and 6 MWD. This may reflect problems with assessment of either. However, a 6 MWD <300 m does predict poorer outcome in stable heart failure patients.

With regards to patients being evaluated for transplantation, a 6 MWD <350 m has a sensitivity of 71% and specificity of 60% for predicting Vo2 max <14 mL/kg/min. In heart failure outpatients with LVEF <40%, the mean changes in 6 MWD associated with moderate and marked improvement in symptoms were 55 m and 107 m, respectively. In this study the mean change in 6 MWD for small improvement in symptoms (19 m) was not statistically different from no change.

The 6 MWD may not be a sensitive marker of change following medical therapy for HF. Treatment with beta-blockers may not significantly improve 6 MWD despite improvements in LVEF and NHYA class. Similarly, ACE inhibitors and angiotensin II blockers do not appear to improve 6 MWD.

However, improvements in 6 MWD with cardiac resynchronization therapy (CRT) have been more consistent, and estimates have ranged from 5 to 74 m. This may be a reflection of selection of sicker patients in trials for CRT. Earlier trials of CRT were also limited by lack of patient or investigator blinding to treatment allocation. In trials where CRT has been compared to left ventricular pacing alone, 6 MWD did not improve.

Pulmonary arterial hypertension

Severe impairment in 6 MWD is seen in PAH. There is an inverse relationship between WHO/NYHA clinical status and 6 MWD in PAH. Six MWD <300 m is an adverse prognostic indicator in PAH. Conversely, 6 MWD >400 m is considered a favorable prognostic indicator.

Most studies that have included patients with severe PAH (mean pulmonary artery pressure >45 mm Hg) have reported baseline 6 MWD <300 m. Six MWD is independently related to mortality in patients who are on treatment as well. Those patients who achieve a threshold of 378 m have improved survival, irrespective of their baseline 6 WMD.

The relationship between walked distance and mortality may not be seen in patients with untreated PAH, where desaturation during the 6 MWD may be a better predictor of mortality. For every percentage point decrease in pulse oximetry during the 6 MWT, there is a 26% increase in risk of death.

Exercise capacity is an important outcome in trials of drugs for PAH. Change in 6 MWD has been accepted by regulatory agencies as an endpoint in randomized controlled trials of drugs for PAH.

Drug effects on the 6 MWT have been mostly slow to manifest and modest in estimate (10% to 15%) in published trials. Reduction in 6 MWD by 15% in combination with worsening in functional class (except for those patients who are Class IV) has been proposed as one of the criteria for measurement of time to clinical worsening in trials.

Cardiac rehabilitation and coronary artery disease

The 6 MWT is commonly used in cardiac rehabilitation for exercise prescription and as a metric to monitor improvement in functional status. In a UAB cohort of cardiac rehabilitation participants, walking distance improved in 73% of participants. Walking distance increased by 178 ± 256 m in participants who completed the rehabilitation program with less improvement noted in women and African Americans; 6 MWD at entry and completion were highly correlated (r = 0.83, P <05).

Peripheral arterial disease

Approximately a third of persons with PAD who report no exertional leg symptoms develop leg symptoms during 6 MWT. These individuals with "asymptomatic" PAD who develop leg symptoms during 6 MWT presumably have restricted their physical activity to avoid exertional leg symptoms during activities of daily living.

Six MWD is reduced in patients with increasing severity of PAD. For example, in a single hospital study of PAD patients, 6 MWD by PAD severity was as follows: ABI <0.5: 290 m; ABI 0.5-0.7:312 m; and ABI 0.7-0.9:355 m. In the same dataset, patients in the lowest quartile of 6 MWD (<273 m) experienced two-fold increased risk of mortality over 4.8 years of follow-up when compared to patients in the highest quartile (>426 m) (HR 2.40; 95% CI; 1.32-4.38).

Six MWD at baseline was also shown to predict mobility loss in PAD patients. Supervised treadmill training increased 6 MWD in PAD patients by 35.9 m (95% CI; 15.3-56.5 m), whereas resistance training increased the 6 MWD by 12.4 m (95% CI; −8.42 to 33.3 m) when compared to a control group.

Performance characteristics of the procedure (applies only to diagnostic procedures)

1.Test-retest reliability

Test-retest reliability has been reported as high. Intraclass correlation coefficients (ICC) were reported as 0.90 at baseline, 0.88 at 18 weeks, and 0.91 at 43 weeks in a cohort of patients with heart failure. ICC for PAD patients has been >0.90. ICC in cardiac rehabilitation patients has been reported to be as high as 0.97.

2. Validity

Moderate to high correlations have been reported (r = 0.56 to r = 0.88) between the 6 MWD and peak Vo2 obtained by maximal exercise testing in persons with heart failure. In a sample of cardiac rehabilitation patients (n = 94), 6 MWD was linearly related to maximum metabolic equivalents estimated using symptom-limited graded exercise test (r = 0.687, P <.001). The 6 MWD was moderately correlated with scores from the Duke Activity Status Index (r = 0.502, P < 0.001), and the Physical Function subscale of the Short Form 36 (r = 0.624, P <.001).

With a good quality-assurance program, with patients tested by the same technician, and after one or two practice tests, short-term reproducibility of the 6 MWD is excellent.

3. Prediction of normal values

Distances reported for healthy individuals ages 40 to 85 years range from 400 to 700 m (see references). It is difficult to publish generalized values because of differing methods used in studies. However, in general non-U.S. populations walk longer distances than U.S. populations.

It should be noted that published studies have been mostly done in Caucasian populations; 20% to 78% of variation in 6 MWD can be explained by prediction equations. Interestingly, the studies reporting the reference values for the 6 MWT have all used differing protocols for 6 MWT in different countries; hence the predicted walk distances differ by up to 30% between studies.

Prediction equations from one study are profiled below.

Men

6 MWD = (7.57 × Height cm) – (1.76 × weight kg) – (5.02 × age) – 309 m

An alternate equation using body mass index (BMI)

6 MWD = 1140 m – (5.61 × BMI) – (6.94 × age)

With both equations subtract 153 m to obtain lower limit of normal (lower 95% confidence interval)

Women

6 MWD = (2.11 × Height cm) – (2.29 × weight kg) – (5.78 × age) + 667 m

An alternative equation using BMI

6 MWD = 1017 m – (6.24 × BMI) – (5.83 × age)

With both equations subtract 139 m to obtain lower limit of normal (lower 95% confidence interval)

4. Factors decreasing 6 MWD include older age, shorter height, female sex, higher body weight, cardiac disease (HF, pulmonary hypertension, CAD, PAD), pulmonary disease (COPD, interstitial lung disease, asthma, cystic fibrosis). Factors that increase 6 MWD include taller height, male sex, increased motivation, past performance of the test, muscle mass and use of supplemental oxygen in patients who develop exercise-induced hypoxemia. Other variables that have been shown to influence 6 MWD include course length, course layout (linear, oval, or continuous circle), and whether a practice walk is done before the final walk.

5. A low 6 MWD is prognostically useful, but nondiagnostic (i.e. does not inform about the reason for the low 6 MWD). It may be due to lung disease, cardiovascular disease, PAD, fatigue, cognitive issues, neuromuscular disease, musculoskeletal problems, poor nutrition, and or lack of motivation.

Outcomes (applies only to therapeutic procedures)

N/A

Alternative and/or additional procedures to consider

The 6 MWT does not provide measurement of Vo2 max (although there is moderate correlation between 6 MWD and Vo2 max), nor does it provide information on the mechanism of exercise limitation or dyspnea.

Prior to description and widespread application of the 6 MWT, a 12-minute walk/run test was described. Its use in clinical studies was limited by the inability of most patients with cardiopulmonary disease to perform exercise for 12 minutes. A two-minute walk test has also been described for use in elderly subjects and stroke patients. Data on its use in patients with cardiovascular disease are limited.

The Shuttle walking test (SWT) is an externally paced test. A 10-meter course is used, with the walking speed dictated by a prerecorded audio signal played on a cassette recorder. The walking pace is increased by 0.17 m/sec every minute, and the test is concluded when an individual is no longer able to keep up the pace or stops because of symptoms. There are a maximum of 12 stages and the primary outcome is the walked distance.

Therefore, the SWT is an incremental and progressive test, which stresses the individual to a symptom-limited maximal performance. No normal reference values have been published by scientific societies for the SWT.

Cardiopulmonary exercise testing measures metabolic gas exchange during rest and exercise, and quantifies aerobic capacity. It provides the most rigorous measurement of exercise capacity. It allows for discrimination between the metabolic, cardiovascular, and pulmonary components of exercise limitation by virtue of measurement of simultaneous gas exchange, and quantification of Vo2 max and anaerobic threshold. However it requires more cumbersome equipment and technology.

Exercise stress testing entails use of either treadmill or bicycle exercise with electrocardiographic and blood pressure monitoring. Ancillary techniques that may be used include imaging and metabolic gas exchange. Many exercise protocols have been described, with the Bruce and modified Bruce protocols being most commonly used. Exercise testing is used to assess the probability and extent of coronary disease, to estimate prognosis and determine functional capacity, and to assess the effects of therapy.

Complications and their management

When performed appropriately, the complication rate with the 6 MWT is very low as patients determine their own pace during the test. Large scale studies have confirmed this.

The test should be terminated if a patient complains of excessive fatigue, angina, or light-headedness during the test. A supervisor can also stop the test if collapse appears imminent. If a test is terminated for angina, and the patient is still symptomatic at rest, sublingual nitroglycerin may be administered unless contraindicated.

What’s the evidence?

ATS guidelines on 6 MWT

"ATS statement: guidelines for the six-minute walk test". Am J Respir Crit Care Med. vol. 166. 2002. pp. 111-7.

Widely referenced 6 MWT protocol

Guyatt, GH, Sullivan, MJ, Thompson, PJ. "The 6-minute walk: a new measure of exercise capacity in patients with chronic heart failure". Can Med Assoc J. vol. 132. 1985. pp. 919-23.

Reference values for 6 MWT

Enright, PL, Sherrill, DL. "Reference equations for the six-minute walk in healthy adults". Am J Respir Crit Care Med. vol. 158. 1998. pp. 1384-7.

Troosters, T, Gosselink, R, Decramer, M. "Six minute walking distance in healthy elderly subjects". Eur Respir J. vol. 14. 1999. pp. 270.

Gibbons, WJ, Fruchter, N, Sloan, S. "Reference values for a multiple repetition 6-minute walk test in healthy adults older than 20 years". J Cardiopulm Rehabil. vol. 21. 2001. pp. 87-93.

Camarri, B, Eastwood, PR, Cecins, NM. "Six minute walk distance in healthy subjects aged 55-75 years". Respir Med. vol. 100. 2006. pp. 658-65.

Chetta, A, Zanini, A, Pisi, G. "Reference values for the 6-min walk test in healthy subjects 20-50 years old". Respir Med. vol. 100. 2006. pp. 1573-8.

(Clinical study that shows that 6 MWD correlates with daily life physical activities in pulmonary hypertension.)

Mainguy, V, Provencher, S, Maltais, F. "Assessment of daily life physical activities in pulmonary arterial hypertension". PLoS One.. vol. 6. 2011. pp. e27993.

(Clinical study that showed moderate correlation between 6 MWD and Vo2 peak determined by maximal cardiopulmonary exercise testing in patients with PAH. Patients with 6 MWD <332 m experienced increased mortality.)

Miyamoto, S, Nagaya, N, Satoh, T. "Clinical correlates and prognostic significance of six-minute walk test in patients with primary pulmonary hypertension. Comparison with cardiopulmonary exercise testing". Am J Respir Crit Care Med. vol. 161. 2000. pp. 487-92.

(Study that evaluated performance characteristics of 6 MWT in heart failure.)

Demers, C, McKelvie, RS, Negassa, A, Yusut, S. "Reliability, validity, and responsiveness of the six-minute walk test in patients with heart failure". Am Heart J. vol. 142. 2001. pp. 698-703.

(Systematic review on utility of 6 MWT-based data from chronic heart failure trials.)

Olsson, LG, Swedberg, K, Clark, AL, Witte, KK, Cleland, JG. "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". Eur Heart J. vol. 26. 2005. pp. 778-93.

Prognostic value of 6 MWT in heart failure

Shah, MR, Hasselblad, V, Gheorghiade, M. "Prognostic usefulness of the six-minute walk in patients with advanced congestive heart failure secondary to ischemic or nonischemic cardiomyopathy". Am J Cardiol. vol. 88. 2001. pp. 987-93.

Bittner, V, Weiner, DH, Yusuf, S. "Prediction of mortality and morbidity with a 6-minute walk test in patients with left ventricular dysfunction". JAMA. vol. 270. 1993. pp. 1702-7.

Studies of historic interest

Cooper, KH. "A means of assessing maximal oxygen intake. Correlation between field and treadmill testing". JAMA. vol. 203. 1968. pp. 201-4.

McGavin, CR, Gupta, SP, McHardy, GJ. "Twelve-minute walking test for assessing disability in chronic bronchitis". BMJ. vol. 1. 1976. pp. 822-3.

McGavin, CR, Artvinli, M, Naoe, H, McHardy, GJ. "Dyspnoea, disability, and distance walked: Comparison of estimates of exercise performance in respiratory disease". BMJ. vol. 2. 1978. pp. 241-3.

Butland, RJ, Pang, J, Gross, ER, Woodcock, AA, Geddes, DM. "Two-, six-, and 12-minute walking tests in respiratory disease". Br Med J (Clin Res Ed). vol. 284. 1982. pp. 1607-8.

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