An official European Respiratory Society/American Thoracic Society technical standard: field walking tests in chronic respiratory disease

Validity of 6MWD

There is a large body of data demonstrating construct and criterion validity for the 6MWD in individuals with chronic respiratory disease [1]. Relationships are strongest with measures of maximal exercise performance and physical activity (correlation coefficients 0.4–0.93). Relationships between 6MWD and respiratory function (forced expiratory volume in 1 s (FEV₁), forced vital capacity and diffusing capacity of the lung for carbon monoxide (D_LCO)) (correlation coefficients 0.31–0.55) or health-related quality of life (HRQoL) (correlation coefficients 0.03–0.65) are of weak to moderate strength across all disease groups examined.

Direct comparisons of the physiological demands of the 6MWT and CPET show that although measures of peak exercise performance (V′_O2peak and heart rate (HR) peak) are similar between the tests, the 6MWT has substantially lower ventilatory requirements (peak carbon dioxide production, peak ventilation and respiratory exchange ratio) [3–8], which may contribute to its tolerability in adults with chronic respiratory disease. Overall, these data lend support to the conceptualisation of the 6MWT as a test of functional exercise capacity.

Reliability and learning effect for 6MWD

The 6MWD is a reliable measure in people with chronic respiratory disease, with excellent intra-class correlation coefficients (ICCs) (range 0.82–0.99) [1]. There are no discernible differences in reliability across groups with different chronic respiratory diseases. Despite its excellent reliability, there is strong evidence of a learning effect for the 6MWD when two or more tests are conducted. 13 studies in patients with COPD show a pooled mean improvement on the second 6MWT of 26.3 m [1]. This estimate does not change when including only the subgroup of studies where the two tests are conducted within 24 h (26.1 m). The largest study to address this issue (n=1514) reported a 95% confidence interval for the learning effect of 24–29 m [9]. There is some variability across individuals: the proportion of participants who walked further on the second 6MWT ranged from 50% to 87% across studies. Fewer data are available in other chronic respiratory diseases, although the available studies tend to support a learning effect across all chronic respiratory diseases [1]. Whether the learning effect is equally significant for individuals who have previously performed multiple 6MWTs is difficult to establish, as few data are available. If the tests are repeated 3 months later, the learning effect appears to persist in individuals with COPD; however, it may be smaller in magnitude [10].

The effect of learning on the 6MWD is large enough to be clinically important when the 6MWT is used to evaluate response to treatment or change over time. In these situations, two 6MWTs should be performed and the best 6MWD recorded. Use of two 6MWDs may also decrease the sample size requirements for clinical trials, due to reduced variability in the pre- and post-intervention measures [11]. Where the 6MWD is used as a one-off measure to stage disease or assess risk (e.g. likelihood of hospitalisation or mortality), the magnitude of the learning effect may be less important and one test may be sufficient. However, clinicians should be mindful of the learning effect if the 6MWD is approaching pre-defined thresholds on which treatment decisions may be based [12, 13]; in this situation, repeat testing should be considered. One test may also be sufficient for patients who have recently performed the test, where the learning effect is smaller (e.g. end of pulmonary rehabilitation) [10].

Relationship of 6MWD to clinical outcomes

The 6MWD has a strong relationship to important clinical outcomes in individuals with chronic respiratory disease. A shorter 6MWD was associated with increased mortality in 13 (93%) out of the 14 COPD studies reviewed [1]. Associations were less consistent in ILD (four (50%) out of eight studies) and PAH (six (66%) out of nine studies) [1]. There is less evidence for the association of 6MWD with hospitalisation, but all studies in which it was evaluated (n=3, COPD and ILD) found a statistically significant relationship [1]. It was outside the scope of this document to identify thresholds for 6MWD to categorise patients according to their risk for these outcomes.

Methodological factors affecting test performance

The 6MWD is highly sensitive to changes in methodology (table 2). Given the impact of encouragement on 6MWD [33] and the use of encouraged tests in the generation of reference equations [1], it is recommended that standardised phrases of encouragement are used (see later section on the testing protocol for 6MWT). Provision of supplemental oxygen [26–29], method for carrying the supplemental oxygen [31, 32] and use of wheeled walkers [20–25] also have an important impact on 6MWD. These factors must be kept constant on repeat testing. The 6MWD generated using an externally paced treadmill is substantially lower than in a hallway [14, 15], which is probably the result of the poor walking efficiency during treadmill walking in subjects unaccustomed to this activity. Externally paced treadmill testing is not recommended. Track layout and length may also affect performance on the test, especially when very short track lengths are used [19]; these factors should be kept constant where within-subject comparison of 6MWD on subsequent occasions is required. For track lengths of >15 m, differences may be small enough such that tests on different track layouts can still be used for risk stratification [1]; however, more data are needed to confirm this.

Monitoring and reporting for 6MWT

Identifying meaningful change in 6MWD

Reference equations for 6MWD

The systematic review identified 16 published studies from 1998 to 2013 that included 6MWD prediction equations from healthy adults [1]. One additional study has been published since the systematic review [74]. These studies were conducted using a wide variety of populations and methodologies. Track lengths ranged from 20 to 50 m and the number of tests ranged from one to four. Application of these equations to an individual gives rise to substantial variation in the predicted distance.

Factors that may affect the 6MWD in healthy adults, and therefore the predicted distance, include methodology for 6MWT, percentage of peak HR achieved, height, age, sex, weight and perhaps race/ethnicity. The utility of % predicted values in assessing clinically meaningful change in 6MWD over time has not been investigated.

Due to the wide variation in predicted 6MWD generated by different equations, reference equations generated and verified in a local population should be applied where possible. A summary of reference equations can be found in the accompanying systematic review [1].

Validity of the ISWT and ESWT

The ISWT is an externally paced maximal exercise test; the speed of walking is controlled by a series of pre-recorded signals. The speed of walking increases until the participant can no longer continue. The maximum duration of the test is 20 min.

Given the progressive nature of the ISWT, a strong relationship with performance on laboratory-based tests might be anticipated. Seven studies in COPD and one in lung cancer show that the V′_O2peak, estimated V′_O2peak, peak work and distance measured on ISWT show moderate to strong correlations with measures of maximal exercise performance on CPET [1], with no difference in measured oxygen uptake (V′_O2) between the tests [6, 75, 76]. Hill et al. [6] performed a minute-by-minute analysis, and both the ISWT and CPET demonstrated a linear response in V′_O2peak, suggesting that the two tests provoke a similar cardiopulmonary response. In summary, the current literature suggests that the ISWT is a valid measure of cardiopulmonary exercise capacity in COPD and provokes a similar physiological response to a CPET. More evidence in other disease populations is required.

The ESWT is a derivative of the ISWT, where patients walk for as long as possible at a predetermined percentage of maximum walking performance as assessed by the ISWT [77]. To set the speed for the ESWT, the ISWT must have been completed previously. The ESWT has been shown to elicit similar end-test HR and dyspnoea responses compared to a treadmill endurance test at the same intensity [77].

Reliability and learning effect for ISWT

Only a handful of studies have reported the reliability of the ISWT, with most data in COPD. One study documented an ICC of 0.88 (95% CI 0.83–0.92) [78]. Another study reported an ICC of 0.89 [79]. These two studies suggest that the association between test–retest walk distances is strong, with the majority of variability being attributable to between-subject differences. More studies are needed to confirm these findings.

There is a small but statistically significant difference between the first two ISWTs performed (mean differences of 20 and 25 m for tests performed on the same day, and 23.5 m for tests performed on different days) [6, 80, 81]. The effect of learning on the ISWT is large enough to be clinically important when evaluating change over time. It is recommended that two ISWTs be performed and the best distance recorded. It remains to be established whether one test would be sufficient where the ISWT is used as a one-off measure to stage disease or assess risk (e.g. likelihood of hospitalisation or mortality). Whether one test is sufficient if the test has previously been conducted, similar to the 6MWT, is still unclear. It is common to repeat the test once after an intervention, but whether this underestimates the impact of the intervention remains to be established. The reliability of the test in chronic respiratory diseases other than COPD has not been explored.

Reliability and learning effect for ESWT

Three studies have examined the reliability of the ESWT, all in COPD [6, 81, 82]. The differences between tests repeated on the same day were generally small and statistically nonsignificant (pooled mean difference +26 s). Two tests do not appear to be necessary, although it is acknowledged that the number of studies is limited. Measurements of HR, S_pO2 and modified Borg dyspnoea scale appear to repeat well during the test [1]. The reliability of the ESWT in other chronic respiratory diseases has not been examined.

Relationship of ISWT and ESWT to clinical outcomes

A small number of studies show that the ISWT is a significant predictor of survival and re-admission in people with COPD, with a lower distance predicting a greater risk of admission [83–85]. The relationship of ESWT to these outcomes is unknown.

Methodological factors affecting ISWT and ESWT performance

Unlike the 6MWT, the track for the ISWT and the ESWT is fixed, and alterations of the course have not been studied. The effect of encouragement has not been directly observed, but the external pacing of the tests may make it difficult to override the test protocol. The impact of supplemental oxygen is influenced by the mode of delivery. If patients are required to support a cylinder independently, oxygen supplementation confers little advantage in walking distance compared to air walking conditions [86], but if the oxygen (or heliox) is carried by a clinician/researcher, the benefit is magnified [87]. Accordingly, if supplemental oxygen is to be used during testing, the same approach should be taken for repeat testing, with the device and the flow rate remaining constant.

Monitoring and reporting for ISWT and ESWT

Identifying meaningful change in ISWT and ESWT

Reference equations for ISWT and ESWT

Three papers have described reference values for the ISWT, two from South America [101, 102] and one from the UK [103]. There is a need for more data, including diversity in the country of origin. Existing data suggest that age, sex and body mass index are important variables. Data from a single centre in the UK [1] included measures of strength (quadriceps maximum voluntary contraction) and physical activity (Duke Physical Activity Status Index and a physical activity monitor), in an attempt to further explain the variability; however, these additions did not improve the reference equation.

To date there has been no attempt to identify reference equations for the ESWT.

Equipment

The equipment required to conduct a field walking test is listed in table 3.

Test location and staffing

The test should be conducted along a quiet course, physiotherapy gym or dedicated exercise testing room. A comfortable temperature is important and air conditioning should be available if needed. Testing should be performed in a location where a rapid, appropriate response to an emergency is possible. The appropriate location of a crash cart should be determined by the physician supervising the facility. Supplies that must be available include oxygen, sublingual nitroglycerine and salbutamol (metered dose inhaler or nebuliser). A telephone or other means of calling for help should be available in case of emergency.

The assessor performing the test should be certified in cardiopulmonary resuscitation with a minimum of Basic Life Support certification. Training, experience and certification in related healthcare fields are also desirable. A certified individual should be readily available to respond if needed. Physicians are not required to be present during all tests. The physician ordering the test or a supervising laboratory physician may decide whether physician attendance at a specific test is required.

Patient assessment

Given that all three field-based exercise tests can elicit a V′_O2peak and peak HR that are similar to the CPET [1], absolute and relative contraindications for exercise testing should be consistent with recommendations for maximal exercise testing (table 4) [108]. Patients with any of these findings should be referred to the physician ordering or supervising the test for individual clinical assessment and a decision about the conduct of the test. Stable exertional angina is not an absolute contraindication for field walking tests, but patients with these symptoms should perform the test after using their anti-angina medication, and rescue nitrate medication should be readily available. All comorbidities and medication use should be recorded prior to the test.

Patient preparation

Patients should wear comfortable clothing and appropriate shoes for walking. Patients should use their usual walking aids during the test and this should be documented on the assessment form. Patients should not have exercised vigorously within 2 h of beginning the test but should have taken their usual medications. All subsequent testing occasions should occur at about the same time of day to minimise intra-day variability, including variability in self-paced tests (6MWT) associated with bronchodilator use [109]. A warm-up is not permitted prior to commencing the test, neither is a shortened version of the test. If respiratory function tests are to be performed on the same day, this should occur prior to exercise testing, to avoid the confounding effects of exercise on these measures. The patients should then rest for at least 15 min before commencing an exercise test.

Use of oxygen

If a patient is on long-term oxygen therapy, oxygen should be given at their standard flow rate or as directed by a physician or a protocol. For any test where the outcome is distance, oxygen flow should be held constant throughout the test. If the purpose of the exercise test is to compare distance walked between tests, any subsequent test should be performed using the same oxygen conditions, in order to make a valid comparison between testing occasions. If oxygen supplementation is needed during the walks and serial tests are planned, then oxygen should be delivered in the same manner (flow rate and delivery device) for all subsequent walks. If the flow rate must be increased for subsequent visits due to worsening gas exchange, this should be noted on the worksheet and considered during interpretation of any changes in performance.

The type of oxygen delivery device should also be noted on the report: for instance, whether the subject carried liquid oxygen or pushed or pulled an oxygen tank, and whether the delivery was pulsed or continuous. Assessors should avoid transportation of the oxygen source where possible; however, if the subject is not able to control/carry/manage their own oxygen cylinder, the assessor should try to walk slightly behind the subject to avoid setting the walking pace. It should be clearly documented how the assessor has assisted with the transport of the oxygen, so any subsequent walk tests with the same subject can be performed in the same manner. Oxygen is not to be titrated during any of the tests where distance is a measured outcome. If oxygen titration is desired, this should be done during a separate test.

Medications

Patients should be instructed to take all usual medications. The type of medication, dose and number of hours it was taken before the test should be noted. Significant improvement in the distance walked or the dyspnoea scale after administration of bronchodilators has been demonstrated in patients with COPD [93–96], although this may be small and clinically insignificant for the 6MWT [95, 110].

Measurements

Patients should rest in a chair, located near the starting position, before the test starts. Absolute and relative contraindications should be checked for prior to test commencement (table 4). The following measurements should be obtained at rest: S_pO2 and HR from pulse oximetry, baseline dyspnoea and fatigue using a reproducible scale, and systemic blood pressure, if not recently documented [108].

Immediately prior to the test

The assessor should provide standardised instructions, either verbally for the 6MWT (table 5) or from the ISWT or ESWT audio recording. The patient should be positioned at the starting line. For the 6MWT, the timer should be started as soon as the patient starts to walk. The recorded instructions of the ISWT and ESWT will prompt the individual to start.

A pulse oximeter should be used for continuous measurement of S_pO2 and HR. The assessor should not “pace” the patient during the test, but should walk behind such that measures of nadir S_pO2 and end-test HR can be recorded without influencing the patient’s movement. The assessor should ensure that the probe is placed such that a quality signal is obtained.

Immediately on test cessation

The S_pO2 and pulse rate should be recorded from the oximeter, and the patient should be asked to rate their dyspnoea and subjective fatigue on the standardised scale. In addition, it is important to understand the reason for test termination/limitation, so patients should be asked why they could not walk any further. It is common for patients to report either dyspnoea or leg fatigue as the primary reason for a restricted performance.

Reasons for the assessor to stop an exercise test

In some individuals, profound desaturation (S_pO2 <80%) may occur during a walking-based exercise test. Use of a 6MWT protocol that directs test cessation when S_pO2 falls to <80% is associated with an extremely low rate of adverse events [35]; this has not been reported for the ISWT or ESWT. Few data are available to determine the risk if the test is not stopped when S_pO2 falls below 80%. Ceasing a test when S_pO2 falls to <80% is also consistent with the recommendations for incremental exercise testing [108]. If S_pO2 recovers to ≥85% during the 6MWT, the patient may be asked to recommence walking.

Other reasons for test cessation include chest pain, intolerable dyspnoea, leg cramps, staggering, diaphoresis, and a pale or ashen appearance. If a test is stopped for any of these reasons, the patient should sit or lie supine as appropriate. The following should be obtained based on the judgment of the assessor: blood pressure, pulse rate, S_pO2 and a physician evaluation. Oxygen should be administered as appropriate.

Test repetition

To establish a stable baseline for the 6MWT and ISWT so that change over time can be detected, two tests must be completed. These can be performed on the same day but there must be an interval between tests of at least 30 min and measures of HR and S_pO2 must have returned to baseline prior to the second test.

Course

The 6MWT should be performed along a flat, straight course with a hard surface with little pedestrian traffic. It is recommended that the walking course be 30 m or more in length, to be consistent with the courses on which reference equations have been generated [1]. The ends of the course should be marked such that they are easily visible to patients.

Conduct

The patient should be encouraged every 60 s using the standard phrases (table 6). Other words of encouragement and other nonverbal prompts should not be used. If the patient stops walking during the test, the timer must not be stopped. The patient should be allowed to rest while sitting or standing, as they prefer. While the patient is stopped, standardised encouragement should be provided every 30 s (table 6). The time that the patient stopped and the time that walking is recommenced should be recorded.

Recording performance of the 6MWT

The primary outcome to be reported is 6MWD. The number of laps and any additional distance covered (the number of metres or feet in the final partial lap) should be recorded. The total distance walked is calculated, rounding to the nearest metre or foot. If the patient stopped during the test, the total time stopped, the number of stops and the average walking speed over the 6 min are also reported [111]. In patients who cannot walk for 6 min, this may provide alternative metrics for detecting change over time [111] and may facilitate exercise prescription [112]. It is optional to report the 6MWD as a percentage of predicted. If the % predicted 6MWD is reported, the reference equations used should be stated. Lowest S_pO2, end-test HR and symptom scores obtained before and after the test should also be reported. A sample recording form can be found in the online supplementary material (table S1).

Course

The course is 10 m in length and is identified for the patient by two cones with an inset of 0.5 m from either end (fig. 1), thus avoiding abrupt changes in direction [113].

• Download figure
• Open in new tab
• Download powerpoint

Figure 1–

Course layout for the incremental and endurance shuttle walk tests. Cones are inset 0.5 m from either end to avoid abrupt changes in direction.

At the beginning of the test the instructions are played to the patient from an audio recording (table 7). Once the instructions have been played, and it is confirmed that the patient has understood the task, the patient is positioned at one end of the course in preparation to start the test. The speed at which the patient should walk is directed by an audio signal and cannot be overridden. There is a triple bleep indicating the test has started: at this point the participant commences walking and the timer is activated.

Conduct of the test

It is important for the assessor to watch the patient but also to ensure they keep count of the number of lengths as the subject completes them, throughout the duration of the test. It is advisable to time the performance as an additional measure, to confirm manual recording of the number of shuttles completed. As the speed of walking increases every minute, indicated by a triple bleep [113], the patient should be advised: “You now need to increase your speed of walking.” During the test, only one verbal cue can be used to encourage the patient to pick up their speed: “You need to increase your speed to keep up with the test.”

Termination of the test

The test is terminated when either 1) the patient indicates that they are unable to continue, 2) the operator determines that the patient is not fit to continue, or 3) the operator assesses that the patient was unable to sustain the speed and cover the distance to the cone prior to the beep sounding [113].

Recording performance of the test

The assessor should calculate the distance walked, in metres, including the last 10-m length that was completed, and record this on the form available (see online supplementary material table S2).

Course

The test is conducted along the same course as described for the ISWT (fig. 1) [113].

Conduct of the test

Unlike the ISWT, the ESWT is not incremental and is performed at a constant speed, but there is a warm-up period of ∼1.5 min [77]. At this point, there are standardised instructions for the participant played from the audio recording, advising the individual that at the next bleep the speed of walking will increase (table 7). This is the speed at which the test is performed. It is important to pre-define the speed at which the test is going to be conducted. This can be calculated from the ISWT: the speed may be taken from a pre-defined percentage of peak performance on the ISWT (e.g. 70–85% estimated V′_O2peak) [114] or a percentage of the peak speed achieved [115]. Once the instructions have been played to the patient, they should be directed to one end of the course. A triple bleep indicates that the test has started [77]. The initial stages of the test are at a slower speed and are a warm-up for the participant. After the warm-up period, the speed of walking increases; this is advised on the audio recording at the end of the warm-up period. The timer is started at the end of the warm-up period. Participants are then paced for the first two shuttles. During the test, only one verbal cue can be used to encourage the patient to pick up their speed: “You need to increase your speed to keep up with the test.”

Termination of the ESWT

The procedure to terminate the test is as described for the ISWT [113].

Recording performance of the test

It is conventional for endurance performance to be recorded as time (in seconds). It is important that the speed of walking and time are recorded. A sample scoring sheet is included in the online supplementary material (table S2).

Absolute and relative contraindications for field walking tests

Research published since 2002 has shown that, in adults with moderate chronic respiratory disease, the 6MWT and ISWT elicit a V′_O2peak that is comparable to that on CPET [3–6, 8]. As a result, the absolute and relative contraindications for CPET have been adopted for field walking tests of exercise capacity.

Continuous measurement of S_pO2

New data show both the importance of nadir S_pO2 during the 6MWT as a marker of prognosis [38, 46, 116–119] and that end-test S_pO2 frequently does not reflect this important value [40, 41]. These factors, together with improvements in pulse oximetry technology in managing motion artefact, have made it feasible and desirable to measure S_pO2 continuously during field walking tests.

6MWT repetition

There are now consistent and compelling data showing a learning effect for the 6MWD [1, 9]. As a result, it is recommended that two 6MWTs are performed when the test is used to measure change over time.

Addition of the ISWT and ESWT

The ISWT and ESWT have emerged as robust and useful tests of exercise capacity in chronic respiratory disease [1]. The standard operating procedures for these tests are described here for the first time.

Common testing procedures for field walking tests

Testing procedures, such as test staffing, patient assessment, patient preparation, use of oxygen and medications, and quality assurance, are now consistent across the three field walking tests described in this Technical Standard.