Six-Minute Walk Test

Abstract

The 6-min walk test (6MWT) is a commonly used test for the objective assessment of functional exercise capacity for the management of patients with moderate-to-severe pulmonary disease. Unlike pulmonary function testing, the 6MWT captures the often coexisting extrapulmonary manifestations of chronic respiratory disease, including cardiovascular disease, frailty, sarcopenia, and cancer. In contrast with cardiopulmonary exercise stress testing, this test does not require complex equipment or technical expertise. In this low complexity, safe test, the patient is asked to walk as far as possible along a 30-m minimally trafficked corridor for a period of 6 min with the primary outcome measure being the 6-min walk distance (6MWD) measured in meters. There has been interest in other derived indexes, such as distance-desaturation product (the product of nadir oxygen saturation and walk distance), which in small studies has been predictive of morbidity and mortality in certain chronic respiratory conditions. Special attention to methodology is required to produce reliable and reproducible results. Factors that can affect walk distance include track layout (continuous vs straight), track length, oxygen amount and portability, learning effect, and verbal encouragement. The absolute 6MWD and change in 6MWD are predictive of morbidity and mortality in patients with COPD, pulmonary arterial hypertension, and idiopathic pulmonary fibrosis and patients awaiting lung transplant, highlighting its use in management decisions and clinical trials. As of January 2018, Current Procedural Terminology code 94620 (simple pulmonary stress test) has been deleted and replaced by two new codes, 94617 and 94618. Code 94617 includes exercise test for bronchospasm including pre- and postspirometry, ECG recordings, and pulse oximetry. Code 94618, pulmonary stress testing (eg, 6MWT), includes the measurement of heart rate, oximetry, and oxygen titration when performed. If 94620 is billed after January 2018 it will not be reimbursed.

Pulmonary function tests have a central role in patient management and research. FEV₁ has traditionally been recommended for rating severity in obstructive and restrictive pulmonary diseases.^1,2 However, correlations of FEV₁ with quality of life, mortality risk, and functional status are only moderate.^3,4 It is likely this single parameter does not capture the often coexisting extrapulmonary manifestations of chronic respiratory disease, including cardiovascular disease, frailty, sarcopenia, and cancer, which contribute to morbidity and mortality.^5,6 This has led to interest in broader measures of functional capacity. Peak oxygen uptake obtained from cardiopulmonary exercise testing (CPET) has proven to be a better predictor of mortality and health-care related quality of life than FEV₁.⁷ Unfortunately, obtaining this measurement requires complex equipment and technical expertise. Additionally, patients with advanced disease are often unable to perform CPET because of severe functional limitation.^8,9

The 6-min walk test (6MWT) is a commonly used test for the objective assessment of functional exercise capacity for the management of patients with moderate-to-severe pulmonary disease. In this relatively low complexity test, the patient is asked to walk as far as possible along a 30-m corridor for a period of 6 min with the primary outcome measure being 6-min walk distance (6MWD).¹⁰

Changes in 6MWD and other derived measurements can be used to determine treatment response and predict morbidity and mortality in chronic respiratory diseases.¹⁰ Relative and absolute contraindications and the specific indications for 6MWT are outlined in Table 1.

Table 1.

Indications and Contraindications for Performing the 6-Min Walk Test

Indications10

Response to a medical intervention (pretreatment and posttreatment comparisons)

Lung transplantation

Lung resection

Lung volume reduction surgery

Pulmonary rehabilitation

COPD

Pulmonary arterial hypertension

Congestive heart failure

Single measurement of functional status

COPD

Cystic fibrosis

Congestive heart failure

Peripheral vascular disease

Fibromyalgia

Predictors of morbidity and mortality

Congestive heart failure

COPD

Idiopathic pulmonary arterial hypertension

Idiopathic pulmonary fibrosis

Absolute contraindications10

Acute myocardial infarction (3-5 d)

Unstable angina

Uncontrolled arrhythmias causing symptoms or hemodynamic compromise

Syncope

Acute endocarditis

Acute myocarditis or pericarditis

Symptomatic severe aortic stenosis

Uncontrolled heart failure

Acute pulmonary embolism or pulmonary infarction

Thrombosis of lower extremities

Suspected dissecting aneurysm

Uncontrolled asthma

Pulmonary edema

Room air Spo2 ≤ 85%

Acute respiratory failure

Acute noncardiopulmonary disorder that may affect exercise performance or be aggravated by exercise

Mental impairment preventing cooperation with examination

Relative contraindications10

Left main coronary stenosis or its equivalent

Moderate stenotic valvular disease

Untreated hypertension at rest (200 mm Hg systolic, 180 mm Hg diastolic)

High degree atrioventricular block

Hypertrophic cardiomyopathy

Significant pulmonary hypertension

Advanced or complicated pregnancy

Electrolyte abnormalities

Orthopedic impairment that prevents walking

Spo₂ = oxygen saturation measured by pulse oximetry.

Test Procedure

The American Thoracic Society (ATS) published guidelines¹¹ for the 6MWT in 2002 with a subsequent joint European Respiratory Society and ATS updated systematic review¹⁸ and technical standard¹⁰ in 2014. A brief summary is outlined here.

Standardized 6MWT methodology is essential for reproducible and reliable results. The test should be performed in a minimally trafficked area along a flat, straight corridor ideally ≥ 30 m in length to be consistent with established reference equations.¹¹ There are reference equations for shorter tracks, including 20 m¹² and 10 m¹³ to reflect space limitations in practice.¹⁴ Both track length and layout affect walk distance. For example, in a crossover study of patients with COPD, a track length of 30 m resulted in significantly greater walk distance in comparison with 10 m.¹⁵ Continuous tracks in either square, circular, or oval layouts resulted in greater walk distances,^16,17 reflecting the time required to make abrupt turns.¹⁸ Treadmills offer the advantage of compact space requirements and easy continuous monitoring, but unfamiliarity with the machinery can lead to significantly lower walk distance.^19,20 Table 2 provides a summary of procedural guidelines.^10,11,15,18,21, 22, 23

Table 2.

Procedural Guidelines

Mark the starting line with brightly colored tape11

Mark the length of the hallway every 3 m11

Mark turn around points with a cone11

Patients should be wearing comfortable clothing and use their usual walking aids11

Patient should be using their prescribed oxygen therapy and manage their oxygen delivery device. If this is not possible, assessor is to walk slightly behind to avoid setting the pace10,18

Notation should be made of how patient was assisted with oxygen because subsequent tests should be performed in the same fashion10,18

Oxygen should not be titrated during the study because supplemental oxygen and its portability affect exercise performance and walk distance11

The patient should rest for at least 10 min prior to commencement of testing11

During this time, BP, heart rate, Spo2, and baseline dyspnea and fatigue should be documented11

Continuous Spo2 should be monitored to capture nadir Spo2, which does not always correlate with end test Spo210,21

If the patient stops during testing, the timer should not be stopped. The time at which the patient stopped and recommenced walking should be noted11

Reasons for premature cessation of testing by the patient include symptoms of chest pain, intolerable dyspnea, or leg pain11

Assessor may terminate testing based on appearance of the patient or if oxygen saturation falls < 80%22

Walk distance is measured by counting the number of full laps and rounding to the nearest meter for the partial final lap11,18

At test cessation, the parameters measured during the pretesting period are repeated11,18

Safety considerations include the following:

•Technician or other testing providers should be certified in basic life support and cardiopulmonary resuscitation

•Access to emergency equipment including a crash cart and medications including sublingual nitrogen, aspirin, and bronchodilators15,23

See Table 1 legend for expansion of abbreviation.

Verbal encouragement is commonly used to enhance participation. Careful attention should be given to the language used to instruct the patient and the frequency of encouragement. The scripted instructions in the guidelines state “the object of this test is to walk as far as possible for 6 minutes.”¹⁰ In a study of patients with idiopathic pulmonary fibrosis (IPF), interstitial lung disease, and pulmonary arterial hypertension (PAH), focus on speed by instructing them to “walk as fast as you can” led to a considerable increase in 6MWD.²⁴ Nevertheless, it is recommended against use of this phrase because the initial benefit of increased speed may be offset by fatigue experienced later and may place undue stress on patients with cardiac conditions.¹¹ The guidelines provide standardized encouragement phrases to be delivered exclusively at 1-min intervals because frequency of encouragement can also affect walk distance.^11,23

Two tests should be performed given the learning effect. The most robust data originate from patients with COPD; however, limited data in other respiratory diseases suggest the same.^25,26 The largest retrospective observational study including 1,514 patients with COPD performing the 6MWT on subsequent days found an average increase in 6MWD of 27 m, with most experiencing an improvement on the second walk.²⁷ Performing tests on subsequent days is not practical in clinical settings. A suggested best of two approach may balance underperformance on the initial test in those lacking familiarity and underperformance on the subsequent test from fatigue.²⁸

Safety Considerations

The 6MWT is a safe test with rare complications (Table 2). In a large study of outpatients with COPD or interstitial lung disease performing the test in pulmonary rehabilitation, the most common adverse event was oxygen desaturation ≤ 80% in 5% of testing. Patient symptoms prematurely terminated the test in 1% of testing.²²

Novel Indexes

There has been interest in other derived indexes with prognostic implications. The distance-desaturation product (DSP) is defined as the product of nadir oxygen saturation measured by pulse oximetry (Spo₂) on room air and 6MWD in meters, easily calculated with continuous pulse oximetry (expressed as meters-percent [m%]).¹⁸ In a prospective longitudinal study assessing clinical outcomes of patients with COPD, a DSP ≤ 290 m% better predicted mortality than 6MWD ≤ 334 m or Spo₂ ≤ 88% after adjusting for age, sex, BMI, FEV₁, FEV₁ to FVC ratio, St. George’s Respiratory Questionnaire, emphysema, and smoking.²⁹ Similarly, in a cohort of patients with IPF, DSP < 200 m% was associated with a 6.5-fold greater mortality and performed better as a screening tool for 12-month mortality than either 6MWD or percent nadir desaturation alone.³⁰

A more complex and less widely used index, the desaturation area, defined as the total area above the curve between Spo₂ observed at every minute during the 6MWT and 100% has been studied in patients with IPF, showing an increased hazard ratio (HR) for mortality for every 10-point increase in the desaturation area.^18,31

The 6-min walk work is the product of 6MWD and body weight, which may better reflect the amount of energy required to complete the test.¹⁸ Studies in patients with COPD have evaluated its correlation with physiological measures of exercise. It correlates more strongly to peak oxygen uptake than 6MWD.^32,33

Interpretation of Change in 6MWD

Change over time in 6MWD can be expressed as absolute difference in meters, percentage change, or change in percent predicted. See Table 3 for details.^11,34,35

Table 3.

Interpretation of Change in 6-Min Walk Distance

•Can be expressed as absolute difference in meters, percentage change, or change in percent predicted.

•ATS recommends change be expressed in terms of absolute value.11

•MID used because statistically significant change may not have clinical relevance.

•MID is smallest change in 6-min walk distance that is perceived to be important enough to prompt modification in management by the physician.34

•Most studies use statistical distribution-based methods to determine this difference rather than anchor-based methods, which incorporates the patient’s perspective on change.35

•In a study of patients with COPD who completed a 6-min walk test before and after a 7-wk rehabilitation program, a MID of 25 m was identified when using the anchor of patient’s perceived change in walk ability. This anchor-based method demonstrated excellent agreement with distribution-based calculation of MID.34

MID = minimal important difference

Current Clinical Use of the 6MWT

COPD

There is a strong correlation between 6MWD and clinical outcomes in COPD, likely because 6MWD captures both the pulmonary and extrapulmonary manifestations of the disease. In fact, comorbid conditions likely account for > 50% of deaths in patients with COPD.⁵ Consequently, the inclusion of 6MWD with FEV₁, dyspnea, and BMI is a better predictor of mortality than FEV₁ alone. The BODE Index is a 10-point scale in which higher scores indicate a greater risk of death. The 6MWD is given 0 points for > 350 m, 1 point for 250 to 349 m, 2 points for 150 to 249 m, and 3 points for ≤ 149 m. Each one-point increment in the BODE Index increases the HR for death from any cause by 1.⁴ An updated BODE Index which assigns greater weight to the 6MWD more accurately predicted mortality than the original scoring system.³⁶

A study of longitudinal changes in 6MWD in 198 patients with severe COPD over a 2-year period found improved survival with increases in 6MWD when divided into discrete 100-m increments. Additionally, nonsurvivors had a significant decline in 6MWD of −40 m compared with −22 m for survivors over 1 year without a parallel change in FEV₁. The rate of decline in 6MWD between survivors and nonsurvivors was significantly different; however, FEV₁ was not. The authors concluded that although FEV₁ is useful in stratifying patients by disease severity, in patients with severe COPD, 6MWD may be a better predictor of mortality.³⁷

In a prospective observational study of 2,110 patients with clinically stable Global Initiative for Chronic Obstructive Lung Disease stage II to IV COPD where baseline and yearly 6MWTs were conducted, the 6MWD thresholds with the highest sensitivity and specificity for hospitalization or 3-year mortality were 357 and 334 m, respectively. Note, however, these thresholds pertained to the group as a whole and when stratified by age, these thresholds changed greatly.³⁸ Nevertheless, these values are consistent with a large meta-analysis revealing a discriminatory value of 350 m below which the risk of death and hospitalization increased quasilinearly.³⁹

IPF

The variable clinical course that is characteristic of IPF has led to interest in reliable and independent predictors of disease outcome. In patients with fibrotic lung diseases, the 6MWT is more reproducible than CPET and correlates strongly with the maximum rate of oxygen consumption measured during incremental exercise at maximum exercise intensity ($\dot{\mathrm{V}}$o₂ max).⁴⁰ Additionally, its simplicity likely results in greater patient acceptance. The 6MWT has been a useful predictor of outcomes in numerous clinical settings in patients with IPF.⁴¹ In a prospective observational study of patients newly diagnosed with IPF, 6MWD ≤ 72% predicted was a significant independent predictor of mortality with an HR of 3.27. When added to a composite physiological index (calculated based on extent of disease on CT scan, diffusing capacity of the lung for carbon monoxide [Dlco], FVC, and FEV₁)⁴² and Medical Research Council dyspnea scale score, it was able to predict 3-year mortality with 100% specificity.⁴³ Data from the effect of interferon gamma-1b on survival in patients with idiopathic pulmonary fibrosis (INSPIRE) clinical trial examining the role of interferon gamma-1b found that a 6MWD < 250 m was associated with a twofold increase in mortality at 1 year, and a decline in walk distance by > 50 m at 24 weeks was associated with close to threefold increase in mortality at 1 year.⁴⁴ A retrospective analysis of patients randomized to the placebo arm of pirfenidone found similar results; however, these were not statistically significant.⁴⁵ These findings highlight the use of the 6MWT as a surrogate marker for mortality in clinical trials.

Other variables measured during testing have prognostic implications in IPF. For example, heart rate recovery < 13 beats/min after cessation of testing is associated with a more than fivefold risk of death.⁴⁶ Heart rate recovery is also a predictor of the presence of pulmonary hypertension as measured via right-sided heart catheterization in patients with IPF.⁴⁷ Oxygen desaturation < 88% is also associated with significant mortality risk (HR, 4.47), even when adjusting for other physiological variables, including Dlco, FVC, and resting saturation.⁴⁸

Lung Transplant

Prior to 2005, allocation of lungs was largely dependent on wait-list times, regardless of disease severity.⁴⁹ This led many physicians to prematurely place patients on the transplant list to accrue time. Under this system, wait-list time was > 2 years and mortality was 10% while awaiting transplantation.⁵⁰ In 1995, to account for higher mortality rates in candidates with IPF, 90 days of wait time was granted at listing. In 2005, a new allocation system was implemented to improve equitable distribution of organs.⁵¹ The lung allocation score is a 0 to 100 scale using criteria predictive of both wait-list and posttransplant mortality. 6MWD has been incorporated into the scoring system as a dichotomous variable (above or below 150 ft [45.7 m]).⁴⁹ Controversy exists regarding the appropriate threshold, or if use as a continuous variable is preferable.⁵⁰

The International Society for Heart and Lung Transplantation guidelines uses disease-specific considerations for lung transplant referrals and listing. For IPF, referral is recommended for abnormal lung function defined by Dlco and FVC, functional limitation, or need for supplemental oxygen. Criteria for listing include desaturation < 88% on 6MWT, 6MWD < 250 m, or a decline by > 50 m in 6 months. For those with COPD, the BODE Index, which includes 6MWD, should be used in determining optimal timing for both referral and listing.⁵²

PAH

The first randomized controlled drug trial for PAH-specific therapy, published in 1996, used 6MWD as the primary outcome.⁵³ Interestingly, this was chosen by the sponsor as a compromise to the Food and Drug Administration’s requirement that the primary end point be a measure of patient symptoms, exercise capacity, or survival. Survival as an end point required a longer study, and World Health Organization functional class was deemed too subjective; therefore, 6MWD was used as an objective end point.⁵⁴ The increase in 6MWD seen in this study led the way for future PAH studies. However, there has been much debate regarding the prognostic value of 6MWD in PAH.

Treatment studies have examined three 6MWD parameters, including 6MWD at treatment initiation, 6MWD posttreatment, and the pre- to posttreatment change in 6MWD.⁵³ In clinical trials of epoprostenol, and epoprostenol with add-on sildenafil therapy, subjects who died had a significantly lower mean 6MWD at baseline.^55,56 A large meta-analysis of 16 randomized controlled trials in PAH including nearly 2,000 patients confirmed the association between 6MWD at baseline and mortality, particularly when the 6MWD was < 330 m.⁵⁷ Absolute 6MWD reached after therapy has also been shown to predict survival. In a French cohort of patients with World Health Organization class III or IV PAH treated with 4 months of bosentan, those who achieved a posttreatment 6MWD > 378 vs < 378 m had improved survival at 1, 2, and 3 years.⁵⁸ Unfortunately, no single absolute threshold value has been validated in the literature; therefore, a specific target threshold to gauge treatment benefit is unknown. In addition, several systematic reviews and meta-analyses of randomized trials have failed to show a survival benefit with improvement in 6MWD.⁵⁹ These data call into question the usefulness of improvement in 6MWD as a surrogate marker for clinical outcomes and stress the need for alternative end points that better represent clinical benefits. The European Cardiology Society and European Respiratory Society guidelines⁶⁰ for diagnosis and treatment of pulmonary hypertension still suggest obtaining 6MWT at baseline, and then every 3 to 6 months after initiation of treatment.⁶⁰

Coding and Billing

The American Medical Association⁶¹ has developed the Current Procedural Terminology (CPT), which consists of identifying codes for uniform reporting of medical services and procedures. The CPT consists of five digits to identify the procedure with optional addition of two-digit modifiers.⁶¹ Medicare and other payers recognize two components to most diagnostic tests, including pulmonary function and exercise testing. The professional component consists of the interpretation of the test and the report subsequently generated by the physician, identified by a 26 modifier to the five-digit procedural code. The technical component which captures the expenses related to the performance of the test, including the cost of technicians, equipment, and space, is indicated by the technical component (TC) modifier. The global service, which includes both the professional and technical components, is identified by the five-digit code alone. This code should be used when the physician who performs the professional component is used by the entity that performs the technical component or when a self-employed physician interprets the test but also owns or leases the equipment and pays the technician.⁶²

Prior to January 2018, the 6MWT was reported under a simple pulmonary stress test code (94620). Under this code, other types of exertional testing could also be reported including oxygen titration with oximetry, excised-induced bronchospasm with pre- and postexercise spirometry, or exercise prescription for pulmonary rehabilitation. The technical performance of the test and the interpretation of the data were included within the code (94620). Significant documentation to support the use of code 94620 was required including, but not limited to, total distance walked, interpretation, recommendations, and data collected during the testing including heart rate, BP, and oxygen saturation. Although it was recognized that spirometry is often performed at baseline and after exercise, this was not an essential requirement to use the global service code of 94620.⁶²

The CPT codes as of January 2018 have since been revised. The code 94620 has been eliminated, and two additional codes (94617 and 94618) have been added. The American Medical Association Relativity Assessment Workgroup,⁶³ which works to identify potentially misvalued services, determined that the two tests falling under 94620, including 6MWT and pre- and postexercise oximetry, were describing two completely different examinations. For this reason, two codes were added to better delineate the different examinations formerly housed under 94620. Code 94617 reflects exercise test for bronchospasm including pre- and postspirometry, ECG recordings, and pulse oximetry. Code 94618 only encompasses pulmonary stress testing (eg, 6MWT) including measurement of heart rate, oximetry, and oxygen titration when performed. Code 94621 remains unchanged to describe cardiopulmonary exercise testing, including measurements of minute ventilation, CO₂ production, oxygen uptake, and ECG recordings. Note that the term “simple” has been removed from the nomenclature. If CPT code 94620 is used after January 1, 2018, the claim will be denied.⁶¹ Table 4 lists Medicare reimbursement for 6MWT and related exercise tests.⁶⁴

Table 4.

Medicare Reimbursement for 6MWT and Related Exercise Tests, Year 2018^a

CPT Code	Region	Global Fee	Technical Fee	Professional Fee
94617: exercise test for bronchospasm	New York City Metro	$114.69	$76.59	$38.10
	Indiana	$90.39	$57.51	$32.89
	Kentucky	$88.45	$55.40	$33.05
94618: pulmonary stress testing (eg, 6MWT)	New York City Metro	$40.26	$14.22	$26.04
	Indiana	$32.91	$10.39	$22.52
	Kentucky	$32.74	$10.12	$22.62
94621: cardiopulmonary exercise testing	New York City Metro	$197.35	$118.67	$78.68
	Indiana	$157.42	$89.60	$67.82
	Kentucky	$154.22	$86.12	$68.09

Fee schedule comes from the Centers for Medicare and Medicaid Services.⁶⁴ 6MWT = 6-min walk test; CPT = Current Procedural Terminology; New York City Metro = New York City boroughs of the Bronx, Brooklyn, and Staten Island and the neighboring suburban counties of Nassau, Suffolk, Westchester, and Rockland.

^aFee schedule rates are in dollars for physicians participating in the Medicare program.

When a diagnostic test is performed on the same day as an evaluation and management (E/M) service (new patient visit, consultation, or subsequent visit), the addition of a 25 modifier (ie, “a separately identifiable E/M service on the same calendar day by the same physician”⁶²) should be used when an E/M service is also billed. There must be reasonable medical necessity to report an E/M code with a procedural code together. Designating separate International Classification of Diseases, Tenth Revision codes for the CPT code and E/M code, if appropriate, may help to delineate this medical necessity.⁶²

Conclusions

The 6MWT has emerged as a reliable measure of functional capacity that is simple to perform and interpret. The test, however, is extremely sensitive to methodology; therefore, a standardized approach is crucial. Careful attention must be given to track layout and length, oxygen therapy, and portability, in addition to strict adherence to protocols of instruction, verbal encouragement, and measurements before and after testing. The primary outcome measure, 6MWD, has been studied as a predictor of mortality across a wide range of chronic respiratory conditions, including COPD, IPF, and PAH, and wait-list mortality for lung transplant. Significant changes from previous ATS guidelines include the recommendation of continuous Spo₂ monitoring during testing and the performance of two tests given the clear learning effect. Previous CPT code of 94620 is no longer valid for billing of 6MWT and will not be reimbursed if billed under this code after January 1, 2018. Rather, 6MWT is appropriately coded under 94618 with significant documentation required beyond 6MWD and Spo_2.