How To Test For A 6Min Walk Test

CHEST Topics in Practice Management The 6-Min Walk Test* Clinical and Research Role, Technique, Coding, and Reimbursement Steve H. Salzman, MD, FCCP FEV 1 is recommended for rating the severity of obstructive and restrictive pulmonary diseases, but it only moderately correlates with quality of life, mortality, and functional status. The 6-min walk test (6MWT) has been increasingly used in clinical practice and research studies as an objective measurement of functional status in patients with moderate-to-severe impairment. This low complexity test measures the distance a patient can quickly walk back and forth in a 30-m (100-foot) corridor in a period of 6 min, referred to as the 6-min walk distance (6MWD). The 6MWD, and in some circumstances oxygen desaturation during the 6MWT, are useful to assess response to medical interventions, to assess prognosis in various conditions, and as a single measurement of functional status. Strictly scripted test instructions and encouragement at baseline and at each minute of exercise is vital to obtain reproducible results. The 6MWT is reported using Current Procedural Terminology code 94620 (simple pulmonary stress test). This code is also appropriate for other simple exercise tests, including oxygen titration (if additional parameters are measured), exercise-induced bronchospasm evaluation using pre- and postexercise spirometry, and exercise prescription for pulmonary rehabilitation. Use of code 94620 to bill for services must be supported by significant documentation. (CHEST 2009; 135:1345 1352) Key words: cardiopulmonary exercise test; congestive heart failure; COPD; correct coding initiative; Current Procedural Terminology; exercise pulse oximetry; exercise tests; exercise tolerance; hypoxemia; idiopathic pulmonary fibrosis; insurance; Medicare; mortality; physician reimbursement; physician services; practice management; pulmonary hypertension; respiratory function tests; 6-min walk distance; 6-min walk test Abbreviations: ATS American Thoracic Society; CHF congestive heart failure; CPET cardiopulmonary exercise test; CPT Current Procedural Terminology; E&M evaluation and management services; HR hazard ratio; ICD-9-CM International Classification of Diseases, ninth revision-clinical modification; IPAH idiopathic pulmonary arterial hypertension; MCID minimal clinically important difference; 6MWD 6-min walk distance; 6MWT 6-min walk test; Spo 2 pulse oximetric saturation *From the Division of Pulmonary and Critical Care Medicine, Winthrop-University Hospital, Mineola, NY; and the Albert Einstein College of Medicine, Bronx, NY. The author has reported to the ACCP that no significant conflicts of interest exist with any companies/organizations whose products or services may be discussed in this article. Manuscript received July 8, 2007; revision accepted February 5, 2009. Pulmonary function tests performed at rest continue to play a central role in patient management and research. The FEV 1 is recommended for rating severity in obstructive and restrictive pulmonary diseases. 1,2 However, correlation of FEV 1 and other single physiologic parameters with quality of life measures, mortality risk, and functional status is only moderate. 3,4 This has led to interest in broader measures of functional status. The 6-min walk test (6MWT) is a test of relatively low complexity that measures the distance that a patient can quickly walk in a 30-m (100-foot) hallway in a period of 6 min, referred to as the 6-min walk distance (6MWD). 5 It has emerged as the most commonly used test for the objective assessment of Reproduction of this article is prohibited without written permission from the American College of Chest Physicians (www.chestjournal. org/site/misc/reprints.xhtml). Correspondence to: Steve H. Salzman, MD, FCCP, Chief, Division of Pulmonary and Critical Care Medicine, 222 Station Plaza North, Suite 400, Mineola, NY 11501; e-mail: ssalzman@ winthrop.org DOI: 10.1378/chest.07-1682 www.chestjournal.org CHEST / 135 / 5/ MAY, 2009 1345

functional exercise capacity for patient management and research in patients with moderate-to-severe cardiopulmonary disease. 5 Alternative exercise tests, such as the cardiopulmonary exercise test (CPET) and the shuttle walk test, are also used (see the Advantages and Disadvantages of Alternative Exercise Tests section). These may provide complementary information, but CPET requires more complex and expensive equipment and higher levels of technologist training. 3,5 The shuttle walk test, an externally paced walk test employing a taped metronome, may be an optimal compromise between the complexity of CPET and the motivational issues in a self-paced 6MWT. However, the 6MWT has the advantage of wide acceptance and experience in both research and clinical settings. Indications and Contraindications The 6MWD, and in some circumstances oxygen desaturation during the 6MWT, are useful to assess response to medical interventions, to assess prognosis in various conditions, and as a single measurement of functional status. Table 1 outlines the specific indications and contraindications for the 6MWT. Test Procedure Table 1 Indications and Contraindications for Performing the 6MWT* Indications Response to a medical intervention (pretreatment and posttreatment comparisons) Lung transplantation Lung resection Lung volume reduction surgery Pulmonary rehabilitation COPD Pulmonary arterial hypertension CHF Single measurement of functional status COPD Cystic fibrosis CHF Peripheral vascular disease Fibromyalgia Predictors of morbidity and mortality CHF COPD IPAH Idiopathic pulmonary fibrosis Contraindications Absolute contraindications Unstable coronary disease Myocardial infarction in the preceding month Relative contraindications Resting tachycardia 120 beats/min Systolic BP 180 mm Hg Diastolic BP 100 mm Hg Arthritis, skeletal, or neuromuscular disease limiting ambulation Syncope during exercise (especially in pulmonary hypertension) *Based on data from references 4, 5, 14 to 22, and 35. The American Thoracic Society (ATS) published an official statement on the 6MWT in 2002. 5 A brief summary of these guidelines is included here. The 6MWT is performed indoors, along a straight flat corridor that is unlikely to be frequented by other people, with orange traffic cones marking the turning point. 5 The recommended length of the walking course is 30 m (100 feet), but a range of 15 to 50 m (mean, 30 m) resulted in no significant difference in 6MWD. 5,6 In addition, reference values for 6MWD are similar in those done on a 20-m and 50-m course. 7,8 Continuous walking courses that are oval or square, without a turnaround point, resulted in a mean 6MWD that was 34 m longer (10% higher) in patients with severe emphysema. 6 Current evidence favors a straight back-and-forth walking course of 30 m (100 feet), with a minimum distance of 15 m (50 feet). 5,6 This arrangement enhances cross-center reproducibility of measurements, 6 but circular courses or courses containing corners are acceptable if a lowtraffic linear course is not available. 6 The corridor should be marked at the starting point and every 3 m for measurement. 5 Patients should wear appropriate exercise attire. While resting in a chair at the starting point for at least 10 min, baseline measurements of pulse, BP, and pulse oximetric saturation (Spo 2 ) [optional, but typically done], and Borg scale ratings of both dyspnea and general fatigue are obtained. 5 These same measurements are repeated at the end of the 6MWT. Strictly scripted test instructions and encouragement at baseline and at each minute of exercise that includes elapsed time is vital to obtaining reproducible results. 5 The technologist should not walk with the subject. Note that the ATS guidelines consider measurement of Spo 2 optional and specifically comment that the technician must not walk with the patient to observe the Spo 2 (because it will interfere with the patient s self-pacing). 5 If used, a lightweight system that the patient does not need to hold, such as a battery-operated unit in a fanny pack, is important so the patient s stride is not disrupted. 5 The 6MWD is measured by counting the number of laps, measured by clicks on a counter or tick marks on a worksheet, plus the distance on the final partial lap, measured by counting the markers placed 3 m apart along the corridor. 5 The total distance walked is rounded to the nearest meter and recorded as the 6MWD. 5 There have been advocates for routine use of a practice test to increase reliability of measurements 1346 Topics in Practice Management

and comparisons with follow-up tests. 5,9,10 Some studies suggest a learning effect, with increases in 6MWD with repeated tests within hours or days. However, current guidelines recommend a practice test is not needed in most clinical settings but should be considered. If a practice test is done, wait for at least one hour before the second test and report the highest 6MWD as the patient s 6MWD baseline. 5 The recommendation not to include a practice test routinely 5 in part reflects the increased time and patient burden it would entail. Contemporary reference values for 6MWD measured without a practice or repeat test are available. 8,11 Familiarity with technical issues related to the reference equations for 6MWD is relevant for interpretation of test results. 6 11 If a patient is receiving long-term oxygen therapy, the 6MWT should be performed with oxygen at the prescribed rate, unless directed by a physician or protocol. Guidelines 5 have recommended that patients carry their own portable oxygen, although some clinical trials had staff members follow behind the patient with supplemental oxygen. 6 If the oxygen flow rate needed during exercise has not previously been determined, a separate oxygen titration study should be performed. This can be performed in a corridor or on a treadmill or ergometer. Because the protocol needed for oxygen titration interferes with an unimpeded 6MWT to determine the 6MWD, these assessments need to be done as sequential tests. Depending on the clinical or research protocol, the exercise oxygen titration test could precede or follow the 6MWT. Billing and coding issues for the oxygen titration are discussed in a separate section later. Safety Considerations The 6MWT should be performed by a technician certified in cardiopulmonary resuscitation at the basic life support level, although advanced cardiac life support certification is desirable. 5 Rapid response to an emergency includes the availability of oxygen, sublingual nitroglycerin, aspirin, and albuterol. 5 A crash cart should be available within the facility. Tests may be stopped due to chest pain, intolerable dyspnea, gait instability, or other signs of severe distress. 5 Under these circumstances the patient should sit or lie down, vital signs and Spo 2 should be measured, and a physician evaluation should be obtained. 5 Interpretation of a Single Measurement of 6MWD A single value of the 6MWD may be used as an objective assessment of functional status to help determine prognosis and aid in treatment decisions including organ transplantation. The measurement can be related to normal populations and expressed as a percentage predicted value 7,8,11 or evaluated as absolute distance walked and compared to diseasespecific cut points of severity that have prognostic significance (examples of the current clinical use of 6MWT are described later in the article). Interpretation of Change in 6MWD A change in 6MWD is usually reported as an absolute value in meters or feet. Alternatives are percentage change or change in percentage predicted. 5 Statistically significant differences in groups from clinical trials are usually much smaller than a clinically important change in an individual patient. Even changes that are statistically significant (larger than the expected test-retest repeatability range) may not be clinically meaningful to the patient. Minimal clinically important difference (MCID) is defined as the smallest meaningful change, judged by the patient or experts, determined by questioning or observing the patient. This change is necessarily larger than the change due to measurement error and day-to-day variability. A recent review 12 has suggested that the MCID for the 6MWD is 54 to 80 m. Using data from patients with severe COPD, 6 this review calculated a coefficient of repeatability of 86 m, suggesting that in this population smaller changes are not likely significant. 12 Others 3 have suggested that changes needed to exceed 107 m to be 95% confident that a short-term improvement had occurred, and 87 m for longer term changes (over 4 weeks), where the learning effect on 6MWT could be discounted. In stable patients with severe COPD, the 6MWD needed to change by 54 m for the average patient to stop rating himself or herself as about the same as other patients in the rehabilitation group. 13 Of note, patients were not very accurate at comparing their performance to a 6MWT done months previously. 13 They also had an optimistic bias, needing a larger drop in their 6MWD ( 70 m) before they rated themselves as a little bit worse than the peer group, but smaller absolute improvements ( 40 m) before rating themselves a little bit better. 13 An asymmetric approach to the interpretation of meaningful change, with a lower threshold for improvement than for worsening, may be relevant in both research and clinical practice. A recent analysis 14 of the interpretation of change in 6MWD in patients with moderate-to-severe COPD (mean FEV 1, 39.2 14.1% predicted) comes from pooled data from nine prospective trials in five countries. Using three statistical methods, they estimated a significant change in 6MWD as 35 m, correwww.chestjournal.org CHEST / 135 / 5/ MAY, 2009 1347

sponding to a change of 10% from baseline 6MWD. This did not vary much by severity, sex, or age. The authors questioned whether a MCID could be found for the 6MWD in this population because there was low correlation between 6MWD and patient-important anchor questions on well-validated questionnaires and other tools that rate symptoms. 14 At this time, no firm conclusions can be made about what constitutes a meaningful change in 6MWD that is broadly applicable. It is likely that a meaningful response may differ depending on the severity of impairment, the underlying disease, or even on more subtle disease subphenotypes, although further research must define these differences. Examples of Current Clinical Use of 6MWT COPD Inclusion of the 6MWD along with FEV 1, dyspnea rating, and body mass index into a 10-point index (the body mass index, obstructive severity, dyspnea score, exercise tolerance [or BODE] index) was better at predicting mortality in COPD than FEV 1 alone. 4 In this index, 0 points are given for 6MWD 350 m, 1 point for 250 to 249 m, 2 points for 150 to 249 m, and 3 points for 149 m. The hazard ratio (HR) for death from any cause increased by 1.34 per 1-point increase in the BODE index and by 1.62 for death from respiratory causes. This demonstrates the important independent role that exercise capacity measured by the 6MWT has in predicting mortality in COPD. In another study, a 6MWD 200 m had a specificity of 84% for prediction of 6-month mortality after lung volume reduction surgery for emphysema. 15 The investigators concluded that patients with unacceptable risk for this procedure were identified by this 6MWT value and a resting Paco 2 45 mm Hg. A recent study of 576 COPD patients with a wide range of airflow obstruction seen at four centers in two countries reported on the value of 6MWD and Spo 2 during the 6MWT. 16 6MWD was a good independent predictor of all-cause and respiratory mortality, particularly for the group with FEV 1 50% of predicted (p 0.0001). Oxygen desaturation during exercise (fall in Spo 2 4% or to 90%) predicted mortality (relative risk, 2.63; 95% confidence interval, 1.53 to 4.51; p 0.001). However, this was not superior to Pao 2 at rest. 16 In a mixed group of patients with advanced lung disease undergoing evaluation for lung transplantation, 17 many of whom had severe COPD, 6MWD 300 m had an 80% mortality rate. Idiopathic Pulmonary Arterial Hypertension In patients with idiopathic pulmonary arterial hypertension (IPAH), 6MWD was significantly but modestly correlated with New York Heart Association functional class, baseline cardiac output, pulmonary vascular resistance, but not mean pulmonary artery pressure. 18 The 6MWD had strong significant correlation with CPET parameters including peak oxygen consumption, oxygen pulse, and ventilatory equivalents for carbon dioxide slope. Among the various parameters, only 6MWD 332 m independently related to mortality by multivariate analysis. In another study 19 of IPAH, 6MWD 300 m and reduction in Spo 2 10% at the end of the 6MWT were associated with increased mortality, with an HR of 2.4 and 2.9, respectively. Idiopathic Pulmonary Fibrosis The importance of measuring oxygen desaturation during the 6MWT was highlighted in a study of patients with idiopathic pulmonary fibrosis, which showed that decreases in Spo 2 during exercise had prognostic value. 20 During the 6MWT the subjects did not wear supplemental oxygen. Continuous measurements of Spo 2 were obtained, and the lowest Spo 2 was recorded. In addition, desaturation area was calculated by summing the difference between 100 and the measured Spo 2 at 1-min intervals during the 6MWT. The patient was allowed to continue the test until a Spo 2 of 86% was reached. For that minute, and all subsequent minutes that the patient would have walked if not stopped due to decrement in oxygen saturation, the patient received a desaturation score of 14 (the maximum score per minute). 6MWD category was a weak predictor of mortality (p 0.038), which was no longer significant when patients were split into presence or absence of Spo 2 88% during exercise. 20 Patients with an Spo 2 88% during their 6MWT had a median survival time of 3.2 years, compared to 6.8 years in those with lesser degrees of oxygen desaturation (p 0.006). 20 Even in those with milder desaturation, a 10-point increase in desaturation area gave a HR for mortality of 1.33 (95% confidence interval, 1.08 to 1.63; p 0.007). 20 In addition, the predictive value of serial changes in physiology varied when patients were stratified by Spo 2 88% during the baseline 6MWT. 20 For patients with baseline 6MWT Spo 2 88%, the best predictor of mortality was serial decreases in diffusing capacity for carbon monoxide. 20 In those with lesser degrees of 6MWT desaturation at baseline, serial decrease in FVC and increases in desaturation area significantly predicted mortality. 20 1348 Topics in Practice Management

These findings highlight the potential importance of continuous measurement of Spo 2 during 6MWT to assess prognosis, an approach specifically not recommended by the ATS guidelines because it might interfere with the patient s self-pacing. 5 To avoid this, the technician should not walk with the patient and lightweight systems that do not interfere with gait should be used, as mentioned earlier. Congestive Heart Failure In patients with congestive heart failure (CHF), those with a 6MWD below the median of 218 m had a 4.6-fold higher mortality risk. 21 For every 100-m increase in 6MWD, the HR for death, hospitalization, and the composite of the two end points was 0.58, 0.85, and 0.75, respectively. 21 In another CHF study, 22 6MWD and short-term change in 6MWD in response to change in therapy were significant independent predictors of survival. Coding and Billing The American Medical Association has developed and annually updates the Current Procedural Terminology (CPT), which consists of descriptive terms and identifying codes for reporting medical services and procedures. The CPT includes a 5-digit code for the procedure and optional 2-digit modifiers to provide additional information. 23 25 CPT, Medicare, and most other payers recognize two components of most diagnostic tests, including most pulmonary function and exercise tests. There is a professional component, consisting of the interpretation and report generated by the physician, indicated by a two-digit modifier (-26) appended to the five-digit code, and a technical component, which captures the technical expenses related to the performance of the test and is indicated by the modifier -TC. The entity billing for the technical component owns or leases the equipment and space, and it pays the technicians performing the tests. A global service, including both the professional and technical components, is identified by using the 5-digit code with no modifier. It is reported when the physician who performs the professional component is employed by the entity that performs the technical component. A selfemployed physician who interprets the tests, owns or leases the equipment and space, and pays the technicians to perform the tests should also use the global service code. A 6MWT is reported using the simple pulmonary stress test code 94620 for the global service, or if billing for the professional and technical components separately, using 94620-26 and 94620-TC, respectively. 23 25 Pulmonary exercise testing is coded as either simple (94620) or complex (94621). For CPT 2007 the descriptor for code 94620 was changed to pulmonary stress testing; simple (eg, 6MWT, prolonged exercise test for bronchospasm with pre- and postspirometry and oximetry). Previously, it was not clear whether 94620 was a valid code for the 6MWT because the descriptor was pulmonary stress testing; simple (eg, prolonged exercise test for bronchospasm with pre- and postspirometry and oximetry), leading some to assume that spirometry might be a required element of this code. 23,24,26 Other forms of simple pulmonary exercise (stress) tests can appropriately be coded using 94620, including oxygen titration, exercise-induced bronchospasm evaluation using pre- and postexercise spirometry, or exercise prescription for pulmonary rehabilitation. 24,26 Appropriate documentation is needed for clinical purposes and to support the use of code 94620. Sample forms for use during a 6MWT and for reporting are available. 5,24 Data that might be included are patient s name, age, height, weight, sex, ordering physician, date of test, diagnosis, 6MWD (or time, speed, and grade if done on a treadmill), comments, interpretation, change from baseline, and recommendations. Other data recorded at rest and after 6 min of exercise included heart rate, BP, Spo 2, Borg scale for dyspnea and general fatigue, the flow of oxygen (in liters) if used, and spirometry findings at baseline and after exercise if relevant. In contrast to the simple exercise test (94620) already described, complex pulmonary stress testing (94621), or CPET, is typically performed on a bicycle ergometer or treadmill, with cardiac monitoring and a metabolic cart to measure oxygen consumption and carbon dioxide production, as well as other measured and calculated variables (eg, peak oxygen consumption, anaerobic threshold, and minute ventilation). CPT uses the following description for 94621: Pulmonary stress testing; complex (including measurements of CO 2 production, O 2 uptake, and ECG recordings). 23,24 The Center for Medicare and Medicaid Services developed a national Correct Coding Initiative to minimize improper coding and avoid unbundling, the intentional or accidental splitting up of services into components that are more appropriately reported by a single comprehensive code. 27,28 For example, 94621 (complex pulmonary stress testing) should not be billed with 94680 (oxygen uptake, rest, and exercise) or 94681 (oxygen uptake and CO 2 output). However, 94620 (simple pulmonary exercise test) and 94621 (complex pulmonary exercise test) can be billed with 93018 (interpretation and report of cardiovascular stress test) when indicated. 24 A separate report for the cardiac portion and an appropriate International Classification of Diseases, www.chestjournal.org CHEST / 135 / 5/ MAY, 2009 1349

Table 2 Medicare Reimbursement Year 2008* CPT Codes Region Global Fee Technical Fee Professional Fee 94620 (simple pulmonary exercise test) New York City metropolitan area $110.14 $77.20 $32.94 Indiana $80.84 $52.67 $28.27 Kentucky $78.59 $50.53 $28.07 94621 (complex pulmonary exercise test) New York City metropolitan area $190.89 $115.89 $75.00 Indiana $144.18 $80.11 $64.07 Kentucky $140.01 $76.31 $63.39 93018 (interpretation and report of cardiovascular stress test) New York City metropolitan area NA NA $17.34 Indiana NA NA $14.61 Kentucky NA NA $14.43 *New York City metropolitan area New York City boroughs of the Bronx, Brooklyn, and Staten Island, and the neighboring suburban counties of Nassau, Suffolk, Westchester, and Rockland; NA not applicable. Fee schedules are from references 29 to 31. ninth revision-clinical modification (ICD-9-CM) code should be used to support medical necessity. 24 Table 2 29 31 presents the Medicare reimbursement for the main exercise testing codes. When a single measurement of Spo 2 is obtained, typically at rest, code 94760 is used. When multiple measurements of Spo 2 are obtained without the documentation of other exercise parameters as required for the simple exercise code (94620), the appropriate code is 94761 (Spo 2, multiple determinations). A representative Medicare reimbursement is $3.59 and $7.82, respectively, for codes 94760 and 94761 (actual amount varies slightly by location). 29 These two codes are designated as T codes, which means they cannot be billed with any other service on the same day by the same provider. Consequently, these codes cannot be billed with simple and complex pulmonary exercise codes (94620 or 94621) or any other pulmonary function testing or evaluation and management services (E&M) codes on the same day. 24,25,27,28 Code 94761 is appropriate if Spo 2 is the sole measurement during exercise testing. When multiple other parameters are measured, 94620 or 94621 (simple or complex exercise testing) is used to report services. 24,28 Appropriate documentation in the medical record is required to support the higher level of service implied by the use of codes 94620 and 94621. When a patient has a 6MWT (or other procedures) on the same day as an E&M service (new patient visit, consultation, or subsequent visit) a -25 modifier is appended to the E&M code to indicate a significant, separately identifiable evaluation and management service by the same physician on the same day of the procedure or other service. 23,32 An attempt should be made to use a distinct ICD-9-CM code for the procedure and E&M code, to demonstrate the medical necessity of each CPT code. 32 For example, the consultation might be linked to dyspnea and the 6MWT might be linked to COPD, as appropriate. The use of the modifier -25 and a distinct ICD-9-CM code may not be necessary in all localities when PFTs and E&M services are performed on the same day. 24 Physician Supervision Requirements The Center for Medicare and Medicaid Services has defined physician supervision requirements for diagnostic testing. 24 Most pulmonary function testing, including the 6MWT and other simple pulmonary exercise tests (94620), requires general supervision (level 1), indicating that the procedure was furnished under the physician s overall direction and control (ie, the physician was available by phone or beeper but physical presence is not required). With direct supervision (level 2), physician presence in the office suite and immediate availability to assist and direct during the procedure are required for the performance of complex pulmonary exercise tests (code 94621). With personal supervision (level 3), attendance in the room during the performance of the procedure is not required for exercise testing. 24 Advantages and Disadvantages of Alternative Exercise Tests Although the 6MWT has been widely adopted due to its simplicity, it has some practical and conceptual limitations that make other tests preferable in some settings. The effects of motivation and strategy on the measured 6MWD need to be minimized by strict technician adherence to the protocol. In addition, finding a sufficiently long corridor (ideally 30 m, acceptably 15 m) that does not have cross traffic can be a challenge in many practice settings. Abnormal 6MWT responses are inherently nonspecific. By comparison, the numerous measurements taken during CPET enhance the ability to make a diagnosis of the pathophysiology causing exercise intolerance. 33,34 CPET is typically per- 1350 Topics in Practice Management

formed as an incremental, symptom-limited exercise test on an ergometer in a laboratory setting. The patient is taken from low levels of exercise intensity progressively up to the maximum tolerated workload (measured both as maximum watts [external work rate] and maximum oxygen consumption achieved). Advantages over the 6MWT include an assessment of maximal aerobic capacity (maximum oxygen consumption), its contribution to the differential diagnosis of exercise intolerance, and its greater discrimination for prognosis assessment in some settings. For example, both low 6MWD and low maximum workload on CPET predicted mortality on univariate analysis in patients with severe emphysema in the National Emphysema Treatment Trial. 35 On multivariate analysis, low maximum workload on CPET remained a strong predictor, but 6MWD lost its predictive value. 35 Due to the wide dynamic range of exercise intensity used during CPET, the test is applicable to patients with mild through severe disease, and even higher exercise intensities as seen in normal subjects and highly trained endurance athletes. As a test that by definition limits subjects to walking speeds, the 6MWT exhibits a plateau effect and will find its greatest utility in assessing the patient with moderateto-severe disease. Less compromised patients will fall in the normal range, with lesser ability to discriminate among mild degrees of impairment, which might affect running but not walking speed. This will limit its ability to measure functional compromise and response to therapy when the population studied has more mild disease. However, the 6MWT has the advantage of simplicity and low cost, and it is focused more on the assessment of functional status and disability in patients with moderate-to-severe disease (eg, lung and heart transplantation candidates and IPAH). CPET can also be performed as a symptomlimited constant work rate cycle ergometry study. This approach is frequently used in pharmacologic studies, for example, comparing measurements of endurance time, symptom intensity, and inspiratory capacity at a constant load of 75% maximum work capacity before and after medication. 36 Alternative field-based walking tests are the endurance and incremental shuttle walks, which involve a tape-recorded metronome to pace walking speed on a 10-m oval course and have properties that reflect true maximal or endurance challenges as opposed to the self-paced performance of the 6MWT. 3,13,37,38 In some ways these combine the simplicity of the 6MWT with the objectivity and incremental aspects of CPET. In the incremental shuttle walk test, the walking speed progressively increases with the audio signal until the patient cannot reach the turnaround point within the required time. The endurance shuttle walk is performed at a constant pace of 85% of maximal oxygen consumption, determined from a prior incremental shuttle walk. The advantages of the shuttle walk tests are that external pacing should remove the motivation, strategy, and learning that can affect the 6MWT, and the incremental shuttle walk test can provide an assessment of maximal aerobic capacity at a lower cost and complexity than CPET. 3,13 Another alternative that some prefer due to its ready availability and safety is to perform a symptom-limited treadmill test with cardiac and Spo 2 monitoring. Clarification of the preferred exercise test in various research and clinical settings is still evolving. 3,13 Conclusions The 6MWT has emerged as an important and relatively simple test of functional capacity that is useful to assess degree of dysfunction, prognosis, and response to therapy in patients who are moderately to severely impaired with a range of cardiopulmonary and other conditions. Despite this seeming simplicity, significant care needs to be taken to use an appropriately long and quiet corridor and to assure that personnel are familiar with and adhere to strict protocols of instruction, encouragement, and measurement prior to and during the test. Measurement of Spo 2 during the 6MWT was considered optional in the ATS guidelines, 5 but subsequent research has shown that it has additional prognostic information in some clinical settings. The 6MWT is appropriately reported using CPT code 94620, but it must be supported by significant documentation beyond 6MWD and Spo 2. References 1 Pellegrino R, Viegi G, Brusasco V, et al. Interpretative strategies for lung function tests. Eur Respir J 2005; 26:948 968 2 Pauwels R, Anthonisen N, Bailey WC, et al. Global Initiative for Chronic Obstructive Lung Disease: global strategy for the diagnosis, management and prevention of chronic obstructive pulmonary disease; NHLBI/WHO workshop report. Bethesda, MD: National Heart, Lung and Blood Institute, 2001; NIH Publication No. 2701 3 Patel SA, Sciurba FC. Emerging concepts in outcome assessment for COPD clinical trials. Semin Respir Crit Care Med 2005; 26:253 262 4 Celli BR, Cote CG, Marin JM, et al. The body-mass index, airflow obstruction, dyspnea, and exercise capacity index in chronic obstructive pulmonary disease. N Engl J Med 2004; 350:1005 1012 5 American Thoracic Society. ATS statement: guidelines for the six-minute walk test. Am J Respir Crit Care Med 2002; 166:111 117 www.chestjournal.org CHEST / 135 / 5/ MAY, 2009 1351

6 Sciurba F, Criner GJ, Lee SM, et al. Six-minute walk distance in chronic obstructive pulmonary disease: reproducibility and effect of walking course layout and length. Am J Respir Crit Care Med 2003; 167:1522 1527 7 Gibbons WJ, Fruchter N, Sloan S. Reference values for a multiple repetition 6-minute walk test in adults older than 20 years. J Cardiopulm Rehabil 2001; 21:87 93 8 Troosters T, Gosselink R, Decramer M. Six minute walking distance in healthy elderly subjects. Eur Respir J 1999; 14:270 274 9 Brooks D, Solway S. ATS statement on six-minute walk test [letter]. Am J Respir Crit Care Med 2003; 167:1287 10 Gibbons WJ. ATS statement on six-minute walk test [letter]. Am J Respir Crit Care Med 2003; 167:1287 11 Enright PL, Sherill DL. Reference equations for the sixminute walk in healthy adults. Am J Respir Crit Care Med 1998; 158:1384 1387 12 Wise RA, Brown CD. Minimal clinically important differences in the six-minute walk test and the incremental shuttle walk test. COPD 2005; 2:125 129 13 Redelmeier DA, Bayoumi AM, Goldstein RS, et al. Interpreting small differences in functional status: the six minute walk test in chronic lung disease patients. Am J Respir Crit Care Med 1997; 155:1278 1282 14 Puhan MA, Mador MJ, Held U, et al. Interpretation of treatment changes in 6-minute walk distance in patients with COPD. Eur Respir J 2008; 32:637 643 15 Szekely LA, Oelberg DA, Wright C, et al. Preoperative predictors of operative morbidity and mortality in COPD patients undergoing bilateral lung volume reduction surgery. Chest 1997; 111:550 558 16 Casanova C, Cote C, Marin JM, et al. Distance and oxygen desaturation during the 6-min walk test as predictors of long-term mortality in patients with COPD. Chest 2008; 134:746 752 17 Kadikar A, Maurer J, Kesten S. The six-minute walk test: a guide to assessment for lung transplantation. J Heart Lung Transplant 1997; 16:313 319 18 Miyamoto S, Nagaya N, Satoh T, et al. 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