Six-Minute Walk Test

Anatomy and Physiology

The interactions between the lungs and heart during exercise are the main physiological phenomena relevant to the test. In general terms, deoxygenated blood returning to the right heart via the venous circulation is pumped by the right ventricle into the pulmonary artery. As it advances through the pulmonary capillaries, gas exchange occurs when the capillaries meet the alveoli, with oxygen entering the circulation while carbon dioxide is released into the alveoli.

Oxygenated blood then circulates to the left heart, where the left ventricle pumps it to the systemic circulation, through which it is eventually delivered to the organs to sustain aerobic metabolism. The neurological and musculoskeletal systems are also involved in this process, particularly with concern to the process of matching minute ventilation to cardiac output in response to varying exercise intensity levels. This occurs via reflex responses that adjust the cardiac and pulmonary activity to the oxygen consumption levels required by the degree of exertion. Impairment of any of these components, the cardiac, pulmonary, neurological, or musculoskeletal systems, will decrease exercise tolerance.

Indications

The 6MWT is primarily indicated for assessing the response to treatment in moderate-to-severe cardiac or pulmonary disease.[2] It can also serve for preoperative risk stratification. For example, patients walking less than 70% of their predicted distance during preoperative assessment for pulmonary surgery had a higher risk of postoperative pulmonary complications.[3]

Other guideline-based indications for the 6MWT include its use to measure a patient's exercise tolerance and prognosis depending on the underlying condition.[2] Since there are many organ systems involved during physical activity, the 6MWT can be used to assess conditions specifically affecting any of these. Among pulmonary diseases, the 6MWT has been widely used in patients with COPD.[4][5] 

Other conditions in which it can be helpful include idiopathic pulmonary fibrosis, cystic fibrosis, and sarcoidosis.[6][7][8] Heart failure and peripheral artery disease are cardiovascular conditions with a significant burden of morbidity and mortality, where the 6MWT can be a useful adjunct in their assessment and management.[9][10] It is often used in the context of cardiac rehabilitation, being reflective of clinical change.[11]

Functional impairment resulting from conditions affecting the neurological and muscular systems, such as fibromyalgia, Parkinson disease, multiple sclerosis, and spinal muscular atrophy, can also be reliably evaluated with the 6MWT.[12][13][14][15] Furthermore, it has been found to be highly predictive of ambulatory capacity after total knee arthroplasty.[16]

Contraindications

The only established absolute contraindication to the 6MWT is having a history of an acute coronary syndrome (either unstable angina or myocardial infarction) within 30 days prior to the test.[2] However, other absolute contraindications applied to field walking tests in general, such as syncope, acute respiratory failure, or the presence of a noncardiopulmonary condition that may impair or become exacerbated by exercise, should also be considered.[1] 

Relative contraindications include severe, uncontrolled hypertension and a resting heart rate greater than 120 beats per minute.[2] While the 6MWT is self-paced, which would reduce the likelihood of untoward events resulting from otherwise excessive exercise levels, clinician judgment needs to be used when deciding to proceed with testing as with any other test.

Equipment

At a minimum, the equipment required for the test should include a pulse oximeter, a portable oxygen device (to be used if needed by the patient), a chair, a validated dyspnea scale (e.g., the Borg scale), and a stopwatch.[1] At least two small cones should be available to mark the turnaround points. An automated electric defibrillator should also be within reach.[2] The test should be done in a comfortable space for the patient and allow accurate measurement of the distance walked. 

Personnel

Medical personnel who must be knowledgeable in the standard protocol for the 6MWT conducts the test. A minimum requirement for staff conducting the six-minute walk test is training in Basic Life Support. Finally, the presence of a physician during the test may be requested at the discretion of the laboratory medical director or the prescribing physician.[2]

Preparation

The following are directives relevant to the preparation for the test, based on statements by the European Respiratory Society and American Thoracic Society [1][2]:

• Comfortable clothing, including walking shoes, should be worn.
• Patients should use any mobility aid device already needed to ambulate (e.g., a cane or walker).
• Patients should continue with their usual medications.
• Patients should rest for at least two hours before the test.
• Perform successive tests at the same time of the day to maintain consistency.
• Warm-ups are not allowed.
• Lung function tests should be done at least 15 minutes before the walk test if scheduled on the same day.

Technique or Treatment

In general, the technician or other staff performing the test should observe the following steps[2]:

• The patient should rest for approximately 10 minutes before starting the test.
• Measure the baseline heart rate and oxygen saturation. These should be continuously monitored to identify the lowest oxygen saturation, which may occur before the end of the test.[1]
• The patient's baseline dyspnea is then rated using the Borg scale.
• Set the lap counter and timer.
• Provide the patient with detailed instructions on how to proceed during the test.
• The patient is then positioned at the starting line and allowed to walk unassisted once the test begins.
• The technician should only address the patient during the test. As each minute passes, the patient should be informed of the time left to complete the test and encouraged to continue.
• At the end of the test, record the Borg dyspnea and fatigue levels, check heart rate and oxygen saturation, the number of laps from the counter or marks on the worksheet, and total distance walked.

Complications

When standard protocols are followed, complications are rare. As noted previously, the self-paced character of the test limits their occurrence. The most common adverse event reported in the literature is oxygen desaturation below 80% resulting in termination of the test.[17] Chest pain and tachycardia have been reported much less frequently. In all, none of the reported adverse events resulted in long-term complications.[1]

Clinical Significance

The primary test outcome is the final distance walked (6MWD).[1] Among healthy individuals, the average 6MWD is between 400 and 700 meters.[18] This range overlaps with data reported in other populations.[19][20] Reference equations are available to determine the 6MWD percent predicted and lower limit of normal for adults.[21] 

Factors associated with variability in test performance among healthy subjects include weight and age.[20][21] However, most of the variability remains unexplained.[2] The longest distance walked by the patient should be taken as the actual result since there is a learning effect associated with improved walking distance when at least two tests are conducted.[18] Other factors that may affect the 6MWD in healthy adults include the methodology for 6MWT and the percentage of peak heart rate achieved.[1]

The 6MWT has limitations as a measure of exercise capacity that needs to be considered. Due to its nature, the study cannot identify the cause of dyspnea, or hypoxemia, or the mechanisms underlying a particular patient's exercise intolerance. Additionally, it cannot determine peak oxygen uptake, though it correlates with this cardiopulmonary exercise test measurement in select patients.[2] 

In the absence of specific clinical clues or an established diagnosis, a low 6MWD is a non-specific finding. In such situations, the cause of the abnormal finding should be investigated, with attention to the cardiac, pulmonary, or musculoskeletal systems, as guided by the patient's medical history.[2]

The 6MWD has been established as a significant marker of disease severity for COPD patients since it directly correlates with lower quality of life indices, respiratory and functional impairment, and survival. It also reflects the clinical response to treatments such as lung volume reduction surgery. However, it does not have the same role for pharmacological interventions.[4] For chronic lung diseases in general, the average 6MWD ranges between 300 and 450 meters.[1] In COPD specifically, a 6MWD of 350 meters or less is inversely correlated with the risk of exacerbation, hospitalization, and mortality.[4] In one study, the minimal difference in walking distance associated with a change in perceived clinical decline or improvement was a mean of 54 meters.[22]

Other measures such as the body weight-walking distance product (D x W) have been proposed as alternative estimates of functional capacity in COPD patients. The DxW product correlates better with a patient's actual work of walking, as well as maximal oxygen uptake and anaerobic threshold, compared to walking distance alone.[23] However, its role in the care of adults with chronic respiratory disease remains undefined.[1]

In idiopathic pulmonary fibrosis, the 6MWD is also an independent predictor of mortality. A baseline 6MWD under 250 meters, and a subsequent decline greater than 50 meters in 24 weeks, are associated with a two- and three-fold increase in mortality, respectively.[24]

As noted, the 6MWT has also been validated for use in cardiovascular diseases. In heart failure, a condition associated with functional disability and exercise intolerance, both in a sense analogous to the impairment of daily activity observed among COPD patients; the 6MWT can provide reliable data about physiological reserve and prognosis. In general, there is an inverse relation between NYHA class and 6MWD, with mean values of approximately 400, 320, and 225 meters for classes II, III, and IV, respectively. Decreased 6MWT performance is also associated with an increased risk of hospitalization and mortality.[25] In patients with heart failure, the 6MWD is more responsive to symptomatic deterioration than improvement.[2]

The 6MWT also correlates with actual physical activity levels in patients with peripheral arterial disease, and it appears to be more so than treadmill testing.[10]

In general terms, the minimal clinically important 6MWD change for patients with either cardiac or pulmonary disease is in the range of 14.0 to 30.5 meters.[26]

While not affecting the organ systems involved in exercise directly, other conditions independently associated with a negative 6MWT performance in relation to the stage of the disease include chronic liver disease and renal failure.[27][28]

Enhancing Healthcare Team Outcomes

Since test performance mainly reflects the integrated activity of the heart and lungs, information derived from the test will be relevant to assessing the degree of impairment associated with conditions affecting either or both of these organs. Therefore, pulmonologists and cardiologists will often be the clinicians requesting the 6MWT and using the data obtained. Other healthcare professionals that may use the 6MWT include primary care providers and anesthesiologists as part of preoperative risk assessments or in the general evaluation of patients with COPD.[29] 

Modified reference equations are available for use in offices with limited space.[30] Considering the value that clinicians in a broad range of specialties can derive from this test, practitioners, in general, should be familiarized with it and the implications of abnormal results.

Proper training of respiratory therapists, nurses, medical assistants, and other personnel conducting the test is an essential aspect of the procedure. Without their dedicated participation through its performance, the test may not be conducted reliably or safely. Accurate and timely notification to the ordering or supervising providers of test results or complications will improve patient outcomes and safety.