Six-Minute Walk Test

Anatomy and Physiology

The primary physiological mechanisms relevant to the 6MWT involve the dynamic interactions between the heart and lungs during exercise. In general terms, deoxygenated blood returning to the right heart through 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.[6]

Oxygenated blood then circulates to the left heart, where the left ventricle pumps it into 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 coordinating minute ventilation to cardiac output in response to varying exercise intensity levels. This coordination occurs through reflex responses that modulate cardiac and pulmonary activity based on oxygen consumption levels required by the degree of exertion. Impairment in any of the cardiac, pulmonary, neurological, or musculoskeletal systems can lead to reduced exercise tolerance.

Indications

The 6MWT is primarily used to evaluate treatment response in patients with moderate to severe cardiac or pulmonary diseases.[5] The test is also useful for preoperative risk stratification. For example, patients who walk less than 70% of their predicted distance during preoperative assessment for pulmonary surgery are at a higher risk of postoperative pulmonary complications.[7]

Other guideline-based indications include the use of 6MWT to measure a patient's exercise tolerance and prognosis depending on the underlying condition.[5] As many organ systems are involved during physical activity, the 6MWT can be used to assess conditions that specifically affect any of these systems. Among pulmonary diseases, the 6MWT has been widely used in patients with chronic obstructive pulmonary disease.[8][9] 

The 6MWT is valuable in evaluating other conditions, including idiopathic pulmonary fibrosis, cystic fibrosis, and sarcoidosis.[10][11][12] Additionally, it serves as a useful adjunct in the assessment and management of cardiovascular conditions, such as heart failure and peripheral artery disease, which are associated with high morbidity and mortality.[13][14] The 6MWT is often used in the context of cardiac rehabilitation, as it reflects clinical change.[15]

The 6MWT is used to evaluate functional impairment resulting from neurological and muscular conditions, such as fibromyalgia, Parkinson disease, multiple sclerosis, and spinal muscular atrophy.[16][17][18][19] Furthermore, it is highly predictive of ambulatory capacity after total knee arthroplasty.[20]

Contraindications

The only established absolute contraindication to the 6MWT is a history of an acute coronary syndrome—either unstable angina or myocardial infarction—within 30 days before the test.[5] However, other absolute contraindications to field walking tests, 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.[2]

Relative contraindications include severe, uncontrolled hypertension and a resting heart rate greater than 120 bpm.[5] Although the 6MWT is self-paced, which reduces the likelihood of untoward events resulting from excessive exercise levels, clinician judgment is still necessary 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, if necessary for the patient; a chair; a validated dyspnea scale, such as the Borg scale; and a stopwatch.[2] At least 2 small cones should be available to mark the turnaround points. An automated electric defibrillator should also be within reach.[5] The test should be conducted in a comfortable space for the patient, allowing for an accurate measurement of the distance walked.

Personnel

The 6MWT should be conducted by medical personnel who are well-versed in the standardized testing protocol. Training in Basic Life Support is a minimum requirement for staff conducting the 6MWT. The presence of a clinician during the test may be requested at the discretion of the laboratory medical director or the prescribing clinician.[5]

Preparation

The following directives are important for preparing for the test, based on guidelines from the European Respiratory Society and American Thoracic Society: [2][5]

• Patients should wear comfortable clothing and appropriate walking shoes.
• Patients should use any mobility aid they normally require for ambulation, such as a cane or walker.
• Patients should continue with their usual medications.
• Patients should rest for at least 2 hours before the test.
• Perform successive tests at the same time of day to maintain consistency.
• Warm-ups are not allowed.
• Lung function tests should be conducted at least 15 minutes before the walk test if scheduled on the same day.

Technique or Treatment

Procedure Steps

Generally, the technician or other staff performing the 6MWT should follow these steps:

• The patient should rest for approximately 10 minutes before starting the test.
• The baseline heart rate and oxygen saturation should be measured. These parameters should be continuously monitored to identify the lowest oxygen saturation, which may occur before the test ends.[2]
• The patient's baseline dyspnea is then rated using the Borg scale.
• The lap counter and timer should be set.
• Detailed instructions on how to proceed during the test should be provided to the patient.
• 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, the Borg dyspnea and fatigue levels, heart rate, oxygen saturation, the number of laps from the counter or marks on the worksheet, and total distance walked are recorded.[5]

Interpretation

The 6MWT assesses functional exercise capacity, particularly in individuals with cardiopulmonary conditions. The distance walked in 6 minutes on a flat, straight course, typically 30 meters long, is recorded. The distance reflects submaximal (not peak) exercise capacity. The 6MWT is useful for tracking disease progression, assessing rehabilitation outcomes, and evaluating treatment response.[21]

Distance walked: A healthy adult typically walks 400 to 700 m. A distance of 300 m walked is associated with poor prognosis in conditions such as chronic obstructive pulmonary disease, heart failure, and pulmonary hypertension. Significant decline over time suggests disease progression. Test results are influenced by demographic factors, including age, sex, height, and weight.

Desaturation: A decrease in peripheral oxygen saturation (SpO₂) greater than 4% is clinically significant. An SpO₂ level below 88% during testing may indicate the need for supplemental oxygen.

Signs and symptoms: Clinicians document dyspnea, fatigue, chest pain, or leg discomfort using the Borg scale. Heart rate and blood pressure are recorded. These data help assess cardiovascular response and recovery after the test.

Complications

When standard protocols are followed, complications from the 6MWT are rare. As noted previously, the self-paced nature of the test limits its occurrence. The most common adverse event reported in the literature is oxygen desaturation below 80%, resulting in test termination.[22] Chest pain and tachycardia have been reported much less frequently. Notably, none of the reported adverse events resulted in long-term complications.[2]

Clinical Significance

The primary test outcome is the final distance walked, referred to as the 6-minute walk distance (6MWD).[2] Among healthy individuals, the average 6MWD is between 400 and 700 m.[23] This reference range overlaps with data reported from other studied populations.[24][25] Reference equations have been formulated to determine the 6MWD percent predicted and lower limit of normal for adults, with median walked distances being 494 m for women and 576 m for men. The lower limit of normal can be obtained by subtracting 139 m for women and 153 m for men from the expected 6MWD when using the corresponding equation.[26]

Factors influencing variability in test performance among healthy individuals include weight and age.[25][26] However, most of the variability remains unexplained.[5] The longest distance walked by the patient should be considered the actual result, as a learning effect is associated with improved walking distance when at least 2 tests are conducted.[23] The method used to conduct the test may also influence the patient's performance.[2]

The 6MWT has limitations as a measure of exercise capacity that should be taken into consideration. Due to its nature, the study cannot identify the cause of dyspnea, hypoxemia, or the mechanisms underlying a particular patient's exercise intolerance. Additionally, it cannot determine peak oxygen uptake, although it correlates with this measurement in selected patients undergoing cardiopulmonary exercise tests.[5]

In the absence of specific clinical clues or an established diagnosis, a low 6MWD is a nonspecific finding. In such situations, the cause of the abnormal finding should be investigated, with a focus on the cardiac, pulmonary, or musculoskeletal systems, as guided by the patient's medical history.[5]

Minimal Clinically Important Difference

According to the European Respiratory Society/American Thoracic Society technical standard, the minimal change in walking distance that can be interpreted as clinically meaningful is 30 m.[2] A systematic review of studies, each including patients with different cardiopulmonary conditions, identified a minimal clinically important difference ranging from 14.0 to 30.5 m.[27]

Interpretation in specific conditions:

• Chronic obstructive pulmonary disease: The 6MWD has been established as a significant marker of disease severity for patients with chronic obstructive pulmonary disease, as it directly correlates with lower quality of life indices, respiratory and functional impairment, and survival. The 6MWD also reflects the clinical response to treatments such as lung volume reduction surgery. However, it does not have the same role for pharmacological interventions.[8] For chronic lung diseases in general, the average 6MWD ranges from 300 to 450 m.[2] A 6MWD of 350 m or less is inversely correlated with the risk of exacerbation, hospitalization, and mortality.[8] In one study, the minimal clinically important difference, measured by a change in perceived clinical decline or improvement, was 54 m for patients with chronic obstructive pulmonary disease.[28]

• Idiopathic pulmonary fibrosis: In patients with idiopathic pulmonary fibrosis, a baseline 6MWD of less than 250 m and a subsequent decline greater than 50 m in 24 weeks are associated with a 2- and 3-fold increase in mortality, respectively.[29]

• Cardiovascular diseases: There is an inverse relation between New York Heart Association class and 6MWD, with mean values of approximately 400 m, 320 m, and 225 m for classes II, III, and IV, respectively. Decreased 6MWT performance is also associated with an increased risk of hospitalization and mortality.[1] In patients with heart failure, the 6MWD is more responsive to symptomatic deterioration than improvement.[5] The 6MWT also correlates with actual physical activity levels in patients with peripheral arterial disease, and this correlation appears to be stronger than that observed with treadmill testing.[14]

Other conditions independently associated with a negative 6MWT performance at the stage of the disease include chronic liver disease and renal failure.[30][31]

Alternative Indexes

• Body weight–walking distance product (D × W): This index has been proposed to have a better correlation between a patient's actual work of walking, maximal oxygen uptake, and anaerobic threshold, compared to walking distance alone.[32] However, its role in caring for adults with chronic respiratory disease remains undefined.[2]

• Distance–desaturation product: This index is the result of multiplying the lowest oxygen saturation while breathing ambient air by the 6MWD, expressed as meters-percent (m%). Values below 290 m% for chronic obstructive pulmonary disease and 200 m% in idiopathic pulmonary fibrosis are better predictors of increased mortality compared to the 6MWD.[3] 

Enhancing Healthcare Team Outcomes

Since test performance mainly reflects the integrated activity of the heart and lungs, information derived from the test is relevant to assessing the degree of impairment associated with conditions affecting either or both of these organs. Therefore, pulmonologists and cardiologists are often the primary clinicians who request the 6MWT and utilize the data obtained. Other healthcare professionals who may use the 6MWT include primary care providers and anesthesiologists as part of preoperative risk assessments or in the general evaluation of patients with chronic obstructive pulmonary disease.[33]

Revised reference equations are available for offices with limited space.[34] 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, the test may not be conducted reliably or safely. Accurate and timely notification to the ordering or supervising providers of test results or complications improves patient outcomes and safety.