PAH Six-Minute Walk Test
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Pulmonary arterial hypertension (PAH) is a rare, progressive disorder characterized by vascular proliferation and remodeling of the small pulmonary arteries that ultimately causes right heart failure and premature death. The predominant symptom of PAH is dyspnea on exertion, with a decrease in exercise capacity.1 A variety of methods are available for the objective assessment of functional exercise capacity, including functional walking tests. The six-minute walk test (6-MWT) is one of the most popular clinical exercise tests and a qualitative systematic overview  of functional walking tests concluded that “the six-minute walk test (6-MWT) is easy to administer, better tolerated, and more reflective of activities of daily living than the other walk tests”.2

Hasan Avcu, MD
H. Avcu, MD earned his Doctor of Medicine degree from Istanbul University Cerrahpasa School of Medicine in Istanbul, Turkey. He has been concentrating on medical communications, thought leader management, and healthcare compliance for the last decade. He has had numerous scientific interactions, implemented educational activities, and organized advisory board meetings with thought leaders in several therapeutic areas mainly focusing on rare diseases.

Uses and Limitations of the 6-MWT

Two important uses of the 6-MWT are pretreatment and posttreatment comparisons and predicting morbidity and mortality in patients with pulmonary hypertension. Research has shown that a six-minute walk distance (6-MWD) threshold of 440 m is associated with longer survival, a threshold of 165 m is associated with increased mortality, and an increase in 6-MWD of more than 42 m is considered a clinically significant improvement.3,4

The American Thoracic Society has published comprehensive guidelines for the 6-MWT, recommending the test be performed indoors, along a long (at least 30 m which is marked every 3 m), flat, straight corridor with a hard surface that is seldom traveled. The turnaround points should be marked with a cone, and some equipment is required, such as a countdown timer, a chair that can be easily moved along the walking course, and worksheets on a clipboard.5 One or two physiologists are usually required for monitoring and record-keeping, and given that patients must go to the hospital, the test can be considered expensive, and it cannot be repeated frequently.


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Addressing the Limitations of the 6-MWT

With the introduction of low-cost digital gadgets and smartphones, it is now feasible to use sensors such as accelerometers or the global positioning system (GPS) to determine distance traveled. Artificial intelligence, mobile health applications, and open-source or proprietary frameworks have become widely available, perhaps leading to more easily accessible alternatives to traditional walking tests with greater patient acceptability. Numerous studies have been conducted to evaluate the use of technological developments, such as the use of GPS to measure distance walked in the 6-MWT with minimal margins of error,6 gait analysis with accelerometers in indoor settings,7-10 and using a mobile phone app to count steps and identify when the user turns while walking back and forth along a corridor.11,12

Salvi et al created a system called SMWT, which was codesigned by engineers, cardiologists, physiologists, and patients in a set of discussion groups. It comprises a mobile phone app for patients, a tablet app used by physiologists, and a server.13 The patient app can work in 2 modalities: indoor, where inertial sensors are used to measure the number of U-turns the patient performs while walking on a straight walkway, and outdoor, where the positioning system, like a GPS, is used to track the user and compute the walked distance. The app also allows users to connect to a Bluetooth pulse oximeter. The server collects the 6-MWT data from the app and provides a web interface to review the data. Physiologists can also add conventional 6-MWT information like total walked distance and symptoms on the patient’s web page.14

In a recent study. Salvi et al assessed the accuracy of the indoor 6-MWTs in clinical settings, validity and test-retest reliability of outdoor 6-MWTs in the community, compliance, usability, and acceptance of the app, and feasibility of pulse oximetry during 6-MWTs by defining a study protocol with the aim of demonstrating that patients are able and willing to use the app for the 6-MWT.14

Thirty patients were eligible and consented to participate in the study. They were actively enrolled in the study for a mean of 244.23 (SD 96.16; range 147-590) days. Each patient performed 3 6-MWTs in the clinic, 3 months apart from each other, using the app in indoor mode while the physiologist reported their observations (distance, oxygen saturation, Borg scale, symptoms) using the dedicated website or app. Between clinic visits, patients were asked to perform tests in the community using the app in outdoor mode.14

The Bland-Altman analysis showed that the limits of agreement (–133.35 and 162.55 m) were above the clinically significant threshold of 42 m in an indoor mode based on the differences between a 6-MWD measured by physiologist and simultaneous 6-MWD measured by the app. Differences between 69 pairs containing a 6-MWD estimated by a physiologist during a clinic test and a 6-MWD measured by the app in outdoor mode within 7 days of the clinic test showed a 0.89 correlation coefficient with 47 m of the standard deviation of the difference. Consecutive tests performed within 7 days showed a high correlation (0.93) with a 12.45% coefficient of variation, 0.91 intraclass correlation, and 36.97 m of the standard error of measurement.

As a result, Salvi et al concluded that while the app-based indoor 6-MWTs cannot be considered equivalent to the conventional ones, the outdoor, community-based 6-MWT measurements are strongly correlated with those performed in the clinic and are repeatable.14

Technological Advancement vs the Human Component

There are limitations of the study, as mentioned by the authors; it was not aimed at obtaining statistical significance, data from outdoor tests that were performed in the wrong conditions were included in the statistics, and the strategy for manual data quality assurance was not always consistent.14 However, despite the insufficient precision demonstrated in the indoor setting and the limits of the study, I anticipate that the fast pace of technological progress will aid in overcoming the problems that have been faced thus far. And when that happens, there’s little question that a technology that provides highly accurate results while being simple for patients to use would have far-reaching implications for both patients and physicians. The study’s findings indicate that the usage of technology has the potential to address unmet needs in this area. It is reasonable to expect that the results obtained by such a technology solution will better represent real-world data, especially considering that 6-MWT may be done at a place preferred by patients.

Patient education, patient compliance, and motivational factors from a patient perspective can be very important in influencing the success of assessment methods such as 6-MWT. The authors address this issue, stating that even if the app has shown to be usable and well-accepted, its use should be stated clearly to patients in order to increase their engagement. Moreover, when we look at the compliance, usability, and acceptance results of the study we see that many patients were happy with the app because it was easy to use (9/18), allowed patients to perform the test within their community instead of having to go to the hospital (4/18), indicated the fitness level and oxygen saturation (4/18), and invited patients to walk regularly (1/18). Some patients perceived the app as useful (7/12), easy to use (3/12), and thought it allowed a better 6-MWT than in the clinic due to not having to turn around (1/12).14

These insights coming from patients with a rare, progressive disorder are particularly important in terms of reflecting how innovative approaches are perceived by patients. While demonstrating the need for patient compliance yet again, it also reminds us that the human component is still essential regardless of how much technology advances. 

References

1.    Galiè N, Brundage BH, Ghofrani HA, et al. Tadalafil therapy for pulmonary arterial hypertension. Circulation. 2009;119(22):2894-2903. doi:10.1161/CIRCULATIONAHA.108.839274

2.    Solway S, Brooks D, Lacasse Y, Thomas S. A qualitative systematic overview of the measurement properties of functional walk tests used in the cardiorespiratory domain. Chest. 2001;119(1):256-270. doi:10.1378/chest.119.1.256

3.    Benza RL, Miller DP, Gomberg-Maitland M, et al. Predicting survival in pulmonary arterial hypertension: insights from the Registry to Evaluate Early and Long-Term Pulmonary Arterial Hypertension Disease Management (REVEAL). Circulation. 2010;122(2):164-172. doi:10.1161/CIRCULATIONAHA.109.898122

4.    Macchia A, Marchioli R, Marfisi R, et al. A meta-analysis of trials of pulmonary hypertension: a clinical condition looking for drugs and research methodology. Am Heart J. 2007;153(6):1037-1047. doi:10.1016/j.ahj.2007.02.037

5.    ATS Committee on Proficiency Standards for Clinical Pulmonary Function Laboratories. ATS statement: guidelines for the six-minute walk test. Am J Respir Crit Care Med. 2002;166(1):111-117. doi:10.1164/ajrccm.166.1.at1102

6.    Wevers LE, Kwakkel G, van de Port IG. Is outdoor use of the six-minute walk test with a global positioning system in stroke patients’ own neighbourhoods reproducible and valid?. J Rehabil Med. 2011;43(11):1027-1031. doi:10.2340/16501977-0881

7.    Tao W, Liu T, Zheng R, Feng H. Gait analysis using wearable sensors. Sensors (Basel). 2012;12(2):2255-2283. doi:10.3390/s120202255

8.    Nishiguchi S, Yamada M, Nagai K, et al. Reliability and validity of gait analysis by Android-based smartphone. Telemed J E Health. 2012;18(4):292-296. doi:10.1089/tmj.2011.0132

9.    Tong K, Granat MH. A practical gait analysis system using gyroscopes. Med Eng Phys. 1999;21(2):87-94. doi:10.1016/s1350-4533(99)00030-2

10.  Shull PB, Jirattigalachote W, Hunt MA, Cutkosky MR, Delp SL. Quantified self and human movement: a review on the clinical impact of wearable sensing and feedback for gait analysis and intervention. Gait Posture. 2014;40(1):11-19. doi:10.1016/j.gaitpost.2014.03.189

11.  Capela NA, Lemaire ED, Baddour NC. A smartphone approach for the 2 and 6-minute walk test. Annu Int Conf IEEE Eng Med Biol Soc. 2014;2014:958-961. doi:10.1109/EMBC.2014.6943751

12.  Capela NA, Lemaire ED, Baddour N. Novel algorithm for a smartphone-based 6-minute walk test application: algorithm, application development, and evaluation. J Neuroeng Rehabil. 2015;12:19. doi:10.1186/s12984-015-0013-9

13.  Salvi D, Poffley E, Orchard E, Tarassenko L. The mobile-based 6-minute walk test: usability study and algorithm development and validation. JMIR Mhealth Uhealth. 2020;8(1):e13756. doi:10.2196/1375614. 

14. Salvi D, Poffley E, Tarassenko L, Orchard E. App-based versus standard six-minute walk test in pulmonary hypertension: mixed methods study. JMIR Mhealth Uhealth. 2021;9(6):e22748. doi:10.2196/22748