**The Six-Minute Walk Test on the Treadmill**

Fryderyk Prochaczek1, Jacek S. Brandt1, Witold Żmuda2, Katarzyna R. Świda1, Zbigniew W. Szczurek1, Jerzy Gałecka1 and Agnieszka Winiarska2 *1Institute of Medical Technology and Equipment, Zabrze 2Clinical Cardiology, Tychy Poland* 

### **1. Introduction**

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Human physical efficiency is shaped by several factors amongst which the functional condition of the heart, lungs and arterial circulation in lower limbs are regarded to be of key importance. Several tests are applied to evaluate human physical efficiency, however, the six-minute walk test (6MWT) deserves special attention due to its usefulness and simplicity. The test builds on the patient's walking capacity, i.e. the natural physical activity. Hence, the performance of the test does not require any individual preparation. The only and at the same time simple evaluation factor in the test is the distance covered within 6 minutes on a flat area. During the six minutes, the patient adjusts the pace of the walk to his or her fatigue, dyspnoea or pain in the lower limbs. According to the recommendations formulated by the American Thoracic Association in 2002, the test should be performed in subjects with at least moderate heart failure, respiratory failure or arterial circulatory failure in lower limbs.

The six-minute walk test is used by physicians as it enables the impartial evaluation of physical efficiency and treatment effectiveness. It also plays a prognostic role in evaluating the life span of patients with heart failure. It is easy to perform the test in a hallway. However, it is difficult to compare the results of the tests in various medical centres because they are performed in hallways which differ in length. In order to overcome this limitation, ATS 2002 guidelines state that the standard length of the hallway should be 30 m and its standard width – 3 m. The hallways in some medical centres comply with the set standards for the 6-minute walk test. However, even in these centres the test cannot be performed because the hallway has been designed for internal circulation purposes. Since several centres do not have a hallway with the required length, the six-minute walk test is performed in halls of non-standard length. Hence, a situation occurs when the right ATS guidelines cannot be complied with for technical reasons.

The idea to circumvent the limitation resulting from various hallway lengths was brought up during a visit to a cardiac rehabilitation centre with very well equipped physical efficiency laboratories in which the patient performing the six-minute walk test disappeared behind the hallway corner changing the direction at a right angle. This rather bizarre situation gave rise to the idea of doing the six-minute walk test in optimal conditions, i.e. in a physical efficiency laboratory, on the treadmills used there. After all, it is enough to adapt the speed of the

The Six-Minute Walk Test on the Treadmill 219

the patient's changing walking speed. It is done quickly enough to keep the patient still on the treadmill. At the beginning, the preparation of such a treadmill seemed difficult for us and a different solution was chosen. We decided to combine the speed of the belt with the patient's position on the treadmill. When the patient is close to the front of the treadmill, the maximum speed of the belt is achieved, when he is close to the end of treadmill, the belt stops. While increasing the walking speed, the patient moves toward the front of the treadmill and the belt speed increases, when he slows down, the belt moves him backwards and the belt speed adjusts again. In order to realize such an algorithm, precise measurements of the patient's position on the treadmill are necessary. Already at the beginning, a decision was made to measure the position wirelessly because all the other

First of all, the use of the ultrasound wave, reflected by the patient on the treadmill, was chosen to measure the distance from the patient to the front of the treadmill. Due to parasitical echoes from the other objects around, measurements were uncertain and the method turned out to be inconvenient. Because transmitting and receiving ultrasonic waves is simple and cheap in practice, we decided to continue using ultrasound after some modification. In the new method, the patient was carrying a transmitter which produced simultaneously a short impulse of ultrasound wave and infrared beam (about 100 milliseconds long). Both signals were received by the receiver at the front of the treadmill and the distance between the transmitter and receiver was calculated from the time delay between the received signals. Distance measurements turned out to be accurate (error less than 10 mm) and due to the shortest direct way of the ultrasound signal, parasitical echoes

Fig. 1 shows the idea of the patient's position measurement using a mixed ultrasound/ infrared method. Carrying a transmitter seemed slightly uncomfortable for the patient, but

methods were rather inconvenient.

did not interfere with the measurements.

Fig. 1. Six-minute walk test on the treadmill

treadmill belt to the pace of the patient's walk by means of the appropriate program and sensors. In this way, a situation equivalent to a hallway walk will be reconstructed where the patient slows down or even stops if tired. The team of constructors from ITAM (Institute of Medical Technology and Equipment) in Zabrze started research into this idea. As a result of their work, a treadmill was constructed which adapts to the walking capacity of a patient suffering from chronic obstructive pulmonary disease (COPD), heart failure (HF) or arterial circulation failure in lower limbs. The algorithm designed to control the speed of the treadmill belt is based on precise, wireless determination of the patient's position on the belt.

The second part of the chapter demonstrates the results of the engineers' work on the construction of the treadmill in order to perform the walk test safely. The third part of the chapter contains the evaluation of adjustment of the treadmill to the walking pace of healthy volunteers, as well as a comparison of the distance covered during the 6-minute walk on the treadmill and in the hallway.

The obtained results demonstrating the advantages of the treadmill, have encouraged us to perform a 6MWT for patients with heart failure in the II-III NYHA classes (unpublished trial). The treadmill test was tolerated equally well by the patients as the hallway test. The fact that a similar distance was covered in both tests demonstrates that the technological barrier preventing us from obtaining credible results of the six-minute walk test on the treadmill, has been overcome. Thus, the possibility of performing 6MWT on the treadmill for patients suffering from obstructive airways disease, heart failure, or intermittent claudication, has appeared.
