Practical stress tests in pneumology

Exercise testing is of great importance in pulmonology: it provides information on physical performance and clues to the reasons for a performance deficit. Which method is used depends on the patient’s condition.

Exercise testing in pulmonology is of particular importance because it not only provides information about a patient’s physical performance, but also provides clues to the cause of a performance deficit when performance is reduced. Spiroergometry has been established as the “Mercedes of exercise testing” for many decades. However, apart from the technical requirements, the effort in terms of personnel and time is not insignificant. Therefore, this examination is usually reserved for specialized centers and special issues. There are, however, quite practicable and nevertheless meaningful stress tests, which can also be carried out in the general practitioner’s or specialist’s practice without extensive equipment.

For routine testing, exercise tests such as the six-minute walk test (6-MWT) or the sit-to-stand test appear to be easier to perform than spiroergometry. For example, the calculation of the deviation from the target walking distance in the 6-MWT provides an indication of the extent of the existing performance deficit. Also, in terms of training control, performing a 6-MWT can be used to calculate the required treadmill speed for gait training. Walking tests are generally, taking into account the absolute and relative contraindications or discontinuation criteria, safe and easy to perform examinations that have minimal equipment, personnel, and time requirements.

The following is an overview of the most important studies in terms of clinical relevance and practicability. For more in-depth knowledge, it is recommended to study the current recommendations of the German Society of Pneumology [4]. In fact, the 6-MWT, the Timed-up-and-Go Test (TUG), and the Sit-to-Stand Test (STST) are most commonly used in practice because these tests are very well validated and standardized. At the same time, they are also easy to perform in a family practice, especially the TUG and STST in this case. Nevertheless, the other tests should also be mentioned, since they are used as endpoints especially in scientific questions.

6-minute walk test (6-MWT)

The walking test, first described in the 1960s and developed over the years into the 6-minute walking test, is a simple and safe test that measures the maximum walking distance a patient can walk in 6 minutes [1]. This allows reproducible and valid assessment of physical performance. The 6-MWT reflects very well the activities of daily living. Walking distance has predictive value for hospitalization rates, morbidity, and mortality [2]. For a valid interpretation, the examination technique should be strictly standardized. Adherence to the European Respiratory Society (ERS) and American Thoracic Society (ATS) guidelines [3] will help in this regard. When deviating from these specifications, the results of the 6-MWT may vary significantly, making interpretation of the result difficult. Even the verbal communication content before and during the test can significantly affect the outcome [5].

On the day of the examination, the medication should be taken unchanged. It is recommended to avoid physical exertion for at least two hours before the examination. In the 6-MWT, the test person walks back and forth between two markers at least 30 meters apart on a flat surface without obstacles after a standardized briefing. The patient himself determines the walking speed. If necessary, breaks are allowed, but the test time is not interrupted. Assistive devices such as rollator, oxygen, etc. should be documented. The examiner can accompany the subject at a distance of about 1 meters, but without interfering with him. By means of a mobile pulse oximeter, _O2 saturation and heart rate can thus be continuously monitored. In addition to a stopwatch, the test equipment also includes a protocol (with clipboard if necessary) and the Borg scale.

After completion of the test, the lowest oxygen saturation value and heart rate are documented and subjective dyspnea is queried using the Borg scale. In case of premature termination by the patient or by the examiner, the distance already walked, the time and the reason for termination are documented. The termination criteria for the investigator are shown in Overview 1 .

For the interpretation of the test result, the comparison with target values has proven to be useful. There are several formulas for calculating the set point. In our clinic, we use the set points according to Troosters et al. [7], which are shown in Table 1 for men and women. Roughly speaking, the age-appropriate norm is about 700 meters for men and about 650 meters for women.

Depending on the disease pattern, thresholds for increased mortality risk are apparent. In COPD, this threshold is 317 meters, in pulmonary fibrosis 254 meters, and in pulmonary arterial hypertension 337 meters [6,8]. It should be mentioned that there is a learning effect when performing the walking test or better results can be achieved when repeating the test on the same day. Therefore, it is recommended to perform two 6-MWTs with a minimum interval of 30 minutes for accurate progression assessment. The better value then counts. To assess the severity of limitation due to a disease and to evaluate a subject’s prognosis, usually only a 6-MWT is performed [3]. A MID (minimally important difference) is considered to be a change in walking distance of at least 30 meters (95% confidence interval 25-33 meters) [9]. For example, a decrease in walking distance of more than 30 meters in serial performance of a 6-MWT over one year in a subject with COPD is associated with a significantly increased risk of mortality in the following year [8].

If the guidelines are followed, only a very low rate of undesirable side effects must be expected with the 6-MWT. Therefore, the 6-MWT is considered a suitable and safe test for the critically ill and elderly [6]. The result of the 6-MWT can also be used for training control e.g. in the context of pneumological rehabilitation. Here, the walking distance in meters in the 6-MWT is multiplied by 0.008. The value gives the walking speed in kilometers per hour (e.g. for setting the treadmill speed in gait training).

Incremental Shuttle Walk Test (ISWT)

First described in 1992, the ISWT was developed to prescribe walking speed for gait tests. Standardization of the ISWT can improve the reproducibility of the test result, as the test format has similarities to controlled exercise testing (such as ergometry) with incremental increases in load [9]. The test is performed on a distance of 10 meters (this corresponds to a shuttle) bounded by two pylons [11]. The walking speed is determined by a preset acoustic signal. The subject must walk around the pylons to time the beeps or should be at the turning point at each beep. The speed set by the beep increases every minute from an initial 1.8 km/h to a maximum of 8.5 km/h. The patient may only walk and not run during this process. DeTer ISWT ends if, among other things, the patient fails to reach the turnaround point to the beep twice in a row, or terminates due to dyspnea or peripheral fatigue. The test arrangement is shown in Figure 1 . Also, the test can be terminated by the investigator at any time according to the termination criteria. The maximum test duration is 20 minutes. At the end of the test, walking distance, heart rate, oxygen saturation value, and subjective dyspnea are documented using the Borg scale [12]. A significant learning effect is also observed with the ISWT, which is why two tests are also recommended here for accurate progress assessment. The MID is reported to be a walking distance of 47.5 meters [9].

Endurance Shuttle Walk Test (ESWT)

The ESWT is performed on the same test track as the ISWT. In ESWT, the walking speed is not continuously increased, but remains constant. After a short slow warm-up period of 90 seconds, an acoustic signal is given, after which a walking speed of 70% to 85% of the maximum value of the ISWT (depending on the protocol) is constantly maintained [4]. There is no formal time limit on this test, but it is usually completed after 20 minutes for practical reasons. The test ends, among other things, if the subject fails to reach the turnpoint to the beep twice in succession, terminates due to dyspnea or peripheral fatigue, or the test is terminated by the examiner according to termination criteria. At the end of testing, time, heart rate, oxygen saturation value, and subjective dyspnea are documented using the Borg scale and (depending on the protocol) walking distance [6]. Time as the most relevant value is documented in seconds. In contrast to the ISWT and 6-MWT, no significant learning effect is observed, so one measurement is sufficient. The MID is usually seen as 180 seconds or 85 meters [14].

Sit-to-stand test (STST)

First described in 1985, the test was initially designed to measure leg strength using a functional everyday movement in the sense of standing up and sitting down. Over time, different variants evolved to answer different questions such as fall risk and balance ability [16]. The ability to stand up and sit down is a fundamental aspect of mobility [20]. The movement sequence is demanding from a neuromuscular point of view and is determined, among other things, by muscle strength and postural stability [19]. Two variants of the STST have become established, the 5-repeat STST and the 1-minute STST [17]. Both correlate significantly with the 6-MWT. The 5-repetition STST focuses primarily on strength ability and coordination, while the 1-minute STST tests more strength endurance and overall physical performance [21]. The test is performed with a chair without armrests. The subject should perform sitting down and standing up as correctly and completely as possible, without using the arms to help. For this, the hands should remain clasped in front of the chest. When standing, the knees and hips must be fully extended, and when sitting, there must be clear contact with the chair. The test starts and ends in a sitting position. The patient stands up and sits down again without delay. This procedure should be repeated as often as possible within one minute during the 1-minute STST at a rate determined by the patient. Rest breaks are allowed without stopping the time. The number of complete repetitions is documented. The 5-repeat STST requires the patient to get up from the chair and sit down again as quickly as possible 5 times in a row [16]. The time required is documented in seconds. The tests are not subject to any relevant learning effect. As MID, one assumes 1.7 seconds for the 5-repetition STST and 1-minute STST from an increase of three repetitions [25]. Fewer than 12 repetitions on the 1-minute STST shows a significantly increased two-year mortality risk [23].

Timed-Up-and-Go-Test (TUG)

The TUG test, developed in 1991 as a modified version of the Get-up-and-Go test, is also used to test subjects’ static and dynamic balance, lower extremity strength, mobility, and fall risk, among other things. In the test protocol usually used for this purpose, a chair with an armrest is used, and a distance marker is placed on the floor at a distance of 3 meters. The subject sits on the chair facing the 3-meter mark and arms on the armrests without further assistance from the examiner [28]. He may use his usual tool such as a stick. The subject must then stand up when prompted, walk or run with a normal safe gait to the distance marker at a distance of 3 meters, turn 180 degrees (turn around), return to the chair, and sit back down in the starting position. The test arrangement is shown in Figure 2. The time in seconds for the entire maneuver is documented. With a threshold of less than 10 seconds (in the literature, however, up to 33 seconds are also described), the subject is not restricted in his everyday mobility or there is no increased risk of falling [29].

Ergometry

Ergometry is a symptom-limited step test usually performed on a cycle ergometer. Among other things, this test can be used to determine individual physical performance in the context of performance diagnostics by means of a step or endurance test as well as in cross-sectional and longitudinal examinations. In competitive sports, sports medicine and occupational medicine, ergometry is used to determine the performance level of the person being examined. The results are used for further planning of the training or stress. In addition, ergometry helps in the detection and progression of cardiac and pulmonary diseases, as well as in risk and prognosis assessment. Rehabilitation physicians use ergometry to make targeted therapy recommendations and to check the effectiveness of the measures taken [4]. Depending on the subject, intention and type of ergometry (bicycle, treadmill, hand crank, etc.), different exercise protocols are used. For medical issues, stepwise stresses of 9-12 minutes duration are performed in combination with continuous ECG and blood pressure recording. In general, cycling ergometry is recommended to start with 25 or 50 watts and increase by 25 watts every 2 minutes. The maximum pulse rate to be achieved is 220 minus age (in years). Age-, sex-, and weight-related target powers are expressed in watts and relative in percent [4]. For training control, the rules of thumb given in overview 2 can be used.

Spiroergometry

The spiroergometric examination of test persons has on the one hand the aim to analyze the performance capacity and on the other hand to be able to estimate limitations and possible causes. For this purpose, comparisons of the measured or calculated results with the target values or limit values are required. In this way, it is also possible to analyze signs of exhaustion or still existing performance reserves. Spiroergometry provides important information in the differential diagnosis of dyspnea and can thus provide information on the genesis of dyspnea [4]. At the same time, however, it is also by far the most complex and expensive examination, which is why its use should be thoroughly considered. Overview 3 shows the indications that usually warrant the performance of spiroergometry.

Before spiroergometry is performed, a number of considerations must first be made, in addition to the indication, to ensure that the examination provides the desired information content. Contraindications to spiroergometry are similar to any other exercise test. In addition to the type of load – usually treadmill or bicycle ergometer – the steepness of the ramp (load protocol) and the maximum capacity of the subject must be estimated. However, the calculated final loads tend to deviate from the actual measured values. The values shown in Table 2 can be used as a rough classification.

For the analysis of spiroergometry, the interpretation in the 9-field graph according to Wasserman has proven successful for many years [34]. The different channels can be assigned to the functions of ventilation [1,4,7], gas exchange [4,6,9] and cardio-circulation [2–5]. By using a blood gas analysis before, during and after the examination, further statements can be made, for example, about gas exchange and the ratio of ventilation to perfusion. The typical response patterns in spiroergometry allow conclusions to be drawn about the underlying disease of the subject, although diagnosis for a particular disease is not and cannot be the domain of spiroergometry [30]. However, as in heart failure, early changes can be detected [33]. The establishment of diagnostic algorhythms, which are often integrated into the computer programs, help the examiner to assign specific disease patterns.

Another application of spiroergometry is in sports medicine, where it also originated. In sports medicine, the focus is less on diseases and more on the analysis of endurance performance and energy metabolism. For endurance performance, the maximum oxygen uptake (_VO2max) is of particular interest [34]. _VO2max reflects the amount of oxygen metabolized. It is a product of all exchanging, transporting, and utilizing systems for oxygen and can be as high as 90 ml/min per kg body weight in highly trained endurance athletes. However, during spiroergometry, indirect calorimetry can also be used to calculate energy expenditure and substrate consumption. Using the respiratory quotient and _VO2, energy metabolism and the ratio of carbohydrate to fat metabolism can be estimated because of the relationship between metabolized substrate, _VO2, carbon dioxide output (_VCO2), and energy production. However, energy expenditure is also influenced by the nutrients currently available, which is why the diet should be standardized before exercise. On the day before, and even better on several days before, make sure to eat sufficient carbohydrate-rich food, especially on the evening before the examination. On the day before and on the day of the examination, it is also important to ensure adequate fluid intake. Nevertheless, the determination of fat and carbohydrate metabolism by indirect calorimetry is subject to some error and should only be used up to a maximum of 75% _VO2max.

Spiroergometry is a low-risk examination method to objectively and continuously record respiratory and cardiovascular parameters during physical exertion. It is the gold standard for assessing physical performance [4]. This performance test not only allows a detailed assessment of performance-limiting factors in healthy subjects, but also provides results that indicate limitation due to cardiac and/or pulmonary disease. It plays an important role in the clarification of unclear performance intolerance, dyspnea or exertion-induced bronchial asthma and helps in the assessment of work capacity and perioperative risk not only of pneumological patients [31].

Summary

Stress testing plays an important role in pulmonology. In addition to assessing the degree of limitation, the results allow conclusions to be drawn about the cause of the performance limitation on the one hand and also about the prognosis for different clinical pictures on the other. Furthermore, when repeated at time intervals, an assessment of the success of, for example, a training intervention or drug therapy can be made. The optimization of a training control requires performance tests, which are on the one hand precise, but on the other hand also practicable. A patient’s caregiver or therapist must select the method from the multitude of possible examinations that is not only meaningful, but also time and cost effective.

Take-Home Messages

For routine performance testing in the office, the sit-to-stand test or the timed-up-and-go test are appropriate.
The 6-minute walk test (6-MWT) allows conclusions to be drawn about the severity of limitation and the risk of mortality for COPD patients, for example. The minimum important difference of the 6-MWT (30 meters) indicates significant changes in the course.
The results of the exercise tests, especially the 6-MWT, the Incremental Shuttle Walk Test, ergometry and spiroergometry can be used for training control.
Spiroergometry is the most complex but also the most informative exercise test, which is why it is usually reserved for special questions or specialized centers.

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