Clinical Exercise Testing (out of print)

Abnormalities in cardiopulmonary responses to incremental cycle ergometer exercise are often similar in obstructive, restrictive and pulmonary vascular disorders. All are characterised by an accelerated ventilatory response, primarily as a result of the effects of a high fixed physiological dead space, combined, in many cases, with attendant arterial oxygen desaturation. Skeletal muscle deconditioning, with earlier metabolic acidosis, is also a common feature of all chronic respiratory disorders and can further amplify ventilatory stimulation during exercise. The breathing pattern is more rapid and shallow in both obstructive and restrictive lung disorders compared to health, and reflects the dynamic mechanical constraints on tidal volume (VT) expansion. In general, respiratory frequency responses to exercise are most pronounced in patients with restrictive lung disease. In chronic obstructive pulmonary disease, a VT plateau with concomitant tachypnoea early in exercise at relatively low ventilation generally indicates the presence of significant resting and additional dynamic lung hyperinflation. Cardiac frequency responses to exercise are variably abnormal, but are broadly similar across these disease groups and, therefore, nonspecific. Resting pulmonary function test results, in association with quantitative exercise flow–volume loop analysis, can accurately delineate abnormalities in resting and dynamic ventilatory mechanics that characterise obstructive and restrictive lung disease and permit easy differentiation from primary pulmonary vascular disorders. Such noninvasive assessments of ventilatory mechanics during exercise, together with the quantification of exertional symptoms, permit comprehensive clinical characterisation of the individual presenting with reduced exercise capacity.

Cardiopulmonary exercise testing (CPET) allows a comprehensive assessment of the mechanisms of exercise intolerance using a test that can generally be performed within 20 min. However, the test relies on a number of devices and sensors, and assessment will only be valid if all function correctly. It is the user’s responsibility to understand the equipment and measurements involved in the testing modality. Performance of the test on a cycle ergometer or a treadmill is an important choice, as each has advantages and disadvantages.

A central focus of the test measurements is calculation of the time course of pulmonary gas exchange. Although breath-by-breath analysis of oxygen uptake and carbon dioxide output is generally performed, this is a complex measurement technique requiring precise knowledge of the oxygen and carbon dioxide sensors and of the flow or volume sensors being used. Advances in the ability of digital computer analysis has led to the successful use of sensors with a number of operating principles and the ability to interpret breathing patterns with less than ideal characteristics. Tests involving elevated inspired oxygen levels present special measurement challenges. Consideration must also be given to heart rate measurement, ECG, blood pressure and pulse oximetry, which are all integral parts of the testing paradigm. A major focus in CPET is quality control, which should include both regular analysis of the performance of individual sensors and the assessment of overall system performance.

This overall analysis may include expired gas collection, use of a metabolic simulator and use of a “standard” human subject. The accuracy of the diagnostic conclusions of CPET will only be assured if the user is confident in the accuracy of the measurements.

It is important to quantify an individual’s exercise capacity. Although laboratory-based tests (with the full complement of equipment) are acknowledged to be the gold standard, it is not always feasible or even desirable to complete such tests in an increasingly elderly and frail population. As a viable alternative, field tests have been developed.

Initially, this was a simple self-paced corridor-walking test. The later development of externally paced tests has imposed a defined protocol upon the participant. It is increasingly possible to collect complex physiological data beyond the laboratory, and studies have demonstrated that the externally paced tests, particularly the incremental shuttle walking test, best reflect the physiological response observed in comparable laboratory tests. Walking tests are simple to perform compared with laboratory-based tests, and require little expensive equipment in order to collect the most basic of data.

Field tests have been described in a number of patient populations with chronic cardiac and respiratory diseases. Studies have identified threshold values that are associated with successful clinical outcomes, or an increased mortality risk. Field test results are used as an outcome measure, particularly in rehabilitation studies, with the 6-min walking test and the incremental shuttle walking test being most frequently reported. Attempts have been made to describe what constitutes an important difference to the patient. In order to identify the maximal change achieved after an intervention, for example rehabilitation or pharmaceutical agents, a high-intensity endurance test yields the greatest improvements. In the field, this is provided by the endurance shuttle walking test.

If standardised, the field test yields information that can be usefully employed to evaluate the level of functional capacity and assess the outcome of therapies. For all patients, it is important that standardised procedures are followed in order to optimise their utility.

Paediatric exercise testing is becoming increasingly important for diagnosis in paediatric respiratory medicine. Testing for exercise-induced asthma (EIA) is still the most important and most frequently used exercise test for children. Standardisation of this test is very important, including standardisation of exercise load at a level of 90–95% of maximum exercise load and standardising environmental conditions, such as temperature, humidity and altitude. Treadmill running is most frequently used, but free range running and cycling are other options.

Exercise testing is important for the differential diagnosis of EIA. The most frequent differential diagnosis is exercise induced vocal cord dysfunction, which demonstrates inspiratory stridor at maximum performance during exercise testing.

Testing for physical fitness has become increasingly popular in paediatric respiratory medicine. Traditionally, a gradual stepwise protocol for exercise intensity has been used, but it has been shown that more short-term intensive protocols may give similar maximum oxygen uptakes and peak ventilation. The protocol may be varied according to the child’s condition. Cardiac illnesses usually require gradual stepwise protocols, whereas children with respiratory disorders manage a quicker protocol.

Of importance for respiratory diagnosis and assessment of possible physical training is measuring tidal breathing during exercise, which is obtainable through modern equipment.

Various whole-body exercise tests (cycle ergometry and treadmill walking) have been used for many years to determine: 1) the level of physical fitness; 2) which factors may limit exercise tolerance; and 3) the safety of exhaustive physical exercise in patients with moderate-to-severe chronic obstructive pulmonary disease (COPD).

Whole-body exercise testing can also be of additional value in designing an exercise-training programme, to prescribe oxygen therapy, estimate daily physical activity levels, prescribe walking aids, assess quadriceps muscle fatigue, and as a stress model to investigate abnormal control of muscle metabolism and oxidative stress induction in patients with moderate-to-severe COPD.

Training load at the start of an exercise-training programme and tailored training components can be partially derived from the results of a symptom-limited incremental cycling test and a 6-min walking distance (6MWD) test. Moreover, 6MWD tests are used to prescribe oxygen therapy in normoxaemic COPD patients who suffer from exercise-induced oxygen desaturation. A 6MWD test can also be used to estimate the level of daily physical activity and to prescribe a walking aid, such as a rollator, to COPD patients with a low unaided 6MWD. In contrast to incremental and endurance shuttle walking tests, a high-intensity symptom-limited constant work-rate cycling test appears to be useful to discriminate between quadriceps muscle fatigue and dyspnoea as the dominant factor limiting exercise tolerance.

In conclusion, whole-body exercise testing can be of additional value in patients with moderate-to-severe COPD in evaluating the responses to different physiological and pharmacological interventions and in targeting novel treatment strategies.