The two most common forms of exercise testing used to evaluate pulmonary disorders are the
- 6-minute walk test
- Cardiopulmonary exercise testing
Six-minute walk test
This simple test measures the maximal distance that patients can walk at their own pace in 6 minutes. The test assesses global functional capacity but does not provide specific information on the individual systems involved in exercise capacity (ie, cardiac, pulmonary, hematologic, musculoskeletal). Neither does it assess patient effort. This test is used for preoperative and postoperative evaluation of patients undergoing lung transplantation and lung volume reduction surgery, to monitor response to therapeutic interventions and pulmonary rehabilitation, and to predict mortality and morbidity in patients with cardiac and pulmonary vascular disorders.
Cardiopulmonary exercise testing (CPET)
This computerized test provides a breath-by-breath analysis of respiratory gas exchange and cardiac function at rest and during a period of exercise, the intensity of which is increased incrementally until symptoms limit testing. Information on airflow, oxygen consumption, carbon dioxide production, and heart rate are collected and used for computation of other variables. Arterial blood gases may also be sampled. Exercise is done on a treadmill or on a bicycle ergometer; the ergometer may be preferable because work rate can be directly measured and the test is affected less by obesity.
CPET primarily determines whether patients have normal or reduced maximal exercise capacity (VO2max) and, if so, suggests probable causes. CPET is used to define which organ systems contribute to a patient’s symptoms of exertional dyspnea and exercise intolerance and to what extent. The test is also more sensitive for detecting early or subclinical disease than are less comprehensive tests that are done at rest. Examples of applications include
- Assessment of exercise capacity for disability evaluation
- Preoperative assessment
- Determination of whether dyspnea symptoms result from cardiac or pulmonary problems in patients who have disorders of both organ systems
- Selection of candidates for heart transplantation
- Assessment of prognosis in selected disorders (eg, heart disease, pulmonary vascular disorders, cystic fibrosis)
CPET can also help gauge responses to therapeutic interventions and guide prescription of exercise in rehabilitation programs. In following the response to therapy or disease progression, a steady-state CPET involving at least 6 minutes of constant work at 50 to 70% of the maximal work rate achieved during a maximal CPET may be more useful than an incremental, maximal CPET. Repeated evaluation at this work rate over time provides comparable data and is sensitive to improvement or decline in cardiopulmonary function.
Several variables are assessed during CPET, and no single one is diagnostic of a cause for exercise limitation. Instead, an integrative approach using clinical data, trends during exercise, and recognition of underlying patterns of physiologic responses is used.