The physiological causes of exercise limitation can be broadly classified into the following categories: i) cardiovascular limitation (incapacity of the heart and the circulation to deliver an adequate blood flow), ii) ventilatory limitation (decreased capacity of the diaphragm, respiratory muscles, thoracic cage and airway to function as a mechanical pump for air), ii) pulmonary gas exchange limitation (inefficient oxygen and carbon monoxide transport between lung alveoli and the pulmonary circulation), and iv) peripheral limitation (abnormal oxidative metabolism in skeletal muscles).
In clinical practice, we often encounter patients who complain of dyspnoea, but either the aetiology of the dyspnoea is obscure, or the severity of dyspnoea is disproportionate to the clinical assessment, lung function test and pulmonary or cardiac imaging findings. or patients have multiple co-morbidities that could have contributed to their symptom.
The cardiopulmonary exercise test (CPET) stresses a patient with maximal exercise so that functional inadequacies in the physiological system that are not detectable at rest can be uncovered. A standard CPET simultaneously measures: the work exerted by the patient against an incremental load, gas exchange at the mouth level (oxygen consumption (V̇O₂), minute ventilation (V̇E), and carbon dioxide production (V̇CO₂)), pulse oximetry, electrocardiogram, blood pressure, as well as the patient's perceived severity of breathlessness and lower limb fatigue. The cause of exercise limitation can be determined by an integrative interpretation of these measurements. In particular, dynamic respiratory mechanics can be assessed with inspiratory capacity manoeuvres, complementing the measurement of breathing reserve to determine the presence of ventilatory limitation. Limitation due to abnormal pulmonary gas exchange can be confidently determined by arterial blood gas sampling during the CPET. Illustrative cases on the use of CPET in evaluation of dyspnoea will be discussed in the lecture.
CPET is an integrative physiological evaluation and it rarely pinpoints a specific clinical diagnosis. While there are published guidelines such as the 2003 American Thoracic Society/American College of Chest Physicians statement and the 2019 European Respiratory Society statement, there remains a large heterogeneity in the interpretative strategies of CPET results. Additionally, high-quality reference equations are lacking. It is important that clinicians are aware of these limitations of CPET.
