Exercise physiology

Physical exercise requires the interaction of physiological control mechanisms to enable the cardiovascular and pulmonary systems (cardiopulmonary system) to couple their behaviors to support their common function. This common function is that of meeting the increased respiratory demands (oxygen [O₂] uptake and carbon dioxide [CO₂] production) of the (contracting muscle) cells. The response towards physical stress (e.g., physical exercise, surgical stress response) is, for example, a direct increase in both heart rate and minute ventilation. During exercise, the cardiopulmonary system is stressed in order to meet the increased need for O₂ by the exercising muscles and the removal of the metabolically produced CO₂.

The clinical exercise physiology uses cardiopulmonary exercise testing to study the response (tolerance) of the cardiopulmonary system towards an increasing load (metabolic stress) using respiratory gas analysis in a clinical setting (e.g., in (chronically) diseased patients, preoperatively). The gold standard to assess the fitness of the cardiopulmonary system is the evaluation of the aerobic exercise capacity by measuring the maximal O₂ uptake (VO_2max or VO_2peak). It is evident that a reduction in VO_2max or VO_2peak can be caused by any disease process affecting skeletal muscle function or the organ systems needed to transport O₂ and CO₂ between the air and the muscle cell. Cardiopulmonary exercise testing does not only determine a person’s VO_2max or VO_2peak, but uses other measures as well in order to define the pathophysiology of exercise limitation.

A cardiopulmonary exercise test is usually performed on a cycle ergometer or treadmill, stressing large muscle groups by an increasing load. Via maximal cardiopulmonary exercise testing it can be evaluated whether a person has a reduced exercise tolerance and, if so, whether this is due to an abnormal cardiovascular, ventilatory, or metabolic response during exercise. Symptoms that stop people from performing a maximal cardiopulmonary exercise test are fatigue, dyspnea, and pain. The importance of cardiopulmonary exercise testing is progressively more accepted in clinical practice. The results can be used at all stages of clinical assessment, for example during diagnosis, characterization of disease severity, prognosis, and response to treatment (e.g. medication, training program). Clinical exercise physiologists work closely with physical therapists and physicians.

Exercise tests in which respiratory gas analysis is not performed cannot realistically evaluate the ability of the cardiopulmonary system to support cellular respiration. For example: an exercise test that restricts its measurements to the ECG can only support a diagnosis of myocardial ischemia. However, a patient may have a pulmonary problem as well. Cardiopulmonary exercise testing can be used to determine which of these effects is responsible for the patient’s symptoms in order to be able to select the most appropriate therapeutic intervention(s).