This corresponding decline in arterial PO 2 can contribute to increased respiratory drive. In a patient already at or near their breathing reserve, this results in severe shortness of breath. Exercise Testing Clinically, the principles of exercise physiology can be applied to diagnosis of patients with respiratory or cardiac problems. Specifically, it is possible to measure various exercise parameters in order to understand why a given patient's exercise tolerance is limited. In principle, both direct and indirect calorimetry can be used for exercise testing but for practical reasons, clinical exercise testing is done exclusively with indirect calorimetry.
Respiratory disease affects exercise performance in a complex fashion.
Many respiratory diseases alter both gas exchange efficiency and respiratory mechanics. The changes in gas exchange efficiency indirectly lead to increased ventilation, thus increasing the demands on the system. The mechanical problems lead to increased workloads for the respiratory muscles, which in turn may not function optimally. Thus, ventilatory capacity is limited or reduced just at the time when an increase in ventilatory function is needed to meet metabolic demands. The net effect is a reduction in exercise tolerance and limitation of exercise by the respiratory system rather than the cardiovascular system.