Cardiopulmonary Fitness

Excerpt

Cardiopulmonary exercise testing (CPET) is a safe, non-invasive assessment of cardiorespiratory function. It allows the determination of crucial prognostic variables and can distinguish pathophysiology not apparent at rest. It can discriminate cardiovascular, ventilatory, and musculoskeletal limitations during exercise by monitoring disturbances in key variable responses such as oxygen, carbon dioxide, minute ventilation, and heart rate. It offers additional interpretive power over conventional stress testing and thus can lead to improved clinical decision-making and risk stratification in patients with cardiometabolic and respiratory disease.

The ability to perform physical exercise is dependent on the cardiovascular system's ability to deliver oxygen to the tissues and eliminate the metabolic by-products produced. The transport of O2 and CO2 in and out of the human body is a sequential phenomenon, and three processes occur in the body that assists with the ability to supply O2 and remove CO2.

1. External Respiration: Lung ventilation and gas exchange facilitate the delivery of O2 into the blood and the elimination of metabolic by-products (CO2) produced by exercise

2. Circulation: Responsible for the transport of O2 and CO2

3. Internal Respiration: Mitochondria undergo oxidative phosphorylation in order to produce ATP, which in turn produces mechanical energy

Ventilation-Perfusion Matching: In healthy individuals, exercise causes blood flow to the apex of the lung to increase due to increased recruitment of previously unused capillaries (increase in tidal volume) along with decreased pulmonary vascular resistance. This results in increased flow to the pulmonary circuit during exercise with relatively small increases in pulmonary arterial pressure. The increase in tidal volume is seen early in exercise and is a contributing factor to increased minute ventilation. The ideal Ventilation (V) - Perfusion (Q) ratio to sustain exercise is approximately 1.0. Progressive exercise will ultimately end in a V/Q mismatch because the cardiac output cannot keep up with the demand of the skeletal muscles. Certain disease states initiate the V/Q mismatch early, contributing to the exercise intolerance commonly observed. In patients with poor cardiac function, perfusion to the lungs becomes reduced, leading to a high V/Q ratio. Conversely, in patients with respiratory disease, ventilation is impaired, causing a low V/Q ratio.