Shortness of breath during exercise (exertional dyspnea) produces a sensation of not being able to "get enough air" and a feeling of being "out of breath." A number of factors can cause exertional dyspnea, but they are usually related to insufficient tissue oxygenation by the blood. This usually results from impaired oxygen exchange by the lungs and can be caused by pulmonary edema or by insufficient blood being pumped by the heart to the lungs and peripheral organs during exertion (i.e., reduced tissue perfusion). Inadequate oxygenation of the blood and diminished organ perfusion causes changes in blood gases - arterial PO2 and pH decrease, and PCO2 increases. When this occurs, a chemoreceptor reflex is triggered, which stimulates respiration. Therefore, exertional dyspnea is accompanied by increased rate and depth of respiration as the body attempts to normalize blood gases. This respiratory stimulation adds to the sensation of not getting enough air.
A very common cause of exertional dyspnea is heart failure, which results in both impaired lung and systemic organ perfusion because of reduced cardiac output. Heart failure is often accompanied by elevations in pulmonary capillary pressure, which leads to pulmonary edema and impaired gas exchange in the lungs.