Dyspnea occurs in such a wide variety of disease states it seems unlikely that a single mechanism can account for the production of the symptom. However, under most (but not all) circumstances in which dyspnea occurs, one or both of the following can be demonstrated: excessive ventilation and increased work of breathing. In 1941, Cournand and Richards1 introduced the term "breathing reserve." This was defined as the difference between the maximum breathing capacity and the ventilation at any instant. They demonstrated in patients with pulmonary insufficiency that dyspnea was not likely to occur until the breathing reserve was reduced to from 60% to 70% of the maximum breathing capacity.
Increased work of breathing has been demonstrated in many diseases commonly associated with dyspnea. These include obstructive emphysema, diffuse pulmonary fibrosis, and pulmonary congestion as seen in mitral stenosis and left ventricular failure. In these examples, however, a decrease in
DYSPNEA OF PREGNANCY. JAMA. 1962;182(11):1114. doi:10.1001/jama.1962.03050500046011
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