To assess factors responsible for the dyspnea of pregnancy, serial treadmill exercise tests were performed in 14 normal women during the course of pregnancy. The method allowed measurement of ventilation, oxygen consumption, and alveolar gas partial pressures at minute intervals during exercise. The stress of the test was such that a normal subject should develop no dyspnea. Five of the subjects experienced dyspnea during the test at some time during their pregnancy; in two dyspnea was severe. Dyspnea was most likely to occur when the alveolar carbon dioxide tension was minimum, but did not correlate with the absolute level of ventilation or with the relationship of ventilation to the maximum ventilatory capacity. Subjects with high alveolar carbon dioxide tensions during exercise when not pregnant seemed most prone to develop dyspnea during pregnancy.
Gilbert R, Epifano L, Auchincloss JH. Dyspnea of Pregnancy: A Syndrome of Altered Respiratory Control. JAMA. 1962;182(11):1073–1077. doi:10.1001/jama.1962.03050500005002
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