Study objective: To examine if the perception of dyspnea during normal pregnancy may be related to an inappropriate ventilatory response to the increased metabolic rate, due to a higher chemosensitivity.
Patients and interventions: At weeks 12, 24, and 36 of gestation and 4 months after delivery, 11 healthy pregnant women with dyspnea and 12 asymptomatic pregnant women were studied. Progesterone plasma levels, lung volumes, diffusion capacity, maximal respiratory pressures, rest oxygen uptake, breathing pattern, and mouth occlusion pressure (P0.1) were measured. Progressive isocapnic hypoxic stimulation and progressive hyperoxic hypercapnic stimulation were performed.
Results: Oxygen ventilation equivalent during pregnancy was significantly higher for the dyspneic group than for nondyspneic pregnant women. Dyspneic patients exhibited greater minute ventilation, tidal volume, and P0.1 than the nondyspneic group. The mean values of ventilatory and P0.1 slopes to hypoxia and CO2 during pregnancy were significantly greater in the patients with dyspnea than in asymptomatic subjects. These changes were not due to differences in progesterone plasma levels. A significant relation among the Borg score, inspiratory drive, and chemosensitivity was found.
Conclusions: In some pregnant women, a higher sensitivity to CO2 and hypoxia may induce excessive ventilation to metabolic demand, which would contribute to dyspnea.