[Pregnancy and the respiratory function].

The respiratory function of pregnant women is changed for more than one reason. There is a mechanical effect due to the increase in the uterine volume and the elevation of the diaphragm. However, there are only modest functional consequences, because the pulmonary volumes are only little changed, with the exception of a reduction in the functional residual capacity. Bronchial permeability is unaltered due to the balancing of constrictors (mechanical, hypocapnia) and dilators (hormonal influences). As regards pulmonary haemodynamics, the hypovolaemia of the pregnant woman has no repercussions on pulmonary vascular pressure. Gas exchange and alveolar-capillary diffusion are normal or even improved as a consequence of chronic hyperventilation. The latter is the most important functional change, sometimes expressed as a sensation of dyspnoea. The origin of this hyperventilation relates to the diminution of the threshold of sensibility in the respiratory centres to CO2 due to the effect of raised progesterone levels.