[Modifications of respiratory function during pregnancy]

During pregnancy, changes in the thoracic configuration subsequent to progressive increase in the abdominal volume have a moderate effect on respiratory function. The cephalic displacement of the diaphragm reduces the expiratory reserve volume and the residual volume. The main consequence is an increase in the closing volume of the airways due to the fact that the lower residual functional capacity occurs during the tidal volume. The increase in the closing volume decreases the ventilation of the adjacent alveolar areas which could explain an increased heterogenicity of the ventilation-perfusion ratios. Increased tidal volume is almost always observed during pregnancy and is the cause of hyperventilation and a major increase in alveolar ventilation which can reach 70% of the post partum value. This hyperventilation is probably due to increased chemosensitivity of the CO2 respiratory centers induced by the hyperprogesteronism observed during pregnancy.