In pregnant woman, the respiratory system is influenced by the hormonal environment and an enlarged gravid uterus. After delivery of the fetus, and contraction of the uterus, changes in the respiratory system produced by the mechanical effect of the gravid uterus may decrease or be reversed. We designed this study to observe changes in the respiratory mechanics in pregnant woman under general anesthesia for cesarean section.

15 pregnant women at term, ASA 1 were studied. Induction was performed with thiopental/succinylcholine and maintained with O2, N2O and enflurane until delivery. Just before delivery, enflurane was discontinued. Vecuronium was given at 0.1 mg/kg iv 4 min after induction, and after delivery, 1 mg was given as needed. Inspired tidal volume, expired tidal volume, inspired minute ventilation, expired minute ventilation, airway peak pressure, plateau pressure, expired volume during the first 1 second/expired tidal volume (V1.0), dynamic compliance and end-tidal CO2 were continuously measured just after induction, at delivery and 5, 10, 15, 20, 25, 30, 40 and 50 min after delivery. Blood pressure, heart rate and oxygen saturation were also measured.

Compared to the value after induction, values after delivery showed no significant changes in inspired tidal volume or minute volume, but significantly increased in expired tidal volume and minute volume and dynamic compliance, and a significant decrease in airway peak pressure, plateau pressure, V1.0 and end-tidal CO2. Blood pressure and heart rate increased significantly after induction, compared to before induction. Oxygen saturation was maintained at 99-100%.

After delivery, the effects of the gravid uterus on the respiratory system rapidly disappeared, in particular changes of pressure in the airway preceded the respiratory volume changes.