Pulmonary pressure/volume relationships during the last phase of delivery and the first postnatal breaths in human subjects.

Intrathoracic pressure and volume changes during delivery and the first postpartum breaths were measured in 17 term infants. The following findings emerged: (1) Infants were often exposed to high pressures during delivery, when significant volumes of lung fluid were expressed. (2) Despite this, none of the infants' lungs reinflated by passive "elastic recoil." (3) The active inspiratory effort required for reinflation was usually small or moderate, and an "opening pressure" did not appear to be necessary for the initial rise in volume. It is believed that diaphragm and intercostal muscle tone falls or disappears during delivery and returns during the first breath.