Intrathoracic pressures in fetal sheep.

Intrathoracic pressures were investigated in the sheep fetus from 122 to 145 days of gestation. Pressures during fetal apnoea were determined by means of open-ended catheters in the trachea, intrapleural space and amniotic cavity. Measurements made with open water manometers were confirmed by differential water manometry as well as by pressure transducers. During apnoea, the pressure in the trachea exceeded that in the intrapleural space by 2.1-2.6 torr. The pressure was attributable mainly to a positive intratracheal pressure of 1.8-2.0 torr and to a lesser extent to a negative intrapleural pressure of 0.2-0.7 torr. Electronic measurements of intrapleural pressure showed that each breath was accompanied by change in pressure from +10 torr relative to atmospheric pressure to a negative pressure of 2-20 torr. Although tubocurarine hydrochloride (0.8 mg), injected as a bolus into the fetal jugular vein, completely abolished both phasic pressure changes and diaphragmatic electromyographic activity for periods of up to 90 min, curarization did not diminish the positive tracheal pressure. It is postulated that the positive pressure is generated by the continuous production of lung fluid and maintained by a resistance to outflow.