Characteristics of fetal lung fluid production

Abstract

The fetal lung secretes fluid into the tracheobronchial lumen, which maintains a positive pressure within the developing lung. Although an essential factor in the development of the fetal lung, little is known about the origin of the fluid or the physical factors controlling the amount of fluid secreted. A series of four experiments were carried out in fetal sheep between 85 and 115 days gestation. The first study measured fetal lung fluid production in six animals under normobaric conditions through a cannula inserted in the end of the transected cervical trachea with exteriorization out the side of the ewe. The second study introduced one-way flow valves of known opening pressure in the tracheal cannula. The third study added intrauterine ligation of either right or left bronchus transthoracically in six animals with cannulation of the transected cervical trachea to measure lung fluid production. The final series involved ligation of either right or left pulmonary artery transthoracically with cannulation of the transected cervical trachea and exteriorization to measure lung fluid production. The experiments reveal the following characteristics of fetal sheep lung fluid: (1) Fluid is produced at a rate of 2–3 cc/kg/hr between 85–115 days gestation. (2) Fluid production is unchanged with outflow resistances from 2–9 cm H 2 O. (3) Fluid production stops when resistance is 10 cm H 2 O or greater. (4) Ligation of one bronchus results in increased lung fluid production from the open side with the normal hourly output. (5) Ligation of one pulmonary artery decreases lung fluid production by half the normal hourly output.