ROLE OF FETAL BREATHING MOVEMENTS IN CONTROL OF FETAL LUNG DISTENSION

Our aim was to determine the role of fetal breathing movements (FBM) i n the maintenance of fetal lung liquid volume. Experiments were perfor med in 14 chronically catheterized fetal sheep. FBM were selectively a bolished for 48 h by the infusion of tetrodotoxin (TTX) onto the phren ic nerves of five fetuses. Lung liquid volumes and secretion rates wer e measured before each treatment, 46-48 h after the start of the TTX i nfusion, and 22-24 h after the end of the infusion. Blockade of the ph renic nerves reduced fetal lung liquid volumes from 27.6 +/- 1.9 to 21 .8 +/- 2.6 ml/kg and increased lung liquid secretion rates from 3.8 +/ - 0.6 to 6.2 +/- 1.1 ml.h(-1).kg(-1). Control experiments confirmed th e lack of effect of TTX infused intravenously and saline infused intra pleurally on changes in fetal lung liquid volume and secretion rate. T o measure the static relaxation volume of the fetal lung, in six fetus es we combined skeletal muscle paralysis with bypass of the upper airw ay for 48 h. This reduced fetal lung liquid volume from 39.1 +/- 3.1 t o 23.0 +/- 2.5 ml/kg and increased lung liquid secretion rates from 4. 1 +/- 0.7 to 5.8 +/- 0.9 ml.h(-1) kg(-1). This experiment demonstrates that the fetal lung is normally maintained at a level of expansion th at is much greater than its static relaxation volume. We conclude that the volume of luminal liquid in the fetal lungs is dependent on the d iaphragmatic contractions associated with FBM. Their effect is to resi st the elastic recoil of the fetal lungs, thereby reducing the loss of liquid from the lungs via the trachea.