Distensibility of the Fetal Lung with Fluid in Sheep

Abstract

Extract: Trachcal ligation was performed in 16 fetal lambs at 74–129 days of gestation (normal term, 138–157 days). Operated lambs were subsequently delivered spontaneously at term. Postmortem procedures were performed on the same day and revealed lungs enormously distended with fluid, weighing approximately 10 times more than those found in normal lambs, and displacing the diaphragm to the point where it was concave downward, bulging into the abdominal cavity. To measure the rate of lung fluid formation in fetal lambs, catheters were placed in the trachea of three fetuses borne by three ewes near term. Periodic aspiration of fluid following the operation revealed rates of lung fluid formation of 0.015, 0.030, and 0.055 ml/kg/min. The day after surgery pressure measurements of lung fluid made via the catheter in two of these animals revealed an initial pressure of 12 and 19 cm saline. This pressure rose only moderately after injecting 150 ml 0.85% NaCl in 20-ml increments into the fetal lung. The injected fluid was readily recoverable, indicating that it had been accommodated by expansion of the fetal lung and had not passed across lung-lining cells into the fetal circulation. Gestation times were within the normal range.Speculation: Fetal lung fluid contains surfactant, which has been biochemically characterized as a phospholipid and as a lecithin. Soya bean phospholipids containing a predominance of lecithins arc capable of sensitizing the rabbit uterus to oxytocin and precipitating premature delivery. The fetus is known to participate in the timing of its own birth, and we speculated whether lung phospholipids formed near term in the fetal lung could reacli the amniotic fluid and precipitate labor. Tracheal occlusion, however, did not alter gestation time in our animals. Other mechanisms of fetal participation in the precipitation of labor must be sought.