Premature changes in surface activity in lung fluid of fetal lambs

The present study was conducted to monitor changes in surface activity of lung and amniotic fluid of premature lambs. Lung fluid was continuously obtained from 10 tracheotomized fetal lambs (gestational age 98-112 days) until (premature) parturition. Amniotic fluid was sampled twice a week from 3 other lambs by catheterization, and during cesarean section at the end of the gestational period from another 3 lambs. Surface activity in lung fluid increased on the first postoperative day, the more so when this occurred at a later fetal age in the period of 98-112 days. These findings show that even before day 110 'stress' can result in a release of surface active material. A return to low surface activity (about 26 mN/m) was observed within 3 to 6 days. Thereafter, between days 110 and 120, surface activity increased steeply within the individual animals but with large variability between the fetuses. The steep and premature increased was followed by a further but slow increase during the mature period and Ymin values of approximately 2 mN/m were reached. Amniotic fluid surface activity changed slowly with gestational age, but did not reach Ymin values of less than 15 mN/m. Lung fluid total phospholipid phosphorus values were low (about 6 micro M) up to 125 days, when they increased rapidly (greater than 16 micro M). Lung fluid production increased from 4.9 ml/h at the beginning of the experiment to 12.1 ml/h after day 130 but decreased dramatically 1-3 days before (premature) parturition. Experiments on fetal lung maturation should be started before day 105 of gestation and we suggest that measurement of surface activity of fetal lung fluid may be useful in monitoring changes in the premature alveolar type II cells of lambs.