TECHNETIUM TC 99M RAPIDLY CROSSES THE OVINE PLACENTA AND INTRAMEMBRANOUS PATHWAY

OBJECTIVE: This study examined the movement of the soluble ion technet ium Tc 99m across the ovine placenta and intramembranous pathway. STUD Y DESIGN: Nineteen fetal sheep at 131 +/- 1 (SE) days' gestation were studied. After a 1-hour control period technetium Tc 99m was injected into either a fetal vein (n = 7), the amniotic cavity (n = 5), or a ma ternal vein (n = 5). Maternal and fetal blood, fetal urine, and amniot ic and allantoic fluid were sampled during the control period and for 8 hours after the injection. Fetal urine was drained externally throug hout the experiment. In five animals technetium Tc 99m was injected in traamniotically after the fetus was killed with air emboli and sampled as described. RESULTS: Intrafetally injected technetium Tc 99m rapidl y crossed the placenta; then it entered and was concentrated in the am niotic cavity. Intraamniotically injected technetium Tc 99m rapidly en tered into the fetal circulation. The maternally injected technetium T c 99m rapidly crossed the placenta into the fetus, suggesting a half-t ime for placental exchange of <50 minutes. The technetium Tc 99m injec ted into the dead fetus group demonstrated significantly less maternal absorption than in the live fetus group. CONCLUSIONS: The soluble ion technetium Tc 99m demonstrated a much more rapid movement in both dir ections across the ovine placenta then previously demonstrated for the smaller ion sodium. Technetium Tc 99m rapidly crossed the intramembra nous pathway bidirectionally, suggesting a high permeability of the in tramembranous pathway. Minimal maternal absorption of technetium Tc 99 m in the dead fetus group suggests little transmembranous absorption b y the mother.