Maternal-Fetal argon transfer using mass spectrometry during fetal hypoxia in pregnant ewes

Objective: To develop a method of maternal-fetal inert gas transfer to assess placental function. Methods: Seven pregnant ewes were prepared, under general anesthesia, for acute and chronic experiments with intravascular mass spectrometer catheters placed into the maternal and fetal femoral arteries. The ratio of peak levels of argon in fetal and maternal circulations following 2 min inhalation of 75% argon in oxygen was used as a test of placental gas transfer to compare different methods of inducing fetal hypoxia. Results: With 2 min inhalation of 75% argon in oxygen, argon appeared in the maternal circulation after 10 sec. The level of argon rose rapidly and peaked by 135 sec before falling less quickly. Argon was identified in the fetal circulation by 40 sec after which it rose slowly to reach a lower peak by 175 sec. Fetal argon was no longer measurable after 20 min. The fetal/maternal (F/M) ratio of peak argon levels was quite reproducible (mean 0.31, SEM 0.008, n = 35). No fetal argon was found following complete umbilical cord occlusion. Fetal hypoxemia, produced by partial uterine artery occlusion, reduced the F/M ratio (0.19, SEM 0.02, P < 0.001). However, a similar degree of fetal hypoxemia, produced by reducing the maternal inspired oxygen fraction, was associated with a rise in the F/M ratio (0.47, SEM 0.03, P < 0.02), implying increased transfer of the inert gas to the fetus. Conclusion: These results suggest that the effective placental exchange area (matching of maternal and fetal circulations within the placenta) may increase as well as decrease, according to circumstances. Maternal-fetal inert gas transfer is a sensitive indicator of effective placental gas exchange area, and mass spectrometry has the potential to assess this measure of fetal-placental reserve.