A multiple infusion start time (MIST) protocol for stable isotope studies of fetal blood

A new approach utilizing multiple infusion start times for two stable isotopes of leucine was applied to seven pregnancies in order to assess equilibration times for isotopic studies when a single fetal blood sample is available. Two infusates, one containing L-[1-13C]-leucine and the other L-[5,5,5-D3]-leucine, were given as a primed constant infusion in the maternal circulation at fetal blood sampling (FBS). In five patients L-[1-13C]-leucine infusion was started at time zero (T0) whereas L-[5,5,5-D3]-leucine infusion began 30 min later, and both were continued until the umbilical sample was obtained at 149.7 ± 8.8 min. In order to assure non-steady state conditions, in two patients the first infusion started at T0 and the second 17 and 6 min before FBS was performed at 115 and 154 min, respectively. The fetal/maternal ratio for L-[5,5,5-D3]-leucine over the fetal/maternal ratio for L[1-13C]-leucine was 0.98 ± 0.03, indicating steady state conditions for both infusions for the first six patients. In the last patient the ratio was 0.51, indicative of non-steady state conditions for the shortest infusion time. Our results show that a single fetal sample can provide data for fetal amino acid enrichments reflecting multiple time points. Leucine steady state is achieved 20 min after a primed continuous infusion both in the maternal and fetal circulations.