Evaluation of the Xe-133 clearance method for indirect placental blood flow determination

The Xe-133 clearance method, chosen by several groups for indirect placental blood flow (PBF) determination, is evaluated using a placenta-fetus compartment model first introduced by Moll (1973). The placenta is modelled as a strong concurrent, counter-current or cross-current exchange system. The fetal circulatory system is simplified by dividing it into two major branches: the umbilical circulation and the fetal tissue or systemic circulation. Following an arbitrary labelling of maternal arterial blood, Xe-133 concentrations in myometrium, placenta and fetus are derived from the model and are used to calculate placental clearance rates. The dependence of clearance rates on maternal and fetal PBF, on the distribution of fetal cardiac output, and on the effect of recirculation times, is considered in the three different arrangements of maternal and fetal placental vessels.