A wave transmission model of the umbilicoplacental circulation based on hemodynamic measurements in sheep.

Electrical analog models of the umbilical circulation were developed based on hemodynamic measurements in fetal sheep. The umbilical artery was represented by a transmission line and the placenta by a resistive load. Model predictions of input impedance and pressure and flow waveforms agreed with in vivo measurements under baseline conditions, following placental embolization, and during angiotensin II infusion. A unique positive impedance phase observed at the heart rate frequency under baseline conditions was best explained by the unusual viscoelastic properties of the umbilical arterial wall and small load reflections. Furthermore, a short, less vasoactive segment of the umbilical artery in the retroperitoneal space had a large impact on the input impedance of the umbilical circulation, which was particularly apparent when the artery was constricted during angiotensin II infusion. The model indicated that reflections arising near the approximate location where the first arterial branches leave the main umbilical artery have a measurable impact on impedance spectra when load reflections are low.