A wave transmission model of the umbilico-placental circulation
based on hemodynamic measurements in sheep.
Hill, A. A., D. R. Surat, R. S. C. Cobbold, B. L. Langille, L. Y. L.
Mo, and S. L. Adamson.
Department of Obstetrics and Gynecology and Institute of Biomedical
Engineering, University of Toronto, Toronto Hospital Research
Institute, and Samuel Lunenfeld Research Institute, Toronto, Ontario,
CANADA, M5G 1X5
APStracts 2:0150R, 1995.
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 that 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.
Received 15 July 1994; accepted in final form 25 May 1995.
APS Manuscript Number R382-4.
Article publication pending Am. J. Physiol. (Regulatory Integrative
Comp. Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 8 June 1995.