18. โขAORTIC ISTHMUS
โข Experimental and clinical data support the
incorporation of Doppler flow velocity waveforms
through the aortic isthmus among the
noninvasive markers of fetal well-being
โข Because of this unique anatomic position, isthmic
flow velocity waveforms are influenced not only
by downstream impedance of the
subdiaphragmatic circulation but also by changes
in arterial tone in the upper part of the body,
especially the brain.
22. Successive and sequential Doppler changes
develop in e-FGR
Cerebral hypoxia
-- cerebral
vasodilatation โ
MCA PI
peripheral fetal
vasoconstriction-
increased
resistance in the
umbilical arteries
โ UA PI
Reversal of flow
in aortic isthmus
absent end-
diastolic/ reverse
flow in umbilical
artery
overload of the
precordial venous
system with "a"
wave absent or
negative on
ductus venosus