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a Chapter 4 Spectral Doppler Sonography: Waveform Analysis and Hemodynamic Interpretation 49
elasticity. Any discontinuity or change in these
properties gives rise to reflections of pressure and
flow waves.
3. Reflection coefficient expresses the magnitude of
the phenomenon of wave reflection in an arterial
tree.
These well-established hemodynamic concepts provide
the bases for hemodynamic interpretation of
fetal Doppler waveforms in various physiologic and
pathologic states. As the arterial Doppler wave represents
the arterial flow velocity wave, it is apparent
that down-stream impedance and wave reflection are
among the principal modulators of the waveform and
therefore of the descriptor indices. As gestation progresses,
umbilical arterial Doppler waveforms demonstrate
a continuing increase in the end-diastolic frequency
shifts (see Fig. 10.5), which is attributable to
a progressive decline in fetoplacental vascular impedance.
The latter is necessarily associated with diminished
wave reflections, although it has not been feasible
to study the phenomenon directly.
Doppler Indices and Peripheral Resistance
Investigations in this field were aimed at establishing a
hemodynamic foundation for the Doppler indices in
terms of peripheral vascular resistance. In vitro and
in vivo models were used. The experimental approach
essentially consisted of increasing the circulatory resistance
and analyzing the changes in the indices.
Spencer and coinvestigators [36] used an in vitro
flow model to validate the Doppler indices in terms
of peripheral resistance (mean pressure/mean flow).
The flow was reduced in a stepwise fashion using a
clamp. The decline in flow led to increases in RI, PI,
and S/D ratio. The pressure and pressure waveform
remained unchanged. The authors noted that rises in
the Doppler indices reflected an increasing peripheral
resistance; the RI and PI had a mostly linear increase,
whereas the S/D ratio demonstrated an exponential
increase. Maulik and colleagues [37] investigated the
relation between the various umbilical arterial Doppler
indices and umbilical arterial resistance in fetal
lambs. The hemodynamic perturbation was induced
by mechanical constriction of the umbilical artery,
which resulted in declines in flow by 74%±89% and
increases in pressure by 8%±23%. The correlation between
the Doppler indices and the hemodynamic parameters
is shown in Table 4.1, which indicates that
with a well-defined obstruction to the downstream
flow the indices correlate highly with the downstream
hemodynamic parameters, including the peripheral
resistance (p