o_1977r8vv9vk1ts2ms0kd8pksa.pdf

548 J. C. Huhta et al.
The equipment for fetal Doppler examination is in a
state of rapid evolution. At the present time, image-directed
or color Doppler-directed pulsed-wave and continuous-wave
Doppler ultrasonography is available to
clinicians. Advances in several technologies have allowed
progressive advances in this field. Better materials
and an understanding of transducers and their design
has led to improved acquisition of signals and an
improved signal-to-noise ratio. The improvement in resolution
for imaging also improves the spatial data collection
for the Doppler setup and gives more confidence
that the blood velocity came from the site where the
sample volume was placed. Advances in broad-band
amplifier design and phased-array and annular technologies
affect modern obstetric Doppler equipment and
allow the same transducer to be used efficiently for both
Doppler sonography and imaging. Integration of imaging
and Doppler sonography is now possible.
Color Doppler sonography is being applied to supplement
other modes of ultrasonic scanning (see below).
The color Doppler technique provides information
about the flow of blood and specifically blood
velocity in the structures being visualized.
The intensities of the various Doppler modalities
are different. With a method of intensity assessment
using the spatial peak temporal average (milliwatts
per square centimeter, or mW/cm 2 ) as a measure of
the heating potential of the ultrasound, the exposure
for pulsed-wave, continuous-wave, and color Doppler
technology can be compared [7].
Peripheral Doppler/Cardiac
Doppler Sonography
The umbilical cord arterial Doppler pattern is characterized
by a peak velocity during late systole, an enddiastolic
velocity measured before the next upstroke,
and the mean velocity, which is the average of the entire
waveform over the cardiac cycle. The energy source
for the umbilical waveform is the fetal heart. The left
ventricle and (predominantly) the right ventricle pump
blood to the placental circulation. The early part of the
upstroke of the umbilical waveform gives information
about the waveform passing from the heart to the peripheral
circulation. It is determined by myocardial
performance, large-vessel compliance and elastic properties,
and arterial reflections along the fetal descending
aorta. The rapidity of the upstroke should therefore
relate to several factors that transmit a pressure waveform
to the cord. Factors that would increase the rate
of systolic velocity increase are increased myocardial
inotropy, decreased compliance of the aortic wall,
and high total impedance. The diastolic portion of
the curve is not sharply demarcated but reflects the
time when the semilunar valves are closed and the circulation
propels blood based on its inertia and pressure
wave propagation during vascular relaxation. The principal
determinant of the end-diastolic velocity is the
distal resistance. The latter parameter is a consequence
of both the systemic and parallel placental circulations.
There is mor pulsation (pulsatility) in an umbilical waveform
when the distal impedance is increased. Therefore
cord pulsatility can be used as an indirect indicator
of placental impedance. Waveforms with a high diastolic
velocity are associated with high flow during diastole,
and those with low or absent diastolic velocity
have low diastolic flow.
The best index to use for analysis and communication
of umbilical cord Doppler information is controversial.
Several authors have presented arguments for
the options [8], which include the pulsatility index
(PI), the resistance (Pourcelot) index (RI), and the
systolic/diastolic (S/D) ratio.
PI ˆ…S
RI ˆ…S
S=D ratio ˆ S=D
D†=mean velocity
D†=S
where S = peak systolic velocity and D=minimum
diastolic velocity. Mean velocity equals the integrated
mean over the cardiac cycle.
The PI is less susceptible to a variation in heart
rate because the heart rate is an intrinsic factor in
the mean velocity. This parameter requires digitalization
of the entire waveform over one cycle and is the
best parameter for longitudinal comparison. The RI
and S/D ratio are popular because they require little
computation. The PI of the peripheral circulation reflects
the distal impedance and therefore is low with
low impedance. In situations where there is kinetic
energy release with discrete obstruction in the circulation,
such as with carotid arterial obstruction or
coarctation of the aorta, the PI may be used to describe
the waveform changes. In this situation, a low
PI is indicative of increased obstruction and pressure
gradient and usually lower flow.
Cardiac Doppler examination is performed by
placing the pulsed Doppler sample volume in the
valve (arch) of interest during the examination as described
below. The three major Doppler techniques
(pulsed-wave, continuous-wave, and color) are complimentary
in the fetal examination. All three are
used during a complete examination of the heart and
the peripheral vessels. Continuous-wave and pulsedwave
Doppler techniques have been applied to the assessment
of fetal hemodynamics by detecting flow in
the umbilical cord and studying the pulsatility of that
waveform. The typical examination includes sampling
many sites in the circulation.