o_1977r8vv9vk1ts2ms0kd8pksa.pdf

38 D. Maulik
flow directionality during echocardiographic assessment
may assist in recognizing pathology. For example,
atrioventricular valvular incompetence may be
diagnosed from the demonstration of reverse flow
during systole at the tricuspid or mitral orifice.
Flow Velocity Profile
Doppler shift spectral range may reflect the velocity
profile of blood flow in the insonated vessel. When
the spectral range is wide it is called spectral broadening,
and when it is slim it is known as spectral narrowing.
Spectral narrowing usually indicates a flat or plug
flow velocity profile in which most RBCs are moving
at a similar speed; it is usually seen in large vessels.
For obstetric applications, this flow profile is encountered
in fetal ventricular outflows (Fig. 4.4). Spectral
narrowing is seen during the early systolic phase
(from the outset to peak systole) in a great vessel as
the ventricular ejection force at its prime imparts
similar speeds of flow to the RBCs; however, such an
appearance may be artifactual because of the steepness
of the trace [5]. Spectral narrowing is also observed
when the sample volume size is smaller than
the lumen, and it is placed in the center of the lumen
where the flow speed is fastest.
Spectral broadening is seen with a parabolic flow
where the RBCs travel at a wide range of speeds and
produce a wide range of Doppler-shifted frequencies.
In a parabolic velocity profile, the RBCs travel at
varying speeds, with the cells at the center of the vessel
traveling at the highest velocity and those near
the vascular wall at the lowest velocity because of the
viscous drag. Such a wide distribution of RBC velocities
provides uniform broadening of the Doppler
spectral display, which is usually encountered in
small vessels, such as the uterine and ovarian
arteries. Figure 4.5 shows an example of spectral
broadening of the Doppler waveform. In obstetrics,
blunted parabolic flow is seen in the umbilical arteries.
It is also encountered in a turbulent flow characterized
by a wide distribution of RBC speed across
the vascular lumen. Finally, as noted with spectral
narrowing, partial insonation of an artery may also
lead to spectral broadening. If a small sample volume
is placed near the wall of a large vessel, spectral
broadening occurs as the flow layers toward the vessel
wall show wider speed distribution.
Doppler Flow Quantification
Measurement of volumetric blood flow remains a parameter
of fundamental hemodynamic importance.
There are various noninvasive sonographic methods
for quantifying flow. Of these, Doppler ultrasound velocimetry
still remains the standard modality. There
are also non-Doppler-based ultrasound technologies
for measuring volumetric blood flow without some
of the limitations of the Doppler technique. These
approaches are discussed below.
Fig. 4.4. Spectral narrowing.
Top: Two-dimensional image of
the fetal heart with the Doppler
sample volume placed at
the left ventricular outflow.
Bottom: Doppler waveforms.
Note the spectral narrowing of
the waveforms