o_1977r8vv9vk1ts2ms0kd8pksa.pdf

a Chapter 10 Umbilical Doppler Velocimetry: Normative Data and Diagnostic Efficacy 137
Table 10.3. Location of measurement and reliability of
Doppler indices (modified from Maulik et al. [1] with permission)
Doppler index Reliability Error variance (%)
coefficient (%)
D/A 71 29
S/D 62 38
PI 54 46
RI 68 32
D/A, diastolic/average ratio; S/D, systolic/diastolic ratio; PI,
pulsatility index; RI, resistance index.
The mechanism of this phenomenon was investigated
by Vieyres and coinvestigators [16] in a computer
model, which showed that placental resistance is
the primary factor for the observed differences between
the Doppler waveforms from the placental end
and from the fetal end of the cord, whereas viscosity
and cord length have secondary influences. The phenomenon
may be explained more adequately by the
concept of impedance and wave reflection (see
Chap. 4). The fetoplacental vascular bed is a low-impedance
system associated with minimal wave reflection,
which explains the presence of continuing forward
flow in the umbilical artery during diastole. The
closer the measurement site is to the placenta, the
less the wave reflection and the greater the end-diastolic
flow. Consequently, the Doppler waveform that
represents arterial flow velocity demonstrates progressively
declining pulsatility and the indices of pulsatility,
such as the RI or the S/D ratio.
Not all studies, however, agree with the above findings.
Ruissen and associates [17] observed that
although the PI values differed among the various locations
in the umbilical artery (within the fetal abdomen,
0±5 cm from the origin of the umbilical cord, in
the free-floating part, 0±5 cm from its insertion in
the placenta) no unequivocal tendency or statistically
significant difference in the PI could be demonstrated.
The authors concluded that possible variations
in the Doppler waveform along the course of
the umbilical artery have no clinical relevance in uncomplicated
pregnancies.
Short-Term Temporal Variations:
Diurnal Effect
Short-term temporal variations in the umbilical arterial
Doppler indices may affect their reproducibility
and efficacy and should be taken into account when
interpreting changes in the indices. FitzGerald and
associates [18] failed to observe any appreciable diurnal
changes in the umbilical arterial Doppler waveform.
This point was further investigated by Hastie
and colleagues [19], who recorded umbilical arterial
Doppler indices on 3 days in each of 97 women within
a 7-day period. No significant day-to-day variations
(p>0.05) were noted in the S/D ratio or the PI
over the period of study. The degree of variability
was greater before 30 weeks' gestation. It appears that
after 30 weeks' gestation diurnal variations in the
umbilical arterial Doppler indices have a range of
daily variability acceptable for clinical or research applications.
Long-Term Temporal Variations:
Gestational Age Effect
As gestation advances, umbilical arterial Doppler
waveforms demonstrate a progressive rise in the enddiastolic
velocity (Fig. 10.4). This phenomenon was
first described by Stuart and associates [20]. The gestational
age at which end-diastolic velocity may be
first noticed in the umbilical arterial circulation depends
on the examination technique. It is imperative
to ensure that the high-pass filter is either turned off
or set at the lowest value irrespective of the sonographic
approach. In addition, the use of transvaginal
Doppler sonography with color flow-assisted pulsedwave
Doppler interrogation significantly improves
our ability to identify the end-diastolic velocity during
early pregnancy.
Arduini and Rizzo [21] utilized color Dopplerguided
transvaginal spectral Doppler sonography and
demonstrated that the end-diastolic forward flow may
be present in the umbilical artery as early as 10
weeks' gestation. After week 10 the proportion of
cases with end-diastolic forward flow progressively
increased with the advancing gestation and it was
present in almost all cases by about 15 weeks. At this
early stage the phenomenon was not noted during
every cardiac cycle, although the percentage of cardiac
cycles in which end-diastolic velocities were present
increased with the progression of pregnancy. In
this preliminary experience, the authors found no association
between the PI and the later development of
pregnancy complications. Other investigators also
noted the presence of end-diastolic velocity in the
umbilical artery by 15 weeks [22].
Early investigators, utilizing the transabdominal
approach, noted the universal presence of the enddiastolic
velocity by about 22 weeks' pregnancy [23].
However, the high-pass filter was set at 200 Hz, which
is unacceptably high for this application. Our own experience
with transabdominal duplex pulsed-wave
Doppler insonation indicates that end-diastolic velocity
is detectable at the placental end of the cord in
most cases by about 18 weeks' gestation. Thereafter