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136 D. Maulik
arterial Doppler indices demonstrate satisfactory precision
for clinical applications.
Continuous-Wave Versus
Pulsed-Wave Doppler Indices
Although one may not expect any discrepancy between
the Doppler indices obtained by continuouswave
or pulsed-wave Doppler interrogation, in practice
there are enough differences between these techniques
to warrant an objective inquiry into the issue.
Brar and associates [8] compared the S/D ratios obtained
by continuous-wave and pulsed-wave Doppler
ultrasonography from the umbilical artery in highrisk
pregnancies during the third trimester and found
no significant difference in the mean S/D ratios obtained
by either method for the entire population
(continuous-wave S/D 2.81 Ô1.79, pulsed-wave S/D
2.71 Ô 1.83; r=0.98), the normal group (continuouswave
S/D 1.96 Ô 0.41, pulsed-wave S/D 1.95Ô0.40,
r=0.91), or the abnormal group (continuous-wave
S/D 6.23Ô1.58, pulsed-wave S/D 6.35Ô1.52, r=0.94).
Mehalek and coinvestigators [9] also observed no significant
differences (p>0.05) between the continuouswave
and pulsed-wave Doppler devices when measuring
the peak S/D ratios in the umbilical arteries. The
S/D ratios of the umbilical artery measured by each
device showed a strong correlation, whether measured
by one observer (r=0.93) or two observers
(r=0.89). Most other investigations corroborated this
finding [5, 10].
In contrast to the above investigators, van Vugt
and coinvestigators [11] found that the AB ratio
(same as the S/D ratio), RI and PI were slightly higher
for the continuous-wave Doppler system compared
to the pulsed-wave Doppler system, but not significantly
(p=0.18, p =0.21, and p=0.44, respectively).
The authors speculated that the difference in the signal-to-noise
ratio (S/N ratio) between the pulsedwave
and continuous-wave Doppler systems was the
reason for the discrepancy. Jorn and associates [12]
also observed differences between duplex pulsed Doppler
and nonduplex continuous-wave and pulsed-wave
Doppler systems in terms of their clinical efficacy.
The sensitivity was higher with the simple nonduplex
Doppler technique (74.3% versus 52.9%), whereas the
specificity was higher with the duplex Doppler technique
(77.9% versus 52.6%). The authors thought that
this finding was attributable to the inability to localize
the vessel precisely using the simpler stand-alone
Doppler devices. Gaziano and colleagues [13] reported
the use of a threshold value of 4.5 for the umbilical
arterial S/D ratio derived with pulsed duplex
Doppler ultrasonography. This value is considerably
higher than the common S/D ratio cutoff value of 3.0
obtained by continuous-wave Doppler insonation.
Despite these differences, most investigators use
the same discriminatory threshold values for the
Doppler indices irrespective of the type of Doppler
instrumentation. Whether it is justifiable remains an
open question.
Site of Doppler Interrogation
Introduction of duplex pulsed Doppler ultrasound for
interrogation of umbilical arteries allowed determination
of the sampling site in the umbilical cord. Utilization
of this approach led to the realization that the
site of Doppler sampling in the umbilical cord may
contribute significantly to variations in the Doppler
indices. Several studies addressed this issue specifically.
Most observed that the location of the Doppler
sampling site in the umbilical cord affects the Doppler
waveform and is reflected in the Doppler indices,
which are higher at the fetal abdominal end of the
cord than at this placental end [14, 15]. Abramowicz
and coinvestigators [15] studied the effect of the examination
site in 30 normal pregnancies and noted
statistically significant differences (p