o_1977r8vv9vk1ts2ms0kd8pksa.pdf

376 D. Maulik, R. Figueroa
Table 25.1. Incidence of absent and reversed end-diastolic velocity in high- and low-risk populations
Study,
first author
No. of
patients
Risk category Doppler type High-pass filter
(Hz)
AEDV
No. %
Johnstone [7] 380 High PW, CW 150 24 6.30
Beattie [27] 2,097 Low CW 200 6 0.29
Huneke [14] 226 High CW 200 18 8.00
Malcolm [15] 1,000 High PW 100 25 2.50
Wenstrom [17] 450 High PW 100 22 4.90
Weiss [19] 2,400 Unselected PW 50 51 2.10
Battaglia [23] a 46 Very high PW 100 26 56.20
Pattinson [22] 342 Very high PW, CW 150 120 34.50
200
Rizzo [25] 6,134 High PW 100 192 3.10
Karsdorp [24] a 459 Very high ?PW Lowest 245 53.40
CW, continuous wave; PW, pulsed wave; AEDV, absent end-diastolic velocity.
a Fetuses with congenital anomalies and dyskaryosis were clearly excluded. The other studies either included these fetuses
or are unclear about it.
Incidence
The frequency with which absent or reversed enddiastolic
velocity (AREDV) is encountered in the umbilical
artery varies according to the risk category of
the obstetric population, the time of gestation at
which the observation is made, and the Doppler examination
technique. For high-risk pregnancies the
incidence varies from 2.1% to 56.0% (Table 25.1).
Such a wide range may be explained by the differing
definitions of high-risk pregnancy used by the investigators
and by the level of the high-pass filter used.
For example, the basis for high-risk categorization of
a pregnancy may range from clearly defined clinical
criteria, such as hypertension, to ill-defined groupings
of various clinical conditions. In contrast to that
in the high-risk population, the incidence of AREDV
may be as low as 0.29% in an obstetric population
with a low prevalence of pregnancy complications.
The following two examples illustrate this point. In
probably the largest reported series on AREDV, Karsdorp
et al. [24] used well-defined criteria for selecting
the population for a multicenter study. Only patients
with hypertension or fetal growth restriction
(or both) were included. Hypertension was defined as
a diastolic pressure of 110 mmHg by a single measurement
or 90 mmHg by two or more measurements.
Also included were patients with hypertension
plus proteinuria; the latter was defined as urinary
protein loss of more than 300 mg in 24 h. Intrauterine
growth restriction (IUGR) was defined as the abdominal
circumference measuring less than the 5th percentile
for gestational age based on local populationspecific
nomograms. The lowest possible high-pass
filter threshold was used. Of the 459 patients selected
in this manner, 245 developed AREDV, for an incidence
of 53.4%. In comparison, Beattie and Dornan
[27] found that only 6 of 2,097 singleton pregnancies
developed AREDV for an incidence of 0.29%. The actual
rate might even be lower if we consider that the
high-pass filter setting was 200 Hz, which is relatively
high for umbilical arterial Doppler insonation.
Technical Considerations
As alluded to above and discussed elsewhere in this
book, the procedure used for Doppler measurement
may affect the measured magnitude of the end-diastolic
frequency shift. It is apparent from the basic
principles of the Doppler shift that shifted frequencies
can only be underestimated, not overestimated.
There are two technical sources of this problem: (1)
the threshold setting of the high-pass filter; and (2)
the angle of insonation between the Doppler beam
and the flow axis. The high-pass filter (see Chap. 3)
eliminates from the Doppler signal the low-frequency/high-amplitude
frequency component and is
used to remove signals generated by movement of the
vascular wall or other adjacent tissues. This filter,
however, also removes low-frequency components
generated from the slow-moving blood flow as encountered
during the end-diastolic phase of the cardiac
cycle. Thus end-diastolic frequencies are removed
from the umbilical Doppler waveform. A relatively
high setting of the filter therefore leads to a false diagnosis
of AEDV. It is strongly recommended that
the high-pass filter should be at the lowest possible
setting, which may not exceed 100 Hz. The second
consideration is related to the angle of insonation,
which is inversely related to the magnitude of the es-