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Indices for the Evaluation of Doppler Sonograms
34
1
toplacental and the uterine vessels, and any abnormal
findings there must be supplemented by follow-up examinations of other vascular beds. The visual classification provides assistance in standardizing this comparison. Special features of the waveform, such as a
notch, must also be noted.
Indices are calculated only after the other two measures. They are used to compare measured values with
reference values for the interrogated vascular bed. Indices are determined by the programming of the in-
Reference Curves
In the early stages of obstetric Doppler sonography all
the leading research teams developed normal or abnormal reference curves using more or less extensive
collections of data. Fortunately, there is general agreement regarding the change in values over the course of
pregnancy. This is evidence for the cross-validity of all
published reference curves. This is especially remarkable in view of the fact that Doppler systems from
different manufacturers were used. Mathematical renderings, however, are still not standardized.
Tables showing normal values and waveforms as
mean values and standard deviations must be rejected,
since there is no symmetrical normal distribution of
these values. A great many measurements are required
to construct percentile graphs. The problem of determining a discriminant depends on the values accepted
as physiological maximal (or minimal) values for the
MCA. Using the 10th/90th percentile or the 3rd/97th
percentile to define the normal range makes a statistically significant difference. The higher the percentile
limits are set, the smaller is the risk of a false positive
diagnosis in any one case.
We present here as an example the reference curves
for the indices PI, RI, and the A/B ratio of the umbilical
a., the aorta, the MCA, and the ascending branch of the
uterine a. The data were drawn from 1550 women in
the 22nd to 42nd weeks of pregnancy. The normal
population was selected according to a strict protocol.
The standardized curves represent the 3rd, 97th, and
50th percentile, i.e., they include the mean. The curves
did not require smoothing. The ordinate scale was
selected to ensure that the waveforms could be com-
igs.
F
pared directly with each other (
Voigt 1991).
When the progression of the waveforms is analyzed,
the percentile curves of the UA/umbilical and MCA
show a tendency to decline during the study interval of
the 22nd to 42nd week of pregnancy. After the 34th
week of pregnancy the curves of the aorta and uterine
a. run almost horizontally. This observation of the
adaptation of the vascular resistance in the respective
vascular beds is consistent with what might be ex-
2.16−2.18) (after
strument used and preferences developed over time.
Caution must be exercised before uncritically accepting waveform-dependent data calculated by some
available instrument. Qualitative differences in Doppler spectrum analysis influence the sharpness of the
waveform edges. Intensity-dependent waveform analyses may lead to totally different values for mean
maximal velocity (T
in calculating PI.
pected in view of their underlying physiology (cf.
Chap
nonparametric independent group comparison (Kruskal−Wallis test). The hypothesis that the fluctuations
corresponded to a curve at the 5 % confidence level had
to be abandoned for all parameters of the aorta and
uterine a. The validity of the declining course of the
curve for indices of the umbilical a. and the MCA was
confirmed.
4). The established data were subjected to a
ter
) than manually obtained values
max
Reference Curves
PI
3.50
3.00
2.50
2.00
1.50
1.00
0.50
0
22 24 26 28 30 32 34 36 38 40 42
a
PI PI
3.50
3.00
2.50
2.00
1.50
1.00
0.50
0
22 24 26 28 30 32 34 36 38 40 42 22 24 26 28 30 32 34 36 38 40 42
b
Weeks of gestation Weeks of gestation
Weeks of gestation Weeks of gestation
PI
1.0
0.8
0.6
0.4
0.2
0
22 24 26 28 30 32 34 36 38 40 42
c
3.50
3.00
2.50
2.00
1.50
1.00
0.50
0
d
Basic Concepts
PI
3,50
3,00
2,50
2,00
1,50
1,00
0,50
0
22 24 26 28 30 32 34 36 38 40 42
e
Fig. 2.16a−e Percentile curves for the PI.
a Umbilical a
b Arcuate a
c Arcuate a. (ordinate shortened in the percentile cur ve to im-
prove definition)
SSW
97th percentile
Median
3rd percentile
d Fetal aorta.
e MCA.
35
Indices for the Evaluation of Doppler Sonograms
1
RI
100
80
60
40
20
0
22 24 26 28 30 32 34 36 38 40 42
Weeks of gestation
RI
100
80
60
40
20
0
22 24 26 28 30 32 34 36 38 40 42
b
Weeks of gestation
RI
100
80
60
40
20
0
22 24 26 28 30 32 34 36 38 40 42
c
RI
100
80
60
40
20
0
22 24 26 28 30 32 34 36 38 40 42
d
Weeks of gestationa
Weeks of gestation
36
97th percentile Median
Fig. 2.17a−d Percentiles of the resistance index RI.
a Umbilical a.
b Arcuate a
c Fetal aorta
d MCA
3rd percentile
Index Quotients
15
12
9
6
3
0
22 24 26 28 30 32 34 36 38 40 42
Weeks of gestation
5
4
3
2
1
0
22 24 26 28 30 32 34 36 38 40 42
b
Weeks of gestation
15
12
9
6
3
0
22 24 26 28 30 32 34 36 38 40 42
c
15
12
9
6
3
0
22 24 26 28 30 32 34 36 38 40 42
d
Weeks of gestationa
Weeks of gestation
Basic Concepts
97th percentile Median
Fig. 2.18a−d Percentiles of the A/B ratio.
a Umbilical a
b arcuate a
c Fetal aorta
d MCA.
Index Quotients
On inspection, the course of the curves for the MCA
and the aorta (AO) is marked by the crossing of their
medians during the 32nd to 34th week of pregnancy.
The median of the AO continues horizontally, while
that of the MCA continues to decline. Hence the
quotient MCA/AO must be 쏜1 before the 32nd to 34th
week of pregnancy and 쏝1 after the 34th week
F
2.19) (after Voigt 1991). Since in cases with
ig.
(
centralization of blood flow the index value of the AO
rises, while the brain-sparing phenomenon leads to a
low value for the MCA, the quotient must also decline.
This suggests that a quotient comprising the values
derived from the vascular beds affected by such a
vascular redistribution should provide a sense of the
extent of the redistribution. We therefore determined
3rd percentile
reference curves for the quotient (Q MCA/AO) from the
two values in the normal population. These show a declining course starting with the 32nd week of gestation, reflecting the declining resistance in the cerebral
vascular bed from this time point to the end of pregnancy. Using this new parameter we defined redistribution of the circulation to the brain consequent on
chronic hypoxia with more confidence than when
using a value below the 3rd percentile in the MCA.
The references for all three indices (Q PI MCA-PI AO,
Q RI MCA-PI AO, Q A/B ratio MCA-A/B ratio AO) are
given below (
Fig. 2.20) (after Voigt 1991).
37
Indices for the Evaluation of Doppler Sonograms
1
350
300
250
200
150
100
50
0
22 24 26 28 30 32 34 36 38 40 42
Weeks of gestation
Median PI for AO
Median PI for MCA
Fig. 2.19 The intersection of the medians of the PI of the AO
and that of the MCA facilitates the development of ratios.
3.5
3
2.5
2
1.5
1
0.5
0
22 24 26 28 30 32 34 36 38 40 42
a
3.5
3
2.5
2
1.5
1
0.5
0
22 24 26 28 30 32 34 36 38 40 42
b
3.5
3
Weeks of gestation
Weeks of gestation
38
2.5
2
1.5
1
0.5
0
22 24 26 28 30 32 34 36 38 40 42
c
Fig. 2.20a−c
a PI ratio of MCA and AO
b RI ratio of MCA and AO
c A/B ratio of MCA. and AO.
Weeks of gestation
97th percentile
Median
3rd percentile
Summary
Summary
The procedures used to evaluate Doppler sonograms
during pregnancy may be systematically classified ac-
Table
cording to the listed values and indices (
Central to this evaluation is the waveform analysis of
the sonograms.
왘 The classification of blood flow in the fetal aorta put
forward by Maršál’s group was presented here as a
means of simple optical classification.
왘 Absolute velocities include, for instance, maximal
peak velocities. In certain clinical interrogations increases as well as decreases may be significant.
왘 Evaluation of systolic/diastolic variability in the
course of the cardiac cycle plays a major role especially in the uteroplacentofetal unit. It can be registered with relative simplicity for a waveform with a
single peak, especially with 2-point indices such as
the RI and the A/B or B/A ratio. Except for the RI
these indices are difficult to evaluate when there is
no end-diastolic flow or reversed flow. On the other
hand, the PI, which is diff icult to calculate because
of its third point—the mean maximal velocity,
(TA)—can differentiate even in cases without
V
max
diastolic flow. For this reason it must be given preference over 2-point indices, at least in theory.
왘 Measures of systolic acceleration, such as the RS and
the RAT have been sparsely used in evaluating pregnancy. This is surely partly because there is no con
vincing clinically relevant concept of “upstream”
impedance.
왘 On the other hand, there is a concept of systolic
delay that includes both variability and velocity re-
duction. However, simple measurements are still
lacking, for the DS is based on a complicated calculation and does not contain any information that
can be easily interpreted. We have therefore pre-
sented some complementary indices using a model
with four measured points. These are easily calculated and interpreted.
왘 A relative reduction in velocity in the whole sono-
gram is quantified by the RSA, the rMIT, the twPI,
and the 4-point model. In pregnancy precisely the
extent of the reduction specifically in diastolic flow
seems to hold greater diagnostic potential than the
increase in systolic/diastolic difference by the same
mechanism. While the concept of a CL is elegant,
but mathematically demanding, that of RSA attracts
by its simplicity.
2.1).
왘 Of the basic quantitative measurements of blood
flow velocity developed from the total sonogram,
mean flow velocity F
quantitative measurements of blood flow and
therefore the most important.
왘 Of the basic qualitative measurements of the total
sonogram the SBI and the SBR should be mentioned,
since they characterize the flow profile in a cross
section of the vessel.
(TA) is the starting point for
max
Suggestions for Obstetric Practice
The Doppler examiner must first gain an acoustic and
optical impression of the Doppler sonogram. The recorded sonogram must be compared to expectations.
The optical classification of Maršál’s group offers an aid
to standardizing this comparison.
Next, any special features of the waveform, such as a
notch, must be noted.
Only after these two observations have been made
will the examiner make use of indices to compare the
data resulting from his examination with reference
values. The RI is easy to calculate and also to interpret,
since it represents the relation between the peak and
trough values of the waveform in a linear fashion. It is
recommended when the points on the waveform are
determined manually so to speak . By contrast the A/B
ratio is in our view much less easy to inspect because
of the exponential increase in its value. If the waveform
can be calculated automatically, it may be feasible to
use the PI, since it includes changes in the waveform
between the two extreme values, and because it allows
more refined conclusions to be drawn even when flow
is absent or retrograde.
If computerized analysis of the sonogram is available, a more precise definition is possible using the CL
indices. Similarly the computer can provide a refined
analysis of the spectrum itself, allowing conclusions
about the type of flow.
This means that the possibilities of Doppler sonography are not limited to developing a few indices. In this
connection Rosemary Thompson noted: “It is not realistic to expect the effects of complex physiological
processes to be neatly separated by a few phenomenologically based waveform indices.” (Thompson et al.
1986)
Basic Concepts
39
1
40
3 Vascular Supply of the Uteroplacentofetal Unit and Tech-
niques for the Examination of Individual Vessels
Vascular Supply of the Uteroplacentofetal Unit
Uteroplacental Blood Supply
The uteroplacental blood supply is shown schematically inFigure
uterus are the uterine aa. and the ovarian aa., which
anastomose around the uterus. The blood supply of the
uterus from the internal iliac aa. (uterine aa., Fig. 3.2)
and directly from the aorta (ovarian aa.) is therefore secure. There are also other possible supply paths, for example, from the external iliac aa. via the round ligaments or more caudally from the internal iliac aa. via
the cervical aa. or the vaginal aa.
The uteroplacental vessels in a narrower sense lie in
the wall of the uterus. The arcuate aa. are tortuous,
forming arcades that anastomose freely, and give rise
to radial aa. that pierce the myometrium radially. They
give rise to basal aa., which supply the stratum basale,
as well as two or more helically wound spiral aa. that
open directly into the intervillous space of the placenta
(
F
3.3).
ig.
The musculoelastic layer in the arterial wall of the
decidual spiral aa. is destroyed by migrating trophoblastic tissue, enlarging the lumen at the placental end
considerably and preventing its narrowing by any
vasomotor stimuli. The uteroplacental blood supply is
3.1. The specific arteries supplying the
Aorta Aorta
Iliac aa.
Uterine aa. Ovarian aa.
Arcuate aa.
XX = systemic
Radial aa. Basal aa.
Spiral aa.
BP = 25 mmHg
V = 30 cm/s
mean
Intervillous space
Perfusion pressure
BP = 15 – 20 mmHg
V = 1 cm/s
mean
Veins
BP = 3 – 5 mmHg
Fig. 3.1 Schematic representation of the uteroplacental blood
supply.
Basic Concepts
Fig. 3.2 Display of the origin of the uterine a. from the iliac a.
with corresponding normal Doppler sonogram.
Fig. 3.3 Display of placental perfusion. This type of examina- 컄
tion is currently not clinically useful.
41
Vascular Supply of the Uteroplacentofetal Unit and Techniques for the Examination
1
controlled by regulation of the uteroplacental aa. There
are some indications that this regulation is initiated by
humoral factors of fetoplacental origin, such as estrogen, even before the trophoblastic invasion.
Drainage of the intervillous space is assured by veins
in the decidual septa and sinuses in the edge of the
placenta. Possibly pulsations in the companion arteries
and perhaps Braxton Hicks contractions enhance
venous drainage. When the inferior vena cava is obstructed, as in the supine position, drainage may continue through the ovarian v.
Every placental cotyledon is supplied by one spiral a.
This has led to speculation that the presence of a
uterine vessel may induce cotyledon formation. According to this model there are some 40−60 such units
combining function and circulation.
The vessels adapt to pregnancy by considerable dilatating. Thus, the diameter of the uterine aa. increases
by one and a half to threefold, the retroplacental arcuate aa. tenfold and the spiral aa. thirtyfold. Uteroplacental hemodynamics are further characterized by
Fig. 3.4 Display of the umbilical vessels by color Doppler and
Doppler sonogram.
blood pressure and flow changes in these vessels. Systemic blood pressure remains almost constant as far as
the arcuate aa. The blood pressure then declines to
25 mmHg at the mouths of the spiral aa. Hence the
perfusion pressure (PP) in the intervillous space is
quite low—about 15−20 mmHg. Finally in the veins a
pressure of 3−5 mmHg may be expected. In the quiescent uterus about 90 % of the blood supply reaches the
intervillous space. The mean flow velocity of the blood
in the spiral aa. is about 30 cm/s. The intervillous space
contains about 150 mL at term, and the blood there
moves at a rate of about 1 cm/s. On an average about
600 mL of blood flow through this open space along a
pressure gradient of about 15−20 mmHg.
Fetoplacental Blood Supply
In contrast to the uteroplacental vascular bed the fetoplacental bed is closed. To a large extent it lies outside
Fig.
the fetus. The two umbilical aa. (
internal iliac aa. Normally they wind helically over a
variable distance of 20−140cm (50 cm on average)
through the amniotic cavity to the surface of the
placenta. There they divide into the vessels of the
chorionic plate, which in turn divide into the vessels of
the villi. These chorionic vessels are characterized by
the absence of nerves and elastic coats. On the other
hand, they have a strongly developed musculature and
swollen epithelial cells. The latter prepare the vessels
for rapid occlusion after delivery. It has been theorized
that the vessels in the villi provide an arteriolar type of
action during pregnancy, reducing pressure and retarding flow before the blood enters the sinusoidally
modified capillaries.
The return flow runs to the unpaired umbilical v.,
parallel to the arterial flow. It then drains intra-abdominally into the portal sinus (sinus venosus), where the
flow is divided into the left hepatic a., the left portal
vein, and the ductus venosus, which drains into the left
3.5).
lobe of the liver (
Fig.
3.4) arise from the
42
Fig. 3.5 Color Doppler display of the umbilical vessels.
Fetal Blood Supply
The fetal circulation (Fig. 3.6) is marked by three major
shunts: the ductus venosus (ductus venosus arantii),
the foramen ovale, and the ductus arteriosus (of
Botalli).
Almost 20−30 % of the oxygenated blood from the
umbilical v. is shunted past the liver to the heart by the
ductus venosus (Fig. 3.7). This well-oxygenated blood
then largely bypasses the pulmonary circulation and
passes from the venous side directly into the great arteries of the systemic circulation. Clearly this route
moves the flow of oxygen- and nutrient-enriched
blood preferentially to the heart and brain (streamlin-
Superior vena cava
Oval foramen
Right atrium
Inferior vena cava
Lung
Vascular Supply of the Uteroplacentofetal Unit
Aortic arch
Ductus
arteriosus
Pulmonary
a.
Pulmonary
v.
Left
atrium
Basic Concepts
Right hepatic v.
Left hepatic v.
Sinus portae
Portal v.
Umbilical v.
Umbilical cord
Umbilical aa.
Urinary
bladder
Ductus venosus
Descending aorta
Sphincter
Intestines
Kidney
Superior vesical a.
Internal iliac a.
Legs
Placenta
Fig. 3.6 Simplified representation of the fetal circulation. The colors indicate the oxygen saturation of the blood, the arrows the
direction of blood flow (after Moore 1996).
Oxygen saturation of the blood:
High Medium Low
43