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a Chapter 37 Evaluation of Pulmonary and Ductal Vasculature 555
continuous infusion. Terbutaline, and premature labor
itself, has a positive inotropic effect on the fetal heart
[9]. Increased right ventricular output causes a higher
blood volume to be pumped from the main pulmonary
artery to the descending aorta, resulting in an
elevated maximal ductal blood velocity.
Terbutaline Effect Versus
Ductal Constriction
An increase in ventricular stroke volume at times increases
the ductal velocities such that the waveform
may simulate ductal constriction. However, the PI increases
in this situation, in contrast to a decreased PI
with ductal constriction [13]. Terbutaline is not
known to constrict the ductus, and we have not
found such constriction. However, because it can increase
fetal cardiac output and increase systolic velocities
in the heart and arches, we have used the PI to
distinguish these causes of increased systolic velocity
(Fig. 37.1, 37.2). Ductal constriction lowers the PI to
less than 2.0 owing to obstruction to flow and high
velocities; increased fetal cardiac output elevates the
PI to more than 3.0. This method of waveform analysis
can therefore distinguish these two causes of increased
ductal velocity.
When terbutaline fails to stop premature labor, indomethacin
is frequently added as a second drug. Increased
maximal velocity in these cases could be due
to indomethacin-induced ductal constriction or the
effect of terbutaline-induced increased right ventricular
output. When using maximal ductal velocity
alone, the latter case may be misinterpreted as fetal
ductal constriction and could lead to discontinuation
of an effective tocolysis regimen.
The flow pattern of a mildly constricted ductus
and that of a wide open ductus with increased flow
may have the same maximal systolic and end-diastolic
velocities, but there is a significant difference in
mean velocity and PI that clearly distinguishes the
two etiologies.
The PI in peripheral vessels is thought to reflect peripheral
impedance. In the ductus arteriosus, however,
it is used only to help in the differential diagnosis of
increased maximal velocity. Ductal constriction causes
decreased pulsatility, with higher than normal velocities
indicating obstruction; whereas decreased pulsability
in the umbilical circulation, for example, is at low velocity
and indicates increased flow and lower impedance
[13]. Mean ductal blood velocity alone may be able to
distinguish ductal construction from other causes of increased
maximal velocity, but it has to be interpreted in
relation to gestational age. The PI, on the other hand,
does not change with gestational age and may therefore
be a more useful parameter in clinical practice.
Fetal Pulmonary Assessment
Using Doppler, fetal pulmonary vascular status can be
assessed. Pulsed Doppler has been shown to be obtainable
in most pregnancies after 20 weeks' gestation. The
typical blood velocity waveform with a systolic reversal
pattern is obtained and can be quantitated with a PI.
This approach was detailed by Rasanen [14] and
showed that the pulmonary vascular resistance drops
near 30 weeks' gestation and then rises near term.
The fetal resistance is known to change with certain
drugs such as indomethacin [15]. Reactivity of the human
fetal pulmonary circulation to maternal hyperoxygenation
was studied with advancing gestation and
it was found that the fetal pulmonary circulation appeared
to vasoconstrict after 31±36 weeks of gestation
[16]. This technique has been applied in a cooperative
study recently [17]. In this study, fetal pulmonary artery
Doppler was used as a test during administration
of 60% oxygen to the pregnant mother after 30 weeks'
gestation to identify fetuses with pulmonary hypoplasia
that would lead to perinatal death. A reactive test
was defined as a decrease of at least 20% in the branch
pulmonary PI with oxygen. Of the 14 fetuses who had a
nonreactive hyperoxygenation test, 11 fetuses (79%)
died of pulmonary hypoplasia. Of the 15 fetuses who
had a reactive hyperoxygenation test, only one fetus
(7%) died in the neonatal period (sensitivity, specificity,
and positive and negative predictive values were
92%, 82%, 79%, and 93%, respectively).
We may speculate that application of this test
could be of use in establishing the prognosis of
fetuses at risk of pulmonary hypoplasia. Such a test
has been performed in fetal lambs and the maternal
hyperoxygenation test did predict those lungs with
hypoplasia [18].
The test should be done by those with experience
in normal and abnormal responses and may be technically
difficult in some cases. Interventions designed
to increase the pulmonary vascular bed cross-sectional
area include occlusion of the trachea.
Summary
Treatment of preterm labor with nonsteroidal antiinflammatory
agents can lead to acute, transient
changes in the caliber of the fetal ductus arteriosus.
These changes can be detected and quantitated using
fetal echocardiography. This complication can be successfully
managed with serial study of the fetus using
Doppler techniques and may be indicated serially.
The b-agonist sympathomimetic medications used to
treat preterm labor, such as terbutaline, have an immediate
effect on the fetal circulation and increase
the heart rate and the inotropic state of the fetal myo-