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Chapter 37
Evaluation of Pulmonary and Ductal Vasculature:
Fetal Echocardiography for Evaluation of Therapy
for Preterm Labor
James C. Huhta, Gerald Tulzer, Sharon R. Weil-Chalker
Studies about the effects of maternal drug administration
on the fetus are limited. In early studies the useful
effects of tocolytic drugs appeared to far outweigh their
possible side effects as they relate to the fetus and any
long-term effects. Data continue to accumulate about
how the effects of a pharmacologic agent on fetal physiology
may affect postnatal life. One tool used to assess
fetal physiology is Doppler ultrasonography. The
clinical investigator thus has immediate feedback about
the effects of a currently used drug on the fetal circulation.
This technique has opened up a new field of fetal
circulatory investigation.
In this chapter we review current knowledge about
how two tocolytic drugs, indomethacin and terbutaline,
affect the fetal circulation acutely. The techniques
are explained to illustrate how the fetal circulation
can be evaluated globally to detect drug side
effects. The effects of these drugs on the fetal ductus
arteriosus are used as an example of the potential of
this type of research to provide information about
the ªsecond patientº in the setting of maternal disease
and preterm labor.
Doppler techniques provide an excellent noninvasive
tool for assessing blood velocity and pressure
gradients across the fetal ductus arteriosus [1]. The
fetal ductus arteriosus may constrict during maternal
indomethacin treatment for premature labor [2±4]. It
was demonstrated in animal models that chronic fetal
ductal constriction/occlusion affects the fetal right
ventricle and leads to morphologic changes in the
pulmonary vasculature, resulting in persistent pulmonary
hypertension of the newborn [5±8]. Thus fetal
echocardiography may play an important role in the
management of drug use during pregnancy: It can be
used to detect and monitor side effects of tocolytic
medications such as indomethacin.
Fetal Examination Technique
Fetal Echocardiography
Complete fetal echocardiographic study generally involves
three investigations: two-dimensional echocardiography,
M-mode echocardiography, and Doppler
interrogation. With current technology, resolution is
best after approximately 16 weeks' gestation. A complete
initial study usually takes 30±45 min unless the
fetus is active or poorly positioned for obtaining clear
views. The most informative approach to integrating
the Doppler technique into the fetal heart examination
is to perform intracardiac Doppler sonography
of the heart valves and a ªperipheralº Doppler study
of the cord and other vessels at the same time.
Ultrasonic imaging is commonplace for assessing
the fetus during obstetric care. Imaging assesses the
morphology, and the shape and movement of fetal/
placental/uterine structures provides information
about changes over the course of the pregnancy. Malformations
can be detected prenatally, and growth
can be calculated from images of the fetus. The placenta
can be evaluated with regard to its position,
size, and ultrasonic tissue characteristics; and the
umbilical cord and its abnormalities can be detected.
Improved ultrasonic imaging techniques have made
imaging of fetal intracardiac and extracardiac anatomy
feasible and reliable after 18 weeks' gestation.
Fetal cardiac imaging requires high resolution at
depth, and the development of real-time, gray-scale
imaging allows imaging of the fetal cardiovascular
system as never before possible. New techniques give
current equipment the ability to image cardiac anatomy
even at fast heart rates (>200 bpm). Experience is
being gained with pre- and postprocessing of the
images to optimize them for each application.
Phased-array technology, computerization of imaging
techniques, advanced digital scan converters, dynamic
focusing, and annular array imaging now allow improved
azimuthal resolution and better image quality.
A wealth of physiologic information is currently
available to the clinician, and fetal physiology can be
further evaluated noninvasively with Doppler techniques.
In contrast to the strong imaging signals collected
by an ultrasound instrument, the Doppler signals
from blood flow are weak and technically more
difficult to optimize. Artifacts still occur in Doppler
recordings because of the limited quality of the Doppler
signal or, more often, the inadequate exclusion
of other signals, such as those from vessel walls or
from structure movements.