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142 D. Maulik
on standing, there was no change in the S/D ratio in
normotensive patients when they were moved to the
upright position. In contrast, the ratio significantly
increased with a postural change in hypertensive patients
and was more pronounced with high resistance
hypertension.
Maternal Exercise and Umbilical
Arterial Doppler Indices
The fetal circulatory response to maternal hemodynamic
stress secondary to maternal exercise may be
an important consideration when timing a fetal Doppler
investigation. Several investigators have addressed
this issue [49±51]. All of these studies, which employed
some form of bicycle ergometry, demonstrated an exercise-induced
response in the maternal cardiovascular
system. The fetal cardiac chronotropic response was
variable. Often the response was tachycardia, although
a lack of variation was also noted. Most studies failed to
show changes in the umbilical arterial Doppler indices.
When such changes were noted, they were associated
with fetal heart rate alterations. From these investigations
it appears that mild to moderate exercise does
not affect flow impedance in the umbilical artery independent
of changes in the fetal heart rate.
Summary
The umbilical arterial circulation is readily accessible
to continuous-wave and pulsed-wave Doppler interrogation.
The procedure is relatively simple. The variance
of umbilical arterial Doppler indices is affected
by both hemodynamic and nonhemodynamic factors.
The latter represent the error component of the measurement
and are attributable to observer and probably
instrumental factors. The hemodynamic factors
include physiologic and pathologic phenomena. Of
the former, the gestational age, fetal breathing, fetal
heart rate, and site of measurement are the most important
contributors.
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