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a Chapter 9 Fetal and Maternal Cardiovascular Physiology 119
Fig. 9.4. Individual values and
calculated 5th, 10th, 50th, 90th
and 95th centiles of blood
flow per cycle (upper trace)
and blood flow per minute
(lower trace) in ductus arteriosus.
(Reprinted from [68])
vascular resistances of the upper and lower circulations,
it may be possible to define circulatory changes
during fetal distress by examining the relative outputs
of the two ventricles by echocardiography or by assessing
the velocity profiles in the aorta and the pulmonary
trunk by the Doppler technique [73±75]. The
acceleration of the aortic flow (dV/dt) is one of the
most reliable indices of myocardial contractility. Measurement
of acceleration time (time to peak velocity)
of both the aorta and the pulmonary artery can be
used as a good index of ventricular performance [76±
78].
During fetal hypoxemia resulting from reduced
oxygen delivery across the placenta (uterine blood
flow reduction, maternal hypoxemia), blood flow to
the fetal body is reduced owing to peripheral vasoconstriction,
but umbilical blood flow does not
change. Under these circumstances the relative output
of the right ventricle, compared to that of the left,
may be expected to increase. However, if fetal hypoxemia
results from umbilical blood flow reduction
(partial cord compression), vascular resistance across
the umbilical-placental circulation is increased; and it
may be expected to be associated with a reduction of
right, relative to left, ventricular output.