o_1977r8vv9vk1ts2ms0kd8pksa.pdf

138 D. Maulik
the end-diastolic velocity continues to increase
throughout the pregnancy, which is expressed by the
umbilical arterial Doppler indices (Fig. 10.5). The indices
that reflect the Doppler wave pulsatility, such as
the RI, S/D ratio, and PI, continue to decrease,
whereas the D/A ratio, which is the normalized enddiastolic
frequency shift, continues to increase. In a
prospective study involving 308 normal pregnant
mothers, Maulik and associates [2] quantified the
contribution of the duration of pregnancy to the total
variance of the umbilical arterial Doppler indices. It
was observed that the gestational age was the single
major contributor to the total variance of the indices,
ranging from 33% for the PI to 46% for the S/D ratio.
These trends in the Doppler waveform are consistent
with the crucial physiological changes that occur
in the fetoplacental circulation characterized by the
gestational age-dependent decline in the fetoplacental
flow impedance [24]. Anderson and Faber [25] estimated
that the fetoplacental circulatory resistance in
the ovine fetus decreased by 2.8% per day, and during
the last third of pregnancy the decrease in resistance
was calculated to be tenfold.
The controlling mechanism for this phenomenon
remains uncertain. It has been shown in human placental
studies that there is continuing expansion of
the fetoplacental vascular system throughout the pregnancy
[26]. Furthermore, the villous vascular system
undergoes a transformation, resulting in the appearance
of sinusoidal dilation in the terminal villous
capillaries as pregnancy approaches term, and more
than 50% of the stromal volume may be vascularized.
The progressive decrease in fetoplacental flow impedance
is associated with a concomitant decline in the
flow wave reflection from the downstream vascular
bed, which results in an increase in the end-diastolic
velocity. The latter ensures continuing forward flow
and perfusion of the fetal placenta during the entire
cardiac cycle.
Fetal Heart Rate
Fig. 10.4 a±g. Gestational age effect on the umbilical arterial
Doppler frequency shift waveforms. Panels are organized
from top to the bottom according to the advancing
gestation. a Waveforms at 16 weeks. b At 20 weeks. c At
24 weeks. d At 28 weeks. e At 32 weeks. f At 36 weeks.
g At 40 weeks. Note the progressive increase in enddiastolic
velocity and the concomitant fall in pulsatility as
the gestation advances
Heart rate influences the configuration of the arterial
Doppler waveform (see Chap. 4). Thompson and colleagues
[27], did not observe any significant effect of
fetal heart rate on the umbilical arterial S/D ratio.
The correlation between the fetal cardiac cycle, which
is the reciprocal of the heart rate, and the S/D ratio
was poor (r=0.1). Other Doppler indices (PI and RI)
demonstrated a similar relation with the heart rate.
This finding was contradicted, however, by Mires and
associates [28], who specifically investigated the relation
between fetal heart rate and the umbilical arterial
Doppler indices in 85 normal pregnancies: 25 of