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a Chapter 32 Introduction to Fetal Doppler Echocardiography 481
Table 32.5. Measures of fetal cardiac output
Stroke volume (ml) =temporal mean velocity
(cm/s) ´ cross-sectional area (cm 2 )
Cardiac or ventricular output (ml/min) = stroke volume
(ml) ´ fetal heart rate (bpm)
Left cardiac output = cardiac output measured at the
aortic or mitral orifice
Right cardiac output = cardiac output measured at the
pulmonic or tricuspid orifice
Combined cardiac output (ml/min) = sum of the right
and left cardiac outputs
Table 32.5. In the fetus the right and the left sides of
the heart work in parallel. Therefore fetal cardiac output
must be determined separately for each side. The
measurements may be performed either at the root of
the great vessels [5, 29, 35, 36] or across the atrioventricular
orifices [31, 38]. The right heart output may
be measured either across the tricuspid orifice or at
the root of the main pulmonary artery just distal to
the pulmonary semilunar valves (Fig. 32.31). Similarly,
the left cardiac output may be measured across
the mitral orifice or at the root of the aorta immediately
distal to the aortic semilunar valves. The precision
of the technique in terms of reproducibility depends
on the site of measurement, the parameters
measured, the imagability of the patient in a given instance,
and operator skill. In our experience, the
roots of the great vessels provide a more reliable
measure than the atrioventricular channels. Groenenberg
et al. [38] investigated this question and noted
high reproducibility of peak systolic velocity and
average velocity measured in the ductus arteriosus,
pulmonary artery, and ascending aorta. The coefficients
of variation between tests within individual patients
were less than or equal to 7%.
Fig. 32.31. Doppler characterization of pulmonary artery
(PA) flow and measurement of right ventricular stroke volume
in a normal fetus. Top: Doppler sample volume, indicated
by a short transverse bar (oblique arrow), was placed
along an M-line cursor in the midlumen of the PA imaged
by two-dimensional echocardiography. Maximal inner PA
diameter (d) (horizontal arrows) at the level of the Doppler
sample volume measured 0.9 cm. The cross-sectional area
at this level was calculated as 0.636 cm 2 (3.14 d 2 /4). PV pulmonary
valve. Bottom: Doppler frequency shifts (D) obtained
from the PA. Deflection above the baseline (B) represents
flow toward the transducer and that below the
baseline denotes flow away from the transducer. As would
be expected, the predominant flow is directed away from
the transducer toward the distal PA and is characterized by
sharp peaks with rapidly accelerating and decelerating
slopes. The maximal area under the flow curve for one
cardiac cycle was measured in kilohertz-seconds (vertical
distance between the two horizontal arrows=0.5 kHz) and
was converted to the maximum velocity curve area (s-cm/
s) (A) using the instrument calibration factor K =25.667 cm/
s-kHz. Multiplying A by the PA cross-sectional area gave a
stroke volume of 3.76 ml. Cardiac output was then calculated
by multiplying the stroke volume by the fetal heart
rate. The vertical lines represent 1-s time markers. M M-
mode tracing, E maternal electrocardiogram
Cardiac Output: Normative Data
Maulik et al. [5], in a preliminary report, noted a
right ventricular output range of 148 ml/min in a
28-week fetus to 451 ml/min in a near-term fetus.
Reed et al. [37] observed a tricuspid flow rate of
307 Ô 30 ml ´ kg ±1´min ±1 and a mitral flow rate of
232 Ô 25 ml ´ kg ±1´min ±1 in fetuses of 26±30 weeks gestational
age. In a comprehensive longitudinal study,
De Smedt and coworkers [39] performed echocardiographic
examinations on 28 normal fetuses at 4-week
intervals from 15±18 weeks to term. The fetal cardiac
output was measured from the tricuspid and mitral
orifices. They observed that with advancing gestation
the mean temporal blood flow velocities increased
linearly, whereas the area of blood flow and calculated
right and left ventricular output increased exponentially.
The combined ventricular output at term
was noted to be 1,735 ml/min and 553Ô153 ml´kg ±1
´ min ±1 when corrected for sonographically estimated
fetal weight.