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a Chapter 31 Doppler Examination of the Fetal Pulmonary Venous Circulation 453
Pulsed Doppler Ultrasound
of Lung Veins
Fig. 31.5. Left pulmonary vein is visualized from a right
lateral view of the heart. Here also the insonation angle is
ideal for a color setting and flow is visualized in red
color Doppler confirmed the correct connection of
the veins in a heart defect [3] as explained later in
this chapter. In these cases we recommend a combination
of color Doppler with pulsed Doppler of the
pulmonary vessels, since flow velocity waveforms are
often abnormal in TAPVC.
Physiologic Conditions
Conversely to postnatal life, where whole blood
passes through the pulmonary vessels to be oxygenated,
the pulmonary circulation in the fetus is only
a small part of cardiac output. Whereas animal experiments
in fetal sheep found that pulmonary flow
is around 8% of combined cardiac output [4], Doppler
studies on human fetuses in vivo suggested that
this flow is larger, being 13% at 20 gestational weeks
and increasing to 20%±25% during the last trimester
[5].
Doppler studies of volume flow were performed
mainly on the arterial side where vessel diameter is
easily measurable, whereas Doppler examination of
the pulmonary venous system focused mainly on the
study of velocity waveform. Pulmonary venous flow
patterns in the adult were demonstrated to be influenced
by dynamic changes in left atrial pressure created
by contraction and relaxation of the atrium and
ventricle [6]. Systolic peak (S) is caused by left atrial
pressure reduction that results from the relaxation of
the left atrium and the downward move of the mitral
valve in systole. Diastolic peak (D) is caused by rapid
emptying of the left atrium during left ventricular relaxation.
During atrial contraction (A) there is a rise
in left atrial pressure which causes in the adult a reversed
flow into the pulmonary vein, i.e., a negative
A-wave [7] which is not present [8, 9] (or occasionally
present [10, 11]) in the fetus.
Fig. 31.6. Left: Power Doppler ultrasound
in a lateral four-chamber view.
With this technique pulmonary veins
and the atria and ventricles are visualized
at the same time. Right: Dynamic
flow is very sensitive and arterial and
venous flow are visualized