The doppler beam should be
positioned close to the junction between the ductus arteriosus and
descending aorta.
The angle between the doppler
beam and direction of blood flow must be £20°.
All measurements appear to be
heart rate independent (1)
Ductal waveforms show a
marked change in shape between early and mid-pregnancy, which appears to
correspond to the change from a high to a low fetoplacental vascular
resistance.
PULSATILITY INDEX (PI) (2)
End-diastolic flow velocities
first appear at 13-14 gestational weeks (coinciding with the appearance of
end-diastolic velocities in the umbilical artery and descending aorta).
End-diastolic velocity is present in 50% of fetuses at 15 weeks and in all
cases after 17 weeks.
The PI varies between 2 and 3
and does not change significantly with gestational age. Unlike the
umbilical arteries in which a progressive decrease in PI occurs with advancing
gestation due to a decrease in placental vascular resistance, a change in
ductal pulsatility index represents a pathophysiological phenomenon as
ductal waveforms do not directly reflect vascular resistance.
PEAK VELOCITY
Increases from about 30cm/sec
at 12 weeks to over 100cm/sec at term (2). The upper limit of normal
suggested by Huhta (3) may be an underestimation as higher values are
described in another series (1) in which peak velocities of 200cm/sec were
occasionally observed in the last few weeks of gestation.
Mean velocity of 44 cm/sec
prior to 18 weeks and 70.8 cm/sec after 18 weeks gestation (2).
Diastole is absent in the
ductus until 13 weeks gestation (2).
REFERENCES
van der Mooren K, Barendregt
LG, Wladimiroff JW. Flow velocity waveforms in the human fetal ductus
arteriosus during the normal second half of pregnancy. Pediatr Res
1991;30:487-490.
Brezinka C, Huisman TWH,
Stijnen T et.al. Normal doppler flow velocity waveforms in the fetal
ductus arteriosus in the first half of pregnancy. Ultrasound Obstet
Gynecol 1992;2:397-401.
Huhta JC, Moise KJ, Fisher
DJ et.al. Detection and quantitation of constriction of the fetal ductus
arteriosus by doppler echocardiography. Circulation 1987;75:406-412.