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a Chapter 35 Doppler Echocardiographic Assessment of Fetal Cardiac Failure 521
vessels, have also been studied. Alterations in the velocity
indices of these vessels, especially in the carotid
and cerebral vessels, have been reported to be a
sensitive indication of fetal well-being. Velocity measurements
at the cardiac level are all absolute values.
Measurements of absolute flow velocities require
knowledge of the angle of insonation and vessel diameter,
each of which maybe difficult to obtain with
accuracy. The following formula can be used to calculate
volume flow per minute:
Volume flow… ml=min†
ˆ p=4 D 2 1=cos0 FVI HR
where D is the diameter (in centimeters) of the vessel
studied, 0 is the angle between the ultrasound beam
and the vessel, FVI is the flow velocity integral (area
under the velocity waveform), and HR is the heart rate
in beats per minute [12]. The error in the estimation of
the absolute velocity depends on the magnitude of the
angle itself and the diameter of the vessel being interrogated.
For angles