Correspondence to: Dr W. P. Martins, Departamento de Ginecologia e Obstetrícia da Faculdade de Medicina de Ribeirão Preto, Universidade de São Paulo. Av. Bandeirantes, 3900, 8 andar. Ribeirão Preto, São Paulo, Brazil (e-mail: email@example.com)
Practical advice on imaging-based techniques and investigations with accompanying slides online
The ductus venosus (DV) is a shunt between the intra-abdominal umbilical vein and inferior vena cava (IVC) that directs well-oxygenated blood preferentially through the foramen ovale into the left heart, thus feeding the coronary and cerebral circulation. The shunt is thin, slightly funnel-shaped, straight or with variable curvature, and ascends towards the IVC at an angle of roughly 50°. The isthmus at its inlet has a regulatory function, but is also responsible for the characteristic high blood velocity and is the standard site of recording. The high velocity ensures high kinetic energy needed for preferential streaming, but also reflects the pressure gradient that drives venous liver perfusion. The pressure gradient between IVC and umbilical vein varies with the phase of the heart cycle, resulting in changes in DV blood velocity. The blood flow velocity waveform, particularly the deflection during atrial contraction (a-wave), is commonly used in hemodynamic evaluation of the fetus, for example in fetal growth restriction. Doppler recording requires some training and patience to reach a reliable level of skill.
Only recordings during fetal quiescence are valid for comparison with currently available reference ranges.
When the fetus is lying on its back, mid-sagittal insonation through the fetal abdomen permits visualization of the DV connecting the umbilical vein to the IVC. Color Doppler confirms correct identification. Alternatively, an oblique transverse plane of the abdomen can be used to visualize the DV. The probe should be in line with the direction of blood flow through the DV, regardless of fetal lie (Figure 1). The DV should not be assessed by scanning through the fetal side, because it will be impossible to obtain an appropriate angle of insonation.
Color Doppler settings should be adjusted to identify the high velocity of the DV with aliasing: pulse repetition frequency (PRF) = 2–3 KHz; or velocity limit = 30–40 cm/s.
The area of interest should be magnified.
Pulsed-wave Doppler settings should be adjusted to encompass the DV waveform unbroken, without aliasing. For this purpose, the observer should use a high PRF (5–7 KHz; or velocity limit of 80–100 cm/s), adjusting the baseline as necessary. The sweep speed should be adjusted to 2–3 s or 5–7 waves, which provide excellent assessment conditions. The observer should also allocate a reasonable space for the Doppler display on the screen.
The sample volume should be positioned over the isthmus and adjacent proximal section of the DV, typically using 2–5 mm. A wide sample volume ensures recording of the highest velocity during the heart cycle but increases the risk of interference from the umbilical and hepatic veins and IVC. If necessary, the sample volume can be reduced to minimize interference, but it should be borne in mind that a small sample volume may prevent the highest velocities from being recorded.
The high-pass filter should be adjusted to the lowest possible setting while avoiding wall motion artifacts along the zero-line and without obscuring the a-wave: for some ultrasound machines this corresponds to 50–100 Hz.
Having recorded waves of even appearance, the observer should check that the a-wave is clearly represented and that the auto-tracing precisely represents velocities during the entire cycle, including when the a-wave is reversed. Manual tracing may be used to calculate the pulsatility index for veins[3, 4] and to document absent or reversed a-wave velocity.
The optimal waveform tracing, i.e. one that clealy represents the flow in the vessel and is easy to measure, should be selected for clinical use. Angle correction is usually not needed. If absolute velocities are being measured, a better option is to repeat the insonation with a more accurate alignment, ideally with a 0° angle of insonation, selecting the wave that contains the highest velocities.