Role of Doppler Sonography in Obstetrics






  • Outline



  • Physical Principles, 733



  • Fetal Arterial Doppler, 734




    • Umbilical Artery, 734



    • Middle Cerebral Artery, 735



    • Uterine Artery, 736




  • Fetal Venous Doppler, 738



  • Fetal Cardiac Doppler, 744




    • Optimizing the Image, 744




  • Fetal Doppler and Fetal Growth Restriction, 745




Summary of Key Points




  • •

    Absent or reverse flow during diastole in the umbilical artery represents an advanced stage of placental compromise and is associated with more than 70% of placental arterial obliteration.


  • •

    In the presence of fetal hypoxemia, central redistribution of blood flow occurs, resulting in an increased blood flow to the brain, heart, and adrenals and a reduction in flow to the peripheral and placental circulations, known as the brain-sparing effect.


  • •

    Middle cerebral artery (MCA) peak systolic velocities (PSVs) are increased in the presence of fetal anemia. The greater velocity results from decreased blood viscosity and increased cardiac output.


  • •

    Pulsed Doppler velocimetry of the uterine artery should be obtained immediately after the vessel crosses the hypogastric artery and before it divides into the uterine and cervical branches. Abnormal uterine artery circulation has been associated with fetal growth restriction (FGR), preeclampsia, preterm delivery, and nonreassuring fetal status in labor.


  • •

    Doppler flow studies of the inferior vena cava and ductus venosus in the fetus provide information with regard to right ventricular preload, myocardial compliance, and right ventricular end-diastolic pressure.


  • •

    Inferior vena cava Doppler waveforms are triphasic in shape, with the first phase corresponding to ventricular systole, the second phase to early diastole, and the third phase to late diastole or the atrial kick. Ductus venosus Doppler waveforms are biphasic in shape, with the first phase corresponding to ventricular systole, the second phase to early diastole, and the nadir of the second phase to late diastole or the atrial kick.


  • •

    Reversed flow in the ductus venosus during atrial systole may be due to worsening placental disease, impaired cardiac function secondary to metabolic compromise, congenital heart defects, redistribution of hepatoportal blood flow through the liver, or a combination of these factors.


  • •

    To obtain accurate Doppler indices in fetal echocardiography, the sample volume is placed distal to the respective valves, the insonating angle should be within 0 to 20 degrees of the direction of blood flow, Doppler waveforms should be obtained during fetal apnea, and multiple measurements should be made.


  • •

    Doppler waveforms across the atrioventricular valves are bicuspid in shape. The first peak (E wave) corresponds to early ventricular filling of diastole, the second peak (A wave) corresponds to atrial systole or the atrial kick, with the E/A ratio being an index of ventricular preload and compliance.


  • •

    Doppler waveforms across the semilunar valves are uniphasic in shape. PSV and the time to peak velocity (TPV) increase with advancing gestation. These Doppler indices reflect ventricular contractility, arterial pressure, and afterload.


  • •

    Umbilical artery Doppler surveillance should be initiated when a growth-restricted fetus is considered viable. Abnormal Doppler waveforms of the umbilical artery have been associated with adverse perinatal outcome.


  • •

    Obtaining umbilical artery Doppler screening routinely in low-risk pregnancies has not been shown to improve outcome.





Physical Principles


The Doppler effect, first reported by Christian Doppler in 1842, describes the apparent variation in frequency of a light or a sound wave as the source of the wave approaches or moves away relative to an observer. The traditional example that is given to describe this physical phenomenon is the apparent change in sound level of a train as it approaches and then departs a station. The sound seems higher in pitch as the train approaches the station and seems lower in pitch as the train departs the station. The actual sound of the train is constant, it is the apparent change in sound pitch to a stationary observer that describes the Doppler effect. This apparent change in sound pitch (Doppler effect), or what is also termed the frequency shift, is proportional to the speed of movement of the sound-emitting source. In clinical applications, when an ultrasound beam with a certain frequency is used to insonate a blood vessel, the reflected frequency or frequency shift is directly proportional to the speed with which the red blood cells are moving (blood flow velocity) within that particular vessel. This frequency shift of the returning signal is displayed in a graphic form as a time-dependent plot. In this display, the vertical axis represents the frequency shift, and the horizontal axis represents the temporal change of this frequency shift as it relates to the events of the cardiac cycle ( Fig. 22-1 ). This frequency shift is highest during systole, when the blood flow is at its fastest, and lowest during end diastole, when the blood flow is at its slowest in the peripheral circulation. Given that the velocity of flow in a particular vascular bed is inversely proportional to the downstream impedance to flow, the frequency shift therefore derives information on the downstream impedance to flow of the vascular bed under study. The frequency shift is also dependent on the cosine of the angle that the ultrasound beam makes with the direction of blood flow in the targeted blood vessel (see formula in Fig. 22-1 ). When the sound beam is parallel to the blood vessel of interest and thus the moving red blood cells, the entire Doppler shift is measured. However, when an angle exists between the direction of flow and the sound beam, the measured velocity is less than the true velocity. The degree to which the velocity is accurately measured depends on the angle of incidence (cosine Q). At an angle of 0 degrees, the measured velocity is equal to the true velocity. When the sound beam is perpendicular to the direction of flow (90 degrees), the measured velocity is zero, because the cosine of 90 degrees is zero. Thus, ideally, one wants to measure the velocity with as small an angle as possible.




FIG 22-1


The Doppler effect (f d ) is dependent on the initial frequency of the ultrasound transducer (f 0 ), the velocity of flow (V) of the blood within a vessel, and the cosine of the angle (A) that the ultrasound beam makes with the direction of blood flow. The frequency shift (f d ) is displayed as a time-dependent plot within a cardiac cycle and is inversely proportional to the constant (c), which reflects a constant related to the medium in which the sound is traversing.


Given that the insonating angle is difficult to measure in clinical practice, indices that rely on ratios of frequency shifts were developed to quantitate Doppler waveforms. By relying on ratios of frequency shifts, these Doppler indices are thus independent of the effects of the insonating angle of the ultrasound beam. Doppler indices that are commonly used in obstetric practice are shown in Figure 22-2 .




FIG 22-2


Doppler indices commonly used in obstetric imaging. c, constant related to the medium in which the sound is traversing; D, diastole; M, mean; PI, pulsatility index; RI, resistance index; S, systole.




Fetal Arterial Doppler


Umbilical Artery


The umbilical arterial circulation is normally a low-impedance circulation, with an increase in the amount of end-diastolic flow with advancing gestation. Umbilical arterial Doppler waveforms reflect the status of the placental circulation, and the increase in end-diastolic flow that is seen with advancing gestation is a direct result of an increase in the number of tertiary stem villi that takes place with placental maturation ( Fig. 22-3 ). Diseases that obliterate small muscular arteries in placental tertiary stem villi result in a progressive decrease in end-diastolic flow in the umbilical arterial Doppler waveforms until absent, and then reverse flow during diastole is noted ( Fig. 22-4 ). Reversed diastolic flow in the umbilical arterial circulation represents an advanced stage of placental compromise and is associated with more than 70% of placental arterial obliteration. The notation of absent or reversed end-diastolic flow in the umbilical artery is commonly associated with severe FGR and oligohydramnios. Only in pregnancies with suspected FGR does the use of umbilical artery Doppler sonography reduce the number of perinatal deaths and unnecessary obstetric interventions.




FIG 22-3


Doppler waveforms of the umbilical artery in a normal fetus in the third trimester of pregnancy. Note the increased end-diastolic velocity (D), consistent with a low impedance circulation. S, peak systole.



FIG 22-4


Abnormal umbilical artery Doppler waveforms; decreased end-diastolic velocity ( A ), absent end-diastolic velocity ( B ), reversed end-diastolic velocity ( C ).


Doppler waveforms of the umbilical arteries can be obtained from any segment along the umbilical cord. Waveforms obtained from the placental end of the cord show more end-diastolic flow and thus lower ratio values (resistive index [RI], systole/diastole [S/D] ratio) than waveforms obtained from the abdominal cord insertion. There are differences in Doppler indices of arterial waveforms obtained from different anatomic locations of the same umbilical cord, but they are generally minor and have no significance in clinical practice ( Tables 22-1 through 22-3 ). In 2013, the International Society for Ultrasound in Obstetrics and Gynecology (ISUOG) recommended taking Doppler measurements from a free loop of cord for the sake of simplicity and consistency.



TABLE 22-1

Reference Values for Serial Measurements of the Umbilical Artery Systolic-Diastolic Ratio














































































































































































































































































Gestation (weeks) SYSTOLIC-DIASTOLIC RATIO, BY PERCENTILE
2.5th 5th 10th 25th 50th 75th 90th 95th 97.5th
19 2.73 2.93 3.19 3.67 4.28 5.00 5.75 6.26 6.73
20 2.63 2.83 3.07 3.53 4.11 4.80 5.51 5.99 4.43
21 2.51 2.70 2.93 3.36 3.91 4.55 5.22 5.67 6.09
22 2.43 2.60 2.83 3.24 3.77 4.38 5.03 5.45 5.85
23 2.34 2.51 2.72 3.11 3.62 4.21 4.82 5.22 5.61
24 2.25 2.41 2.62 2.99 3.48 4.04 4.63 5.02 5.38
25 2.17 2.33 2.52 2.88 3.35 3.89 4.45 4.83 5.18
26 2.09 2.24 2.43 2.78 3.23 3.75 4.30 4.66 5.00
27 2.02 2.17 2.35 2.69 3.12 3.63 4.15 4.50 4.83
28 1.95 2.09 2.27 2.60 3.02 3.51 4.02 4.36 4.67
29 1.89 2.03 2.20 2.52 2.92 3.40 3.89 4.22 4.53
30 1.83 1.96 2.13 2.44 2.83 3.30 3.78 4.10 4.40
31 1.77 1.90 2.06 2.36 2.75 3.20 3.67 3.98 4.27
32 1.71 1.84 2.00 2.29 2.67 3.11 3.57 3.87 4.16
33 1.66 1.79 1.94 2.23 2.60 3.03 3.48 3.77 4.06
34 1.61 1.73 1.88 2.16 2.53 2.95 3.39 3.68 3.96
35 1.57 1.68 1.83 2.11 2.46 2.87 3.30 3.59 3.86
36 1.52 1.64 1.78 2.05 2.40 2.80 3.23 3.51 3.78
37 1.48 1.59 1.73 2.00 2.34 2.74 3.15 3.43 3.69
38 1.44 1.55 1.69 1.95 2.28 2.67 3.08 3.36 3.62
39 1.40 1.51 1.64 1.90 2.23 2.61 3.02 3.29 3.54
40 1.36 1.47 1.60 1.85 2.18 2.56 1.96 3.22 3.48
41 1.33 1.43 1.56 1.81 2.13 2.50 2.90 3.16 3.41

From Acharya G, Wilsgaard T, Bernsten GKR, et al: Reference ranges for serial measurements of umbilical artery Doppler indices in the second half of pregnancy. Am J Obstet Gynecol 192:937, 2005.


TABLE 22-2

Resistance Index of the Umbilical Artery Between 20 and 40 Weeks of Gestation




















































































































Gestation (weeks) Resistance Index, by Percentile
5th 50th 95th
20 0.567 0.690 0.802
21 0.557 0.680 0.793
22 0.548 0.671 0.784
23 0.539 0.663 0.776
24 0.530 0.655 0.768
25 0.522 0.646 0.760
26 0.514 0.639 0.752
27 0.506 0.631 0.745
28 0.498 0.623 0.737
29 0.490 0.615 0.730
30 0.482 0.608 0.723
31 0.474 0.600 0.715
32 0.465 0.592 0.707
33 0.457 0.584 0.700
34 0.449 0.576 0.692
35 0.440 0.567 0.684
36 0.431 0.559 0.675
37 0.422 0.550 0.667
38 0.412 0.540 0.657
39 0.402 0.530 0.648
40 0.390 0.519 0.637

From Merz E (ed): Ultrasonography in Obstetrics and Gynecology, vol 1. Stuttgart, Thieme, 2005, pp 469-480, 614.


TABLE 22-3

Pulsatility Index of the Umbilical Artery Between 20 and 40 Weeks of Gestation




















































































































Gestation (weeks) PULSATILITY INDEX, BY PERCENTILE
5th 50th 95th
20 0.940 1.216 1.505
21 0.913 1.189 1.476
22 0.890 1.165 1.450
23 0.869 1.142 1.427
24 0.849 1.122 1.405
25 0.831 1.102 1.385
26 0.813 1.084 1.365
27 0.798 1.065 1.346
28 0.780 1.048 1.327
29 0.764 1.031 1.308
30 0.748 1.014 1.290
31 0.732 0.997 1.272
32 0.716 0.980 1.254
33 0.700 0.963 1.236
34 0.684 0.946 1.218
35 0.668 0.928 1.199
36 0.651 0.910 1.180
37 0.634 0.891 1.160
38 0.615 0.872 1.139
39 0.595 0.851 1.117
40 0.573 0.828 1.093

From Merz E (ed): Ultrasonography in Obstetrics and Gynecology, vol 1. Stuttgart, Thieme, 2005, pp 469-480, 614.


Middle Cerebral Artery


The MCA is the most accessible cerebral vessel to ultrasound imaging in the fetus, and it carries more than 80% of cerebral blood flow. The cerebral circulation is normally a high-impedance circulation with continuous forward flow throughout the cardiac cycle. In the presence of fetal hypoxemia, central redistribution of blood flow occurs, resulting in increased blood flow to the brain ( Fig. 22-5 ), heart, and adrenals and a reduction in flow to the peripheral and placental circulations. This blood flow redistribution is known as the brain-sparing effect and plays a major role in fetal adaptation to oxygen deprivation.




FIG 22-5


Doppler waveforms of the middle cerebral artery in a normal fetus ( A ) and in a hypoxemic fetus ( B ). Note the increase in end-diastolic velocity in fetus B resulting from a low-impedance cerebral circulation as part of the brain-sparing reflex.


The right and left middle cerebral arteries represent major branches of the circle of Willis in the fetal brain. The circle of Willis, which is supplied by the internal carotid and vertebral arteries, can be imaged with color flow Doppler sonography in a magnified axial plane of the fetal head obtained at the base of the skull, at the level of the thalami and wings of sphenoid bone ( Fig. 22-6 ). In this plane, the proximal and distal middle cerebral arteries are seen in their long axis, with their course almost parallel to the ultrasound beam. As such, the insonating beam, which is parallel to the vessel and thus has an angle of insonation of 0 degrees, will result in a measured velocity that accurately reflects the true velocity of blood in this vessel (cos 0 = 1). MCA Doppler waveforms, obtained from the proximal third portion of the vessel, immediately after its origin from the circle of Willis, have shown the best reproducibility ( Tables 22-4 and 22-5 and Fig. 22-7 ).




FIG 22-6


Circle of Willis shown on color Doppler imaging. ACA, anterior cerebral artery; MCA, middle cerebral artery; PCA, posterior cerebral artery.

(From Ultrasound in Obstetrics & Gynecology: A Practical Approach. Available at www.openultrasound.com . Used with permission.)


TABLE 22-4

Reference Values for the Resistance Index of the Middle Cerebral Artery





































































































































Gestation (weeks) RESISTANCE INDEX, BY PERCENTILE Gestation (weeks) RESISTANCE INDEX, BY PERCENTILE
5th 50th 95th 5th 50th 95th
18 0.544 0.687 0.787 31 0.652 0.798 0.907
19 0.574 0.708 0.808 32 0.645 0.792 0.902
20 0.592 0.727 0.828 33 0.636 0.783 0.894
21 0.608 0.744 0.846 34 0.625 0.773 0.885
22 0.622 0.758 0.861 35 0.612 0.761 0.873
23 0.633 0.771 0.874 36 0.597 0.747 0.86
24 0.643 0.782 0.886 37 0.579 0.73 0.844
25 0.651 0.79 0.895 38 0.56 0.712 0.826
26 0.656 0.796 0.902 39 0.539 0.692 0.807
27 0.659 0.801 0.907 40 0.515 0.669 0.785
28 0.661 0.803 0.91 41 0.489 0.644 0.761
29 0.66 0.803 0.911 42 0.462 0.618 0.735
30 0.657 0.801 0.91

From Bahlmann F, Reinhard I, Krummenauer F, et al: Blood flow velocity waveforms of the fetal middle cerebral artery in a normal population: reference values from 18 weeks to 42 weeks of gestation. J Perinat Med 30:490, 2002.


TABLE 22-5

Reference Values for Umbilical Artery (UA) and Middle Cerebral Artery (MCA) Resistance Indices, as Well as Their Ratio, by Percentile




































































































































































































































Gestation (weeks) UA RESISTANCE INDEX MCA RESISTANCE INDEX RATIO
5th 50th 95th 5th 50th 95th 5th 50th 95th
24 0.615 0.717 0.828 0.778 0.867 – 0.696 0.809 0.968
25 0.605 0.707 0.819 0.789 0.881 – 0.676 0.791 0.955
26 0.594 0.697 0.810 0.795 0.892 – 0.658 0.775 0.945
27 0.583 0.687 0.802 0.798 0.898 – 0.642 0.761 0.937
28 0.572 0.678 0.793 0.797 0.901 – 0.628 0.750 0.932
29 0.562 0.668 0.785 0.793 0.900 – 0.616 0.740 0.929
30 0.551 0.658 0.776 0.786 0.897 – 0.606 0.732 0.928
31 0.540 0.648 0.767 0.776 0.891 – 0.597 0.726 0.929
32 0.530 0.638 0.759 0.764 0.883 – 0.590 0.722 0.931
33 0.519 0.629 0.750 0.750 0.872 – 0.585 0.719 0.936
34 0.508 0.619 0.742 0.734 0.860 – 0.581 0.717 0.941
35 0.498 0.609 0.733 0.717 0.846 – 0.578 0.717 0.949
36 0.487 0.599 0.724 0.698 0.831 – 0.576 0.718 0.957
37 0.476 0.589 0.716 0.677 0.814 – 0.575 0.720 0.967
38 0.465 0.580 0.707 0.655 0.795 – 0.576 0.724 0.978
39 0.455 0.570 0.699 0.632 0.776 – 0.577 0.728 0.991
40 0.444 0.560 0.690 0.607 0.755 – 0.580 0.734 1.004
41 0.433 0.550 0.681 0.582 0.734 – 0.583 0.740 1.018
42 0.423 0.540 0.673 0.556 0.711 – 0.588 0.747 1.034

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Feb 16, 2019 | Posted by in ULTRASONOGRAPHY | Comments Off on Role of Doppler Sonography in Obstetrics

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