Doppler in obstetrics

In recent years, Dopplerography has become one of the leading research methods in obstetrics. The essence of the Doppler effect is as follows. Ultrasonic vibrations generated by piezoelectric elements with a given frequency propagate in the object under study in the form of elastic waves. Upon reaching the boundary of two media with different acoustic resistances, part of the energy passes into the second medium, and part of it is reflected from the boundary between the media. In this case, the frequency of vibrations reflected from a stationary object does not change and is equal to the original frequency. If an object moves at a certain speed towards the source of ultrasonic pulses, its reflecting surface comes into contact with ultrasonic pulses more often than when the object is stationary. As a result, the frequency of reflected vibrations exceeds the original frequency. On the contrary, when reflecting surfaces move away from the radiation source, the frequency of reflected vibrations becomes less than the emitted pulses. The difference between the frequency of generated and reflected pulses is called the Doppler shift. The Doppler shift has positive values when an object moves towards the source of ultrasonic vibrations and negative values when moving away from it. The Doppler frequency shift is directly proportional to the speed of the reflecting surface and the cosine of the scanning angle. When the angle approaches 0°, the frequency shift reaches its maximum values, and when there is a right angle between the Doppler beam and the direction of the reflecting surface, the frequency shift is zero.

In medicine, the Doppler effect is mainly used to determine the speed of blood flow. In this case, the reflecting surface is mainly erythrocytes. However, the speed of erythrocytes in the blood flow is not the same. The parietal layers of blood move at a significantly lower speed than the central ones. The spread of blood flow velocities in a vessel is usually called the velocity profile. There are two types of blood flow velocity profiles: parabolic and cork-shaped. With a cork-shaped profile, the speed of blood movement in all sections of the vessel lumen is almost the same, the average blood flow velocity is equal to the maximum. This type of profile is displayed by a narrow frequency range on the Dopplerogram and is typical of the ascending aorta. The parabolic velocity profile is characterized by a large spread of velocities. In this case, the parietal layers of blood move much more slowly than the central ones, and the maximum speed is almost 2 times higher than the average, which is reflected on the Dopplerogram by a wide frequency range. This type of velocity profile is typical of the umbilical arteries.

Currently, a filter with a frequency of 100–150 Hz (recommended by the International Society for the Application of Doppler Ultrasound in Perinatology) is used to conduct research in obstetrics. The use of higher-frequency filters when studying the blood flow velocity in the umbilical arteries often leads to false-positive results in the diagnosis of critical fetal condition.

To obtain high-quality blood flow velocity curves, the scanning angle should not exceed 60°. The most stable results are achieved with a scanning angle of 30–45°.

The following indicators are currently mainly used to assess the state of blood flow:

• systolic-diastolic ratio (A/B) - the ratio of maximum systolic velocity (A) to end diastolic (B);
• resistance index - (A–B)/A;
• pulsation index - (A–B)/M, where M is the average blood flow velocity during the cardiac cycle.

It has been established that the most valuable information about the state of the fetoplacental complex can be obtained by simultaneously examining the blood flow in both uterine arteries, the umbilical arteries, the internal carotid arteries, or the main arteries of the brain.

There are several classifications of uteroplacental and fetoplacental blood flow disorders. In our country, the following is the most widely used:

1. 1st degree.
   • A - violation of uteroplacental blood flow with preserved fetoplacental blood flow;
   • B - violation of fetoplacental blood flow with preserved uteroplacental blood flow.
2. II degree. Simultaneous disruption of uteroplacental and fetoplacental blood flow, not reaching critical values (end diastolic blood flow is preserved).
3. III degree. Critical disturbance of fetoplacental blood flow (zero or negative diastolic blood flow) with preserved or impaired uteroplacental blood flow. An important diagnostic sign is the appearance of a diastolic notch on the blood flow velocity curves in the uterine artery, which occurs at the beginning of diastole. Only such a change in blood flow should be considered a pathological diastolic notch when its peak reaches or is below the level of the final diastolic velocity. In the presence of these changes, it is often necessary to resort to early delivery.

A decrease in diastolic blood flow in the uterine arteries indicates a violation of uteroplacental circulation, while a violation of fetoplacental circulation is indicated by a decrease in diastolic blood flow in the umbilical arteries, with its zero or negative value.

From a physiological point of view, the determination of zero diastolic blood flow in the umbilical arteries means that the blood flow in the fetus in these cases is suspended or has a very low velocity in the diastolic phase. The presence of negative (reverse) blood flow indicates that its movement is carried out in the opposite direction, i.e. towards the fetal heart. Initially, the absence of the terminal diastolic component of blood flow in individual cycles has a short duration. As the pathological process progresses, these changes begin to be recorded in all cardiac cycles with a simultaneous increase in their duration. Subsequently, this leads to the absence of a positive diastolic component of blood flow for half of the cardiac cycle. The terminal changes are characterized by the appearance of reversal diastolic blood flow. In this case, reversal diastolic blood flow is initially noted in individual cardiac cycles and has a short duration. Then it is observed in all cycles, it occupies most of the duration of the diastolic phase. Usually, no more than 48–72 hours pass before intrauterine fetal death from the moment of registration of constant reversal blood flow in the umbilical artery at the end of the second and third trimesters of pregnancy.

Clinical observations indicate that in more than 90% of cases, the absence of end-diastolic blood flow velocity in the umbilical artery is combined with fetal malnutrition.

There are reports that if, in the absence of fetal hypotrophy, zero or negative blood flow persists for 4 weeks or more, then in a significant number of observations this may indicate chromosomal pathology and developmental anomalies, most often trisomy 18 and 21.

Certain additional information can be provided by studying cerebral blood flow. Pathological curves of blood flow velocity in the cerebral vessels of the fetus (in the middle cerebral artery), unlike the aorta and umbilical artery, are characterized not by a decrease, but by an increase in diastolic blood flow velocity. Therefore, when the fetus suffers, a decrease in the vascular resistance index is noted.

An increase in cerebral blood flow indicates compensatory centralization of fetal circulation during intrauterine hypoxia and consists of a redistribution of blood with preferential blood supply to such vital organs as the brain, myocardium, and adrenal glands.

Subsequently, during dynamic observation, “normalization” of blood circulation may be noted (reduction of diastolic blood flow on the Dopplerogram). However, such “normalization” is in fact pseudo-normalization and is a consequence of decompensation of cerebral circulation.

It was noted that increased cerebral blood flow is characteristic only of asymmetric fetal hypotrophy, while this is not observed in the symmetric form.

It was established that the resistance index in determining the uteroplacental blood flow in healthy fetuses in the third trimester of pregnancy is on average 0.48±0.05; with initial disturbances - 0.53±0.04; with pronounced ones - 0.66±0.05; with sharply pronounced ones - 0.75±0.04. In the study of fetoplacental blood flow, the resistance index was on average 0.57±0.06, 0.62±0.04, 0.73±0.05, 0.87±0.05, respectively.

In general, when using Doppler ultrasound, the accuracy of diagnosing a healthy fetus or a disorder of its condition is on average 73%. A fairly clear correlation has been noted between changes in Doppler ultrasound parameters and fetal hypotrophy. Thus, with a disorder of fetoplacental blood flow, fetal hypotrophy can be established in 78% of cases. With a decrease in uteroplacental blood flow, on the one hand, hypotrophy develops in 67%, and with a bilateral decrease in blood flow - in 97%. With a simultaneous decrease in uteroplacental and fetoplacental blood flow, hypotrophy also occurs in almost all cases.

Color Doppler sonography can provide valuable information in diagnosing umbilical cord entanglement around the fetus's neck. Umbilical cord entanglement is the most common complication that obstetricians encounter (it occurs in approximately every fourth woman in labor). Acute fetal hypoxia due to umbilical cord pathology occurs 4 times more often than during normal labor. Therefore, diagnosing umbilical cord entanglement around the fetus's neck is of great practical importance. Color Doppler sonography is used to detect umbilical cord entanglement. Initially, the sensor is placed along the fetus's neck. In the case of a single entanglement, three vessels (two arteries and one vein) can usually be detected in this scanning plane. In this case, due to the different directions of blood flow, arteries and veins are depicted in blue or red and vice versa. Using this scanning method in most cases also allows one to determine the number of entanglements. Transverse scanning of the fetus's neck should also be used to confirm the diagnosis. In the plane of this scan, the umbilical cord vessels will be depicted as linear tubular structures in red and blue. However, the disadvantage of this scanning method is that it is impossible to determine the number of entanglements.

It should be noted that in some cases certain difficulties may arise in differentiating between double entanglement and the location of the umbilical cord loop in the fetal neck area. It should be borne in mind that if, with umbilical cord entanglement, two vessels of one color and four of another are determined on scanograms, then in the presence of a loop, three vessels will be depicted in one color and three in another.

The accuracy of correct diagnosis of the presence or absence of umbilical cord entanglement around the fetus's neck 2 days before delivery was 96%. A week before delivery (6–7th day), the accuracy of correct diagnosis decreased to 81%. The latter circumstance is explained by the fact that during pregnancy, both the appearance and disappearance of umbilical cord entanglement can occur due to rotational movements of the fetus.

In conclusion, it should be noted that Dopplerography is a valuable method, the use of which allows obtaining important information about the condition of the fetus, as well as diagnosing umbilical cord entanglement and, based on the data obtained, outlining the most rational tactics for managing pregnancy and childbirth.

Recommended reading

Prenatal diagnostics of congenital malformations of the fetus / Romero R., Pilu D., Genty F. et al. - M.: Medicine, 1994.

Clinical guidelines for ultrasound diagnostics / edited by V.V. Mitkov, M.V. Medvedev. - M.: Vidar, 1996.

Congenital malformations. Prenatal diagnostics and tactics / edited by B.M. Petrikovsky, M.V. Medvedev, E.V. Yudina. - M.: Realnoe Vremya, 1999.

Ultrasound fetometry: reference tables and standards / edited by M.V. Medvedev. - M.: Realnoe Vremya, 2003.

Clinical visual diagnostics / edited by V.N. Demidov, E.P. Zatikyan. - Issues I–V. - Moscow: Triada-X, 2000–2004

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