Doppler ultrasound in obstetrics

In recent years, dopplerography has become one of the leading methods of research in obstetrics. The essence of the Doppler effect is as follows. Ultrasonic vibrations generated by piezoelements 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 media interface. In this case, the frequency of oscillations reflected from a stationary object does not change and is equal to the original frequency. If the object moves at a certain speed toward the source of the ultrasonic pulses, then its reflecting surface is in contact with ultrasonic pulses more often than when the object is stationary. As a result, the frequency of the reflected oscillations exceeds the original frequency. On the contrary, when reflective surfaces move from the radiation source, the frequency of the reflected oscillations becomes less than the emitted pulses. The difference between the frequency of the generated and reflected pulses is called the Doppler shift. The Doppler shift has positive values when the object moves toward the source of ultrasonic vibrations and negative ones - when moving from it. The Doppler frequency shift is directly proportional to the velocity of the reflecting surface and the cosine of the scanning angle. At a magnitude approaching 0 °, the frequency shift reaches its maximum values, and in the presence of a right angle between the Doppler beam and the direction of motion of the reflecting surface, the frequency shift is zero.

In medicine, the Doppler effect is mainly used to determine the rate of movement of blood. The reflecting surface in this case is mainly erythrocytes. However, the speed of movement of erythrocytes in the flow of blood is not the same. The near-wall layers of blood move at a much slower rate than the central ones. The spread of blood flow velocities in the vessel is usually called the velocity profile. There are two types of velocity profile of the blood flow: parabolic and corky. With a cork profile, the blood flow velocity in all parts of the lumen of the vessel is almost the same, the average blood flow velocity is equal to the maximum. This type of profile is displayed by a narrow spectrum of frequencies on the dopplerogram and is characteristic for the ascending aorta. The parabolic velocity profile is characterized by a large spread of velocities. At the same time, the blood layers of the blood move much more slowly than the central layers, and the maximum velocity is almost 2 times higher than the mean, which is reflected in the Dopplergram by a wide spectrum of frequencies. This type of velocity profile is characteristic of the umbilical arteries.

At present, a filter with a frequency of 100-150 Hz (recommended by the International Society for the Application of Dopplerography in Perinatology) is used to conduct research in obstetrics. The use of higher-frequency filters in the study of blood flow velocity in the umbilical arteries often leads to false-positive results in the diagnosis of the critical condition of the fetus.

To obtain qualitative curves of blood flow velocities, one should strive to ensure that the scanning angle does not exceed 60 °. The most stable results are achieved with a scanning angle of 30-45 °.

To assess the status of the blood flow, currently the following indicators are mainly used:

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

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

There are several classifications of abnormalities of uteroplacental and placental-placental blood flow. The most widespread in our country is the following:

1. I degree.
   • A - a violation of the uteroplacental blood flow with the preserved fetal-placental blood flow;
   • B - violation of the placenta blood flow with the utero-placental blood flow preserved.
2. II degree. Simultaneous disturbance of utero-placental and fetoplacental blood flow, not reaching critical values (the final diastolic blood flow is preserved).
3. III degree. Critical impairment of the fetoplacental blood flow (zero or negative diastolic blood flow) with stored or disturbed uteroplacental blood flow. An important diagnostic feature is the appearance of a diastolic dredge on the curves of blood flow velocities in the uterine artery that occurs at the beginning of the diastole. For a pathological diastolic depression, only such a change in blood flow should be taken when its apex 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.

The violation of uteroplacental circulation is indicated by a decrease in diastolic blood flow in the uterine arteries, a violation of the fetoplacental - a decrease in the diastolic blood flow in the arteries of the umbilical cord, zero or its negative values.

From the physiological point of view, the determination of zero diastolic blood flow in the umbilical arteries means that the fetal blood flow in these cases is suspended or has a very low rate in the diastole phase. The presence of negative (reversible) blood flow indicates that its movement is carried out in the opposite direction, i.e. To the heart of the fetus. At first, the absence of a terminal diastolic component of the 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 the blood flow during the half of the cardiac cycle. The appearance of reversion diastolic blood flow is characteristic for terminal changes. In this case, reversion diastolic blood flow is initially noted in individual cardiac cycles and has a short duration. Then it is observed in all cycles, it takes up most of the duration of the diastolic phase. Usually passes no more than 48-72 h before fetal death of the fetus from the moment of registration of a constant reversion blood flow in the cord artery at the end of II and in the III trimesters of pregnancy.

Clinical observations indicate that in more than 90% of cases the absence of a terminal diastolic blood flow velocity in the cord artery is combined with fetal hypotrophy.

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

Certain additional information can be given by the study of cerebral blood flow. Pathological curves of blood flow velocities in the cerebral vessels of the fetus (in the middle cerebral artery), in contrast to the aorta and the umbilical artery, are characterized not by a decrease, but by an increase in the diastolic blood flow velocity. Therefore, when the fetus suffers, the index of vascular resistance decreases.

An increase in cerebral blood flow testifies to compensatory centralization of the fetal blood circulation in intrauterine hypoxia and consists in the redistribution of blood with the predominant blood supply of such vital organs as the brain, myocardium, adrenals.

In the future, with dynamic observation, the "normalization" of the circulation can be noted (a decrease in the diastolic blood flow on the dopplerogram). However, such a "normalization" is in fact a pseudo-normalization and is a consequence of decompensation of the cerebral circulation.

It is noted that the increase in cerebral blood flow is characteristic only for asymmetric fetal hypotrophy, while in a symmetrical form this is not observed.

It was established that the index of resistance in determining uteroplacental blood flow in healthy fetuses in the third trimester of pregnancy is on the average 0.48 ± 0.05; with its initial violations - 0.53 ± 0.04; at expressed - 0,66 ± 0,05; at sharply expressed - 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 Doppler is used, the accuracy of diagnosing a healthy fetus or disturbing its condition is on average 73%. There is a fairly clear correlation between changes in dopplerography and fetal malnutrition. Thus, in case of violation of fetoplacental blood flow fetal hypotrophy can be established in 78% of cases. With a decrease in uteroplacental blood flow, on the one hand, the development of hypotrophy is 67%, while in the bilateral reduction of blood flow - in 97%. With simultaneous decrease in uteroplacental and fetoplacental blood flow, hypotrophy also appears in almost all observations.

Valuable information, color dopplerography can give a diagnosis of umbilical cord entanglement around the neck of the fetus. Umbilical cord engagement is the most common complication with which midwives have to be met (it is observed in approximately 4 of each birth). Acute fetal hypoxia in umbilical cord pathology occurs 4 times more often than with normal birth. Therefore, the diagnosis of umbilical cord circumscription around the neck of the fetus is of great practical importance. For the detection of umbilical cords, color dopplerography is used. Initially, the sensor is placed along the neck of the fetus. In the case of a single entrapment, three vessels (two arteries and one vein) are usually detected in this scanning plane. In this connection, due to the different directionality of the blood flow, arteries and veins are depicted in blue or red and vice versa. The use of this method of scanning in most cases makes it possible to establish also the multiplicity of the entanglement. To confirm the diagnosis, a transverse scan of the fetal neck should also be used. In the plane of this scan, the umbilical vessels will be depicted as linear tubular structures of red and blue. However, the disadvantage of this method of scanning lies in the impossibility of determining the multiplicity of the entanglement.

It should be noted that in some cases, there may be certain difficulties in the differentiation of double entrapment and the location of the umbilical cord in the neck of the fetus. In this case, it must be borne in mind that if, with the umbilical cords on the scans, two vessels of one and four are identified - another color, then in the presence of a loop the three vessels will be represented by one and three - by another color.

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

In conclusion, it should be noted that Doppler is a valuable method, the use of which allows you to obtain important information about the fetal condition, as well as to diagnose the cord with the umbilical cord and, based on the findings, to outline the most rational tactics for managing pregnancy and childbirth.

Recommended literature

Prenatal diagnosis of congenital malformations of the fetus / Romero R., Pilu D., Genti F. Et al. - M.: Medicine, 1994.

Clinical Manual for Ultrasound Diagnostics / Ed. V.V. Mitkova, M.V. Medvedev. - M .: Vidar, 1996.

Congenital malformations. Prenatal diagnosis and tactics / Ed. B.M. Petrikovsky, M.V. Medvedeva, E.V. Yudina. - M.: Real time, 1999.

Ultrasonic Fetometry: Ref. Tables and standards / under. Ed. M.V. Medvedev. - M.: Real time, 2003.

Clinical visual diagnostics / ed. V.N. Demidova, E.P. Zatikyan. - Вып. I-V. - M.: Triad-X, 2000-2004

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