Cardiotocography in labor

It has been established that fetal cardiac activity in the first period of labor in the absence of hypoxia is not subject to significant changes and the heart rate is on average 120-160 beats/min. According to the authors, rupture of the amniotic sac does not affect it either.

More dangerous situations may arise in the second period of labor. G. M. Savelyeva et al. (1978) believe that during cardiac monitoring, the criteria for initial and pronounced signs of fetal hypoxia are different in the first and second periods of labor. In the first period, the authors consider bradycardia up to 100 beats/min and tachycardia no more than 180 beats/min, as well as periodically occurring monotony of the rhythm and short-term late slowing of the heart rate to be initial signs of hypoxia. In the second period of labor, the initial signs of fetal hypoxia are bradycardia (90-110 beats/min), arrhythmia, late and Y-shaped slowing of the heart rate outside of contractions.

During labor, three parameters should be systematically taken into account when analyzing the cardiotocogram (CTG): the level of the basal frequency of the fetus's heartbeat, the variability of the basal line, and deviations associated with uterine contractions. Decelerations are the most important parameter of the fetus's condition. They are defined as a decrease in the basal frequency of the cardiotocogram, are associated with uterine contractions, and should be differentiated from bradycardia, which is simply manifested as a decrease in the basal level of the cardiotocogram without uterine contractions. When assessing the condition of the fetus, it is extremely important to determine the time relationships between uterine contractions and decelerations.

Currently, three classifications of deceleration are most widely used in scientific and practical activities of obstetricians throughout the world:

• Caldeyro-Barcia classification (1965);
• Hone's classification (1967);
• Sureau classification (1970).

Caldeyro-Barcia classification. When chronologically comparing the time phases of uterine contraction with the beginning, duration and end of fetal deceleration, three most typical curve variants were identified. There are two types of deceleration: deep I and deep II. According to the Caldeyro-Barcia classification, deceleration is the time relationship between the lower point of deceleration and the apex of the corresponding uterine contraction.

With the first type, shortly after the onset of the contraction, a slowdown in the fetal heartbeat is observed, which quickly passes, and with the cessation of the contraction, the fetal heartbeat returns to normal (dip I). Decelerations of this type usually last no more than 90 seconds and the heart rate is not less than 100 beats per minute.

In the second type, deceleration in the fetus begins 30-50 seconds after the peak of the contraction and lasts for some time after the contraction has ended (dip II). In this case, the fetal heart rate is rarely less than 120 beats/min. Very rarely, deceleration can be deeper - up to 60 beats/min or less. The duration of such deceleration usually also does not exceed 90 seconds. In such cases, so-called compensatory tachycardia is possible after the contraction has ended. This type of deceleration is often combined with acidosis in the fetus.

Hone's classification. This classification takes into account two main criteria - the relationship between the time of the onset of contraction and the onset of deceleration and its form. Hone identifies three types of deceleration:

• early decelerations begin with uterine contractions and have a regular shape. These decelerations are currently considered physiological due to compression of the fetal head;
• late decelerations begin 30-50 seconds after the onset of uterine contraction and also have a regular shape. They are caused by fetal hypoxia;
• Variable decelerations are characterized by different times of occurrence in relation to the onset of uterine contractions and are a combination of the first two types of decelerations. They are variable in form and in the relationship of one deceleration to another. In addition, they are different in relation to uterine contractions. The occurrence of such decelerations is associated with compression of the umbilical cord. If the compression of the umbilical cord does not last long, it does not have a damaging effect on the fetus. Long-term compression of the umbilical cord or a significant increase in intrauterine pressure can have a damaging effect on the fetus. Variable decelerations can also be observed in inferior vena cava syndrome.

Suro classification. There are 3 types of deceleration: simultaneous deceleration, residual deceleration and amplitude of deceleration.

With simultaneous deceleration, the end of the contraction coincides in time with the end of deceleration.

Residual deceleration is characterized by the fact that after the end of the contraction, so-called residual deceleration remains.

Deceleration amplitude is the amplitude of deceleration relative to the basal level.

There are 3 types of deceleration amplitude: moderate, threatening and dangerous.

For simultaneous decelerations, a moderate amplitude is within 30 beats/min, a threatening amplitude is up to 60 beats/min, and if it is more, the amplitude is dangerous.

For residual decelerations, moderate amplitude is already within 10 beats/min, threatening amplitude is up to 30 beats/min, and 30-60 beats/min is considered a dangerous amplitude.

The classification of Suro is based on the following principles:

• all decelerations must be taken into account;
• Decelerations should be considered the most informative if they take the form of late deceleration or are prolonged in relation to uterine contraction;
• the danger to the fetus increases with an increase in the amplitude of decelerations (this pattern has been established for both late and variable decelerations);
• At present, there are still significant disagreements about the pathophysiological origin of decelerations, so first of all it is necessary to know their prognostic value, and if there is data on compression of the umbilical cord, then the obstetrician should regard this type of deceleration as a danger to the fetus.

Based on the presented data, it is advisable to take into account the following points when monitoring women in high-risk groups and choosing the most rational method of delivery, especially when deciding on abdominal delivery:

• if there is an admixture of meconium in the amniotic fluid and good fetal CTG results, there is no need for urgent surgical intervention;
• less severe types of deceleration are often difficult to interpret, but additional determination of the pH value of capillary blood from the skin of the fetal head in combination with monitoring determination of CTG allows us to establish the degree of its suffering;
• various options for deviations on the cardiotocogram are the earliest sign indicating the possibility of fetal distress, but a change in pH is a more accurate indicator of its condition. Therefore, when the pH numbers from the skin of the fetal head are normal, then even in the presence of pathological CTG, a cesarean section can be avoided.

According to the classification of Syuro, 4 options for managing pregnant and laboring women are recommended.

I. Normal or moderate amplitude of decelerations:

A) norm:

• CTG baseline - 120-160 beats/min;
• curve variability - 5-25 beats/min;
• there are no decelerations.

B) moderate amplitude of decelerations:

• CTG baseline - 160-180 beats/min;
• curve variability is more than 25 beats/min;
• simultaneous decelerations - less than 30 beats/min, residual - less than 10 beats/min;
• accelerations.

II. Threatening condition for the NLOD:

• CTG baseline - more than 180 beats/min;
• curve variability is less than 5 beats/min;
• simultaneous decelerations - 30-60 beats/min, residual - 10-30 beats/min.

III. Onas state for the fetus:

• several threatening signs on CTG;
• basal line - less than 100 beats/min;
• simultaneous decelerations - more than 60 beats/min, residual - more than 30 beats/min.

IV. Extreme condition of the fetus:

• tachycardia combined with a flattened CTG curve and residual decelerations;
• residual decelerations - more than 60 beats/min for longer than 3 minutes.

In the first option, the woman in labor is in the process of giving birth and does not need any interventions.

In the second option, vaginal delivery is not excluded, but if possible, the Zading test should be performed - to determine the pH value of capillary blood from the skin of the fetal head. Taking into account the obstetric situation, it is advisable to carry out the following measures: change the position of the woman in labor, placing her on her side, reduce uterine activity, perform oxygen inhalations and treat maternal hypotension. If these measures are ineffective, it is necessary to carry out appropriate preparation for a cesarean section.

In the third option, the same treatment measures and diagnostic methods are carried out.

In the fourth option, immediate delivery is necessary.

When performing the Zaling test, it is necessary to take into account not only the current pH values, but also the time of performing repeated tests: a pH value greater than 7.25 should be regarded as an indicator of the normal state of the fetus; pH values within 7.20-7.25 indicate a threatening state of the fetus and a repeated determination of pH must be performed no later than 20 minutes after the first Zaling test; if the current pH is less than 7.20, a repeated analysis is performed immediately, and if no tendency towards an increase in these values is noted, a cesarean section must be performed.

At present, there is no single objective method on the basis of which one can accurately determine the degree of fetal suffering, as well as decide on the issue of surgical delivery.

Computerized assessment of cardiotocograms during labor

Currently, some countries have developed programs for computerized evaluation of intranatal CTG. Some programs also include analysis of uterine activity, which is of great importance when prescribing oxytotic agents during labor.

E. A. Chernukha et al. (1991) developed a computerized assessment of CTG during labor. Multifactorial analysis of CTG involves the inclusion of the main parameters of fetal cardiac activity and uterine activity in the discriminant equation.

Based on the data set, the computer issues conclusions about the condition of the fetus at intervals of 2-3 minutes:

• from 0 to 60 conventional units - the fetus is in normal condition;
• from 60 to 100 conventional units - borderline;
• above 100 conventional units - severe fetal distress.

If the fetus is in a borderline condition, the display shows the message "Determine the fetal COS". After the mother has been given the appropriate medications, the message disappears. However, if the fetus's condition worsens progressively, the message "Consider the possibility of labor termination" appears. The computer only notes a significant deterioration in the fetus's condition that requires emergency measures, but the scope and direction of the measures are entirely determined by the doctor conducting the labor. Uterine activity is calculated by the computer in Montevideo units. If the level is below 150 EM for 45 minutes, a conclusion about decreased uterine activity appears, and after another 10 minutes - an indication of the need to prescribe uterotonic drugs. If the uterine activity level is above 300 EM, after 20 minutes the message "Increased uterine activity" appears, and after another 10 minutes (i.e., 30 minutes after exceeding the uterine activity standards) - "Tocolysis".

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