Cardiotocography

Currently, cardiotocography (CTG) is the leading method for assessing the functional state of the fetus. A distinction is made between indirect (external) and direct (internal) cardiotocography. During pregnancy, only indirect cardiotocography is used. A classic cardiotocogram is two curves superimposed in time. One of them displays the fetal heart rate, and the other - uterine activity. The uterine activity curve, in addition to uterine contractions, also records the motor activity of the fetus.

Information about the fetus's cardiac activity is obtained using a special ultrasound sensor, the operation of which is based on the Doppler effect.

Direct cardiotocography is used in labor. The study is based on fetal ECG recording. With this method, after the amniotic fluid has been released and the cervix has opened to 3 cm or more, a spiral ECG electrode is placed on the fetus's head, and another electrode is attached to the woman's thigh. It should be noted that this method allows for a higher-quality fetal heart rate curve to be obtained.

Modern cardiac monitors are also equipped with a strain gauge sensor. With the help of such a sensor, in addition to the contractile activity of the uterus, the motor activity of the fetus is recorded.

During the examination, the ultrasound sensor is placed on the woman's anterior abdominal wall in the place where the fetal heartbeat can be best heard and secured with a special belt. The sensor is installed when the sound, light or graphic indicators in the device begin to show stable fetal cardiac activity. The external strain gauge sensor is installed on the woman's anterior abdominal wall and secured with a belt.

There are also antenatal cardiac monitors, where two curves are simultaneously recorded using one ultrasound sensor: the fetus's heart rate and its motor activity. The feasibility of creating such devices is due to the fact that when using an ultrasound sensor, significantly more fetal movements are recorded than when using a strain gauge.

Cardiotocography recording is performed with the woman lying on her back, on her side, or sitting.

Reliable information on the fetus's condition using this method can be obtained only in the third trimester of pregnancy (from 32-33 weeks). This is due to the fact that by this period of pregnancy the myocardial reflex and all other types of fetal activity reach maturity, exerting a significant influence on the nature of its cardiac activity. Along with this, it is during this period that the fetus's activity-rest (sleep) cycle is established. The average duration of the active state of the fetus is 50-60 minutes, the calm state - 15-40 minutes. The leading period in assessing the fetus's condition using cardiotocography is the active period, since changes in cardiac activity during the rest period are almost identical to those observed when the fetus's condition is disturbed. Therefore, taking into account the sleep-like state of the fetus, in order to avoid errors, the recording duration should be at least 60 minutes.

When decoding cardiotocograms, the amplitude of instantaneous oscillations and the amplitude of slow accelerations are analyzed, the value of the basal heart rate is assessed, and the value of decelerations is taken into account.

The decoding of the cardiotocogram usually begins with the analysis of the basic (basal) heart rate. The basal rhythm is the average heart rate of the fetus, which remains unchanged for 10 minutes or more. Accelerations and decelerations are not taken into account. In the physiological state of the fetus, the heart rate is subject to constant small changes, which is due to the reactivity of the autonomic system of the fetus.

Heart rate variability is assessed by the presence of instantaneous oscillations. They are rapid, short-term deviations of the heart rate from the basal level. Oscillations are counted for 10 minutes of examination in areas where there are no slow accelerations. Although determining the frequency of oscillations may have some practical value, counting their number during a visual assessment of the cardiotocogram is almost impossible. Therefore, when analyzing the cardiotocogram, they are usually limited to counting only the amplitude of instantaneous oscillations. A distinction is made between low oscillations (less than 3 heartbeats per minute), medium (3–6 per minute), and high (more than 6 per minute). The presence of high oscillations usually indicates a good condition of the fetus, while low oscillations indicate a disorder.

When analyzing a cardiotocogram, special attention is paid to the presence of slow accelerations. Their number, amplitude, and duration are counted. Depending on the amplitude of slow accelerations, the following cardiotocogram variants are distinguished:

• silent or monotonous with low amplitude of accelerations (0–5 contractions per minute);
• slightly undulating (6–10 contractions per minute);
• inducing (11–25 contractions per minute);
• saltatory or jumping (more than 25 contractions per minute).

The presence of the first two rhythm variants usually indicates a disturbance in the fetus’s condition, while the last two indicate its good condition.

In addition to oscillations or accelerations, when decoding cardiotocograms, attention is also paid to decelerations (slowing of the heart rate). Decelerations are understood as episodes of slowing of the heart rate by 30 contractions or more lasting 30 seconds or more. Decelerations usually occur during uterine contractions, but in some cases they can be sporadic, which usually indicates a significant disturbance in the condition of the fetus. There are 3 main types of decelerations.

• Type I - deceleration occurs with the onset of contraction, it has a smooth beginning and end. The duration of this deceleration either coincides with the duration of the contraction or is somewhat shorter. Often occurs with compression of the umbilical cord.
• Type II - late deceleration, occurs 30 seconds or more after the onset of uterine contraction. Deceleration often has a steep onset and a more gradual leveling off. Its duration is often longer than the duration of the contraction. It mainly occurs in fetoplacental insufficiency.
• Type III - variable decelerations, characterized by different times of occurrence in relation to the beginning of the contraction and have different (V-, U-, W-shaped) forms. At the top of the decelerations, additional fluctuations in heart rate are determined. Based on numerous studies, it has been established that the following signs are characteristic of a normal cardiotocogram during pregnancy: the amplitude of instantaneous oscillations is 5 contractions per minute or more; the amplitude of slow accelerations exceeds 16 contractions per minute, and their number should be at least 5 per 1 hour of the study; decelerations are either absent or are the only ones with a deceleration amplitude of less than 50 contractions per minute.

At a meeting in Zurich, Switzerland in 1985, the FIGO Perinatal Committee proposed to evaluate antenatal cardiotocograms as normal, suspicious, and pathological.

The criteria for a normal cardiotocogram are the following signs:

• basal rhythm not less than 110–115 per minute;
• amplitude of basal rhythm variability 5–25 per minute;
• decelerations are absent or sporadic, shallow and very short;
• two or more accelerations are recorded during 10 minutes of recording.

If this type of cardiotocogram is detected even during a short period of examination, then the recording can be stopped. Suspicious cardiotocogram is characterized by:

• basal rhythm within 100–110 and 150–170 per minute;
• amplitude of basal rhythm variability between 5 and 10 per minute or more than 25 per minute for more than 40 minutes of study;
• absence of accelerations for more than 40 minutes of recording;
• sporadic decelerations of any type except severe.

When this type of cardiotocogram is detected, other research methods must be used to obtain additional information about the condition of the fetus.

Pathological cardiotocogram is characterized by:

• basal rhythm less than 100 or more than 170 per minute;
• basal rhythm variability of less than 5 per minute is observed for more than 40 minutes of recording;
• marked variable decelerations or marked repetitive early decelerations;
• late decelerations of any type;
• prolonged decelerations;
• sinusoidal rhythm lasting 20 minutes or more.

The accuracy of determining a healthy fetus or its abnormal condition with such a visual assessment of the cardiotocogram is 68%.

In order to increase the accuracy of cardiotocograms, scoring systems for assessing the condition of the fetus were proposed. The most widely used of these was the system developed by Fisher in the modification of Krebs.

A score of 8–10 points indicates a normal condition of the fetus, 5–7 points indicates initial disorders, 4 points or less indicates severe intrauterine fetal distress.

The accuracy of the correct assessment of the fetus's condition using this equation was 84%. However, significant subjectivity in manual processing of the monitor curve and the impossibility of calculating all the necessary indicators of the cardiotocogram to some extent reduced the value of this method.

In this regard, a completely automated monitor ("Fetal Health Analyzer") was created that has no analogues. During the study, two curves are displayed on the display screen: the heart rate and the motor activity of the fetus. Registration of the specified parameters of fetal activity, as in other devices, is carried out using a sensor based on the Doppler effect. After the end of the study, all the main necessary calculation indicators, as well as the fetal health indicator, are displayed on the display screen.

The main advantages of the automated monitor compared to other similar devices.

• Higher (by 15–20%) information content compared to traditional methods of cardiotocogram analysis.
• Full automation of the received information.
• Unification of results and absence of subjectivity in the analysis of cardiotocograms.
• Almost complete elimination of the influence of fetal sleep on the final result.
• In doubtful cases, automatic extension of the research time.
• Taking into account the motor activity of the fetus.
• Unlimited long-term storage of information and its reproduction at any time.
• Significant cost savings due to the absence of the need to use expensive thermal paper.
• Can be used in any maternity hospital, as well as at home without the direct involvement of medical personnel.

The accuracy of correct assessment of the fetus condition using this device turned out to be the highest and amounted to 89%.

An analysis of the impact of the use of an automated monitor on perinatal mortality showed that in those institutions where this device was used, it was 15–30% lower than the baseline.

Thus, the presented data indicate that cardiotocography is a valuable method, the use of which can contribute to a significant reduction in perinatal mortality.

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