Fetal photography and electrocardiography

The most common methods for assessing fetal cardiac activity are electrocardiographic (ECG) and phonocardiographic (PCG) studies. The use of these methods allows for a significant improvement in the diagnosis of fetal hypoxia and umbilical cord pathology, as well as the antenatal diagnosis of congenital cardiac arrhythmias.

Direct and indirect fetal ECG are distinguished. Indirect ECG is performed by placing electrodes on the anterior abdominal wall of the pregnant woman (the neutral electrode is placed on the surface of the thigh) and is used mainly in the antenatal period. Normally, the ventricular QRS complex is clearly identified on the ECG, sometimes the P wave. Maternal complexes are differentiated by simultaneously recording the mother's ECG. Fetal ECG can be recorded starting from 11-12 weeks of pregnancy, but in 100% of cases this is possible only by the end of the third trimester. Thus, indirect ECG is used after the 32nd week of pregnancy.

Direct ECG is recorded directly from the fetal head during labor when the cervix is 3 cm or more dilated. Direct ECG is characterized by the presence of an atrial P wave, a ventricular PQ complex, and a T wave.

When analyzing an antenatal ECG, the heart rate and r., rhythm character, size and duration of the ventricular complex, as well as its shape are determined. Normally, the fetal rhythm is regular, the heart rate fluctuates within 120-160 /min, the P wave is sharpened, the duration of the ventricular complex is 0.03-0.07 sec, and its voltage varies from 9 to 65 μV. With increasing gestational age, a gradual increase in the voltage of the ventricular complex is noted.

The fetal PCG is recorded by placing a microphone at the point where the stethoscope can best listen to the fetal heart sounds. The phonocardiogram is usually represented by two groups of oscillations, which reflect the first and second heart sounds. Sometimes the third and fourth sounds are determined. The fluctuations in the duration and amplitude of the heart sounds are quite variable in the third trimester of pregnancy and average: first sound - 0.09 sec (0.06-0.13 sec), second sound - 0.07 sec (0.05-0.09 sec).

With simultaneous recording of fetal ECG and PCG, it is possible to calculate the duration of the cardiac cycle phases: the phase of asynchronous contraction (AC), mechanical systole (Si), general systole (So), diastole (D). The phase of asynchronous contraction is detected between the beginning of the Q wave and the first tone, its duration is within 0.02-0.05 sec. Mechanical systole reflects the distance between the beginning of the first and second tones and lasts from 0.15 to 0.22 sec. General systole includes mechanical systole and the phase of asynchronous contraction and is 0.17-0.26 sec. Diastole (the distance between the second and first tones) lasts for 0.15-0.25 sec. It is important to establish the ratio of the duration of general systole to the duration of diastole, which at the end of an uncomplicated pregnancy averages 1.23.

In addition to the analysis of fetal cardiac activity at rest, functional tests are of great help in assessing the reserve capacity of the fetoplacental system using antenatal CTG. The most widely used are non-stress (NST) and stress (oxytocin) tests.

The essence of the non-stress test is to study the reaction of the fetal cardiovascular system in response to its movements. During a normal pregnancy, in response to fetal movement, the heart rate increases on average by 10 minutes or more. In this case, the test is considered positive. If accelerations occur in response to fetal movements in less than 80% of observations, the test is considered negative. In the absence of changes in heart rate in response to fetal movements, the NST is negative, which indicates the presence of intrauterine fetal hypoxia. The appearance of bradycardia and monotony of the heart rhythm also indicate fetal distress.

The oxytocin test is based on the study of the reaction of the fetal cardiovascular system in response to induced uterine contractions. To perform the test, an oxytocin solution is administered intravenously (0.01 U/1 ml of 0.9% sodium chloride solution or 5% glucose solution). The test is assessed as positive if at least 3 uterine contractions are observed within 10 minutes at an oxytocin administration rate of 1 ml/min. With sufficient compensatory capabilities of the fetoplacental system, a mild short-term acceleration or early short-term deceleration is observed in response to uterine contractions. Detection of late, especially W-shaped, decelerations indicates fetoplacental insufficiency.

Contraindications to the oxytocin test are: abnormal attachment of the placenta, its partial premature detachment, the threat of termination of pregnancy, the presence of a scar on the uterus.

The task of monitoring during labor is to promptly recognize deterioration in the condition of the fetus, which allows for adequate therapeutic measures to be taken and, if necessary, to speed up labor.

To assess the condition of the fetus during labor, the following parameters of the cardiotocogram are studied: the basal rhythm of the heart rate, the variability of the curve, as well as the nature of slow accelerations (accelerations) and decelerations (decelerations) of the heart rate, comparing them with data reflecting the contractile activity of the uterus.

In uncomplicated labor, all types of basal rhythm variability may be encountered, but slightly undulating and undulating rhythms are most commonly present.

The criteria for a normal cardiotocogram in the intranatal period are considered to be:

• basal heart rate 110-150 beats/min;
• amplitude of basal rhythm variability 5-25 beats/min.

Signs of a suspicious cardiotocogram during labor include:

• basal rhythm 170-150 beats/min and 110-100 beats/min;
• amplitude of basal rhythm variability of 5-10 beats/min for more than 40 minutes of recording or more than 25 beats/min;
• variable decelerations.

Diagnosis of pathological cardiotocogram during labor is based on the following criteria:

• basal rhythm less than 100 or more than 170 beats/min;
• basal rhythm variability of less than 5 beats/min for more than 40 minutes of observation;
• marked variable decelerations or marked repetitive early decelerations;
• prolonged decelerations;
• late decelerations;
• sinusoidal type of curve.

It should be emphasized that when using CTG during labor, a monitoring principle is necessary, i.e. constant dynamic observation throughout labor. The diagnostic value of the method increases with careful comparison of CTG data with the obstetric situation and other methods of assessing the condition of the fetus.

It is important to emphasize the need to examine all women in labor admitted to the maternity ward. Subsequently, cardiotocogram recordings can be made periodically if the initial recording is assessed as normal for 30 minutes or more, and labor proceeds without complications. Continuous cardiotocogram recordings are made in case of pathological or suspicious types of the primary curve, as well as in pregnant women with a burdened obstetric history.

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