Clinical and biophysical evidence on the coordination of uterine contractions in labor

Recognition of the initial symptoms of uterine motor dysfunction during labor, comparative assessment of the effectiveness of treatment of labor abnormalities based on clinical observations alone are very difficult, therefore, at present, methods of monitoring during pregnancy, even at home, during labor are becoming increasingly important - external and internal hysterography, cardiotocography.

In recent years, methods of recording uterine contractions by external multichannel hysterography, as well as internal hysterography (tocography) using the radio telemetry device of the Capsule system, transcervical method of recording intrauterine pressure using the open polyethylene catheter technique, and transabdominal method of studying intrauterine pressure have become widespread in obstetric practice. Steer et al. developed a more advanced catheter for recording intrauterine pressure by the transducer type, which is devoid of the disadvantages of an open catheter. In 1986, Svenningsen and Jensen developed a fiber-optic catheter for measuring intrauterine pressure. Currently, the Utah Medical Systems company has developed the Intran 2 catheter.

Great attention to this problem and its solution is due to the serious importance of studying the contractile activity of the uterus for the diagnosis and prognosis of labor in complicated cases.

The first person to attempt to measure the strength of uterine contractions during labor was the Russian scientist N. F. Tolochinov (1870), who proposed a spring manometer mounted in a cylindrical vaginal speculum. The manometer was brought to the fetal bladder and measured the force of its pressure. In 1913-1914, the French obstetrician Fabre first conducted a parallel recording of uterine contractions using external and internal hysterography and concluded that the curves obtained when recording contractions using both methods corresponded to each other. In 1872, Schatz used internal hysterography, which is still widely used today.

It is important to note that the data obtained during the simultaneous recording of amniotic pressure with a catheter inserted through the abdominal wall and transcervically showed complete identity of the obtained curves. According to Mosler, the basal tone is 15 mm Hg, the value of intrauterine pressure in the first period of labor is 60 mm Hg, in the second period - 105 mm Hg. According to Alvarez, Caldeyro-Barcia, these indicators were 8 mm, 35-100 mm Hg and 100-180 mm Hg, respectively. According to Williams, Stallwoithy, the indicators of uterine contractility were 8 mm Hg, 40-90 mm Hg, 120-180 mm Hg, respectively. Williams and Stallworthy point out that internal hysterography has the advantage of reflecting the pressure in the hydrostatic cavity, so that indicators based on hydrodynamic calculations reflect the true activity of the contractile function of the uterus.

Some authors use closed polyethylene tubes with one sensor and a pressure sensor, which is located between the wall of the uterus and the fetal head along the largest circumference of the fetal head, to measure intrauterine pressure. However, there are many examples in obstetric practice showing that there is often no correspondence between the clinical course of labor and hysterography indicators.

Over the last 50 years, a large number of factors (hormones) and various pharmacological substances on the uterus have been studied. Mechanical factors also have a fairly long history. As early as 1872, Schatz showed that a sudden increase in the volume of the uterus leads to the occurrence of uterine contractions. Reynolds in 1936 proposed a theory of uterine tension ("a uterine distention theory"), in 1963 Csapo - the theory of "progesterone block", considered by the author as a mechanical factor in pregnancy.

At the same time, the physical laws of hydrodynamics can and should undoubtedly be applied to the study of the contractile activity of the uterus. For the first time in 1913, Sellheim in his monograph "Childbirth in Man" made a number of calculations on a hydrodynamic basis; these studies were reflected in many textbooks of domestic and foreign obstetricians. In the monograph Reynolds (1965), devoted to the physiology of the uterus, detailed calculations are given, showing the role of physical factors in uterine activity with a hydrodynamic justification according to the laws of Laplace and Hooke. Referring to the research of Haughton, conducted in 1873, showed that the proportion of the radius of curvature in the fundus of the uterus and the lower segment of the uterus is equal to 7:4, i.e. the difference in the tension of the uterus in its upper and lower sections has a ratio of 2:1 and therefore in the process of normal labor there is a clear difference in the tension of muscle fibers in the area of the fundus and the lower segment of the uterus, equally this applies to the thickness of the myometrium in the specified sections, which is related as 2:1. Therefore, the force is proportional to the thickness of the uterine tissue according to Haughton. Based on the calculations and ideas of Haughton and his own data based on the method of three-channel external hysterography developed by Reynolds in 1948, the author believes that the opening of the cervix is observed only when rhythmic activity in the fundus of the uterus predominates over its other areas. In this case, in the middle zone of the uterus (body) in relation to its fundus, contractions are less intense and they are usually shorter in duration, and their frequency decreases as labor progresses. The lower segment of the uterus remains inactive throughout the first stage of labor. Thus, the opening of the cervix during labor is the result of a decrease in the gradient of physiological activity from the fundus to the lower segment of the uterus. The functional components of this activity are the intensity and duration of uterine contractions. In this case, uterine contractions in the fundus area are 30 s longer than in the body of the uterus, i.e. the so-called "triple descending gradient" is observed. These author's judgments were confirmed by the works of Alvarez, Caldeyro-Barcia (1980), who measured and assessed intrauterine and intramuscular pressure in the uterus at different stages of pregnancy and labor using complex microballoon equipment. With the help of this method, it was possible to confirm the concept of the "triple descending gradient" characteristic of the normal course of labor. In addition, it was shown that the wave of contractions began in one of the tubal angles of the uterus, and the theory of the dominant role of the fundus of the uterus and the presence of a triple descending gradient was confirmed.

Similar judgments on the application of the laws of hydrodynamics in the study of uterine dynamics are also given in the monograph by Mosier (1968). According to the author's concept, two opposite forces control and complete the labor process: the force of tension and elasticity. However, the author emphasizes that it is impossible to transfer the results of studying uterine contractions to animals and to the human uterus without reservations, as given in the work of Csapo et al. (1964), since animals have a bicornuate uterus, and humans have a simplex. Therefore, both studies on the human uterus and consideration of some discrepancies between the laws of hydrodynamics and clinical observations are needed. Thus, with maximum tension of the uterine walls, a decrease in the resistance of the cervical walls is simultaneously observed. In this case, contractile activity of the uterus during labor occurs not due to an increase in intrauterine pressure, but due to increased tension of the uterine walls, which occurs as a reaction to an increase in the total volume (diameter) of the uterine cavity. It should be noted here that the increase in the volume of the uterus that occurs during pregnancy occurs without a noticeable increase in pressure in the uterus, where the pressure varies from 0 to 20 mm Hg and an increase in pressure is noted only at the end of pregnancy. Bengtson (1962) recorded average values of intrauterine pressure at rest, during pregnancy, equal to 6-10 mm Hg. The nature of this "resting pressure" - residual or basal pressure according to Mosler is not entirely clear in detail, but is obviously causally partially related to the intrauterine pressure itself and intra-abdominal pressure, as Sellheim pointed out back in 1913.

Mosler emphasizes that the measurement of intrauterine pressure is an indirect determination of the tension of the uterine wall, caused by contractions of the uterine muscles and also dependent on the radius of the uterine cavity. The tension of the uterine wall can be described by the Laplace equation. At the same time, one cannot help but notice the fact that when using micro-balloon technology (from 1 to 15 mm in volume), the rubber balloon, with long-term recording, gives relatively inaccurate pressure data based on changes in elasticity.

An important point for obtaining identical data is, from our point of view, the exact determination of the depth of insertion of the catheter into the uterine cavity, which, unfortunately, is not taken into account when conducting internal hysterography, since the authors proceed from the incorrect idea of the same pressure in the uterine cavity during labor, if we proceed from Pascal's law. Only in the work of Hartmann, when studying intrauterine pressure outside pregnancy, it is indicated that all catheters have a ring attached at a distance of 5 cm, showing the depth at which the catheter is in the uterine cavity. However, as will be shown below, when determining the intrauterine pressure indicators, it is necessary to take into account the height of the hydrodynamic column - the height of the uterus and the angle of inclination of the uterus in relation to the horizontal line and, depending on the angle of inclination of the uterus, the pressure in the lower parts of the uterus will be higher than in the upper parts of the uterus (fundus).

The study of the contractile activity of the uterus using five-channel external hysterography during normal labor, even accompanied by painful contractions, allowed us to reveal the absence of discoordination of labor. Those minor differences in the duration and intensity of contractions of both halves of the uterus at one level (in one segment) are of no importance, since its contractions remain coordinated and the amplitude of contractions reaches its highest point simultaneously in all recorded segments of the uterus, which allowed us to move on to three-channel external hysterography, placing the sensors accordingly in the area of the fundus, body and lower segment of the uterus.

The analysis of the obtained data was carried out by quantitative processing of hysterograms for every 10 minutes. The main parameters of the contractile activity of the uterus were studied (duration and intensity of contractions, frequency and duration of pauses between them, coordination of different parts of the uterus with each other, etc.). At present, electronic integrators are used for this purpose, when the area of active pressure under the curve of intrauterine pressure is measured, especially when using internal hysterography.

In order to rationalize calculations and save time, we have proposed a special ruler for analyzing hysterograms.

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