Characteristics of childbirth

To understand the nature of the disturbance of the contractile activity of the uterus in the early stages of its development, along with studying the coordination, strength and frequency, duration and rhythm of uterine contractions, it is also necessary to take into account disturbances in the tone of the uterus.

When studying the dynamics of cervical dilation during normal labor using internal hysterography, Lindgren believed that during labor there is the same pressure everywhere in the uterus, since with a sufficient amount of amniotic fluid in the uterine cavity during contractions and in the pauses between them, the same pressure arises. In addition, when using the Malmstroma recorder, the same pressure was also found during contractions both in the uterine cavity and behind the lower pole of the head. Lindgren, when quantitatively recording the pressure between the fetal head and the uterine wall, revealed other pressure ratios that did not correspond to the values of amniotic pressure.

As is known, the increase in the frequency of cesarean sections in many countries is due to dystocia during labor or lack of progress in cervical dilation. In order to reduce the frequency of cesarean sections in these women, active management of labor with high doses of oxytocin is offered, but many obstetricians are very wary of these recommendations. This is due to ignorance of the physiology of cervical dilation. It has been shown that with ineffective induced labor, the pressure between the head and cervix was low, despite adequate intrauterine pressure, and therefore, for the normal course of labor, it is necessary to establish the correct relationship between the pressure between the head, lower segment and cervix. However, these conclusions of the authors were purely speculative, without sufficient factual data. The main difficulty in interpreting earlier works of a number of authors is that they measured not so much the force as the pressure between the head and cervix. Studies have shown that active intrauterine pressure fluctuated within 5-121 mm Hg. (mean 41.75 ± 16.16 mm Hg), and the active force is 0-ISO gwt (mean 35 ± 30.59). This work was the first study in which the forces existing between the fetal head and the cervix during labor were measured at a number of points using a special catheter. The active force developed between the head and the cervix is independent of the intrauterine pressure. Therefore, the development of the highest force between the fetal head and the cervix is a real chance of completing labor through the natural birth canal, even with an adequate level of uterine activity in different women in labor. The process of cervical dilation is the result of the harmonious action of three main components:

• isometric mode of contraction of the smooth muscle elements of the functional parts of the uterus;
• the volume of blood deposited in the vascular reservoirs of the myometrium, decidua and cervix;
• optimal value of resistance to deformation of the cervix.

Variants of cervical dilation during term labor have been studied and their clinical significance has been determined. In this case, the movement of the presenting part of the fetus along the birth canal occurs synchronously with the process of cervical dilation and with an increase in the dilation of the os, the movement of the presenting part along the birth canal accelerates. Progressive movement of the fetus in the active period of labor is noted after 3 cm of dilation of the uterine os.

The uterus consists of a very large number of muscles, and according to the general laws of physiology, the purpose of muscles in an animal organism is to perform work. Therefore, during childbirth, the muscles of the uterus in all sections are active and form peristaltic movements.

Modern research has shown the possibility of two mechanisms of cervical dilation during labor: longitudinal contraction of the uterine walls, causing an increase in intrauterine pressure, and radial tension as the head moves along the cervix.

Until now, there was no method for separately measuring intrauterine pressure and radial tension. The authors designed a voltage transducer that responded minimally to an increase in intrauterine pressure. A probe with 4 such transducers was placed between the fetal head and the mother's cervix along the long axis of the fetus. The intrauterine pressure transducer at the end of the probe allowed for the simultaneous measurement of amniotic pressure. Preliminary studies in 20 women in labor confirmed the possibility of radial tension in the dilation of the cervix.

For recognition of contractions during pregnancy, it is characteristic that there is no uniform general compaction of the uterus, and at the same time, it occurs periodically. In addition, the following criterion is true: if the internal os is still palpable, therefore, if the cervix has not begun to smooth out, then labor has not yet begun, contractions, even if they are felt quite strongly, should be considered contractions during pregnancy. The beginning of smoothing of the cervix (from the side of the opening internal os) is the first sign of the beginning of labor.

Among the additional clinical criteria, it is recommended to conduct an ultrasound examination for 45 minutes to distinguish true labor from "false": the presence of fetal breathing with a Bishop cervical maturity score of less than 9 points clearly indicates "false" labor. It is believed that "false" labor is more often observed with a higher position of the fetal head and about 10% of women with a prolonged latent phase can be attributed to "false" labor. Pathological contraction (slow relaxation) of the isthmus is an important cause of delayed entry of the head into the pelvic cavity and delayed smoothing of the cervix.

Labor with pathological position of the contraction ring is observed due to localized pathological contraction of the upper or lower sphincter. It is important to take into account the transition from the latent phase to the active phase of labor. In uncomplicated labor, primiparous and multiparous women have the same dynamics of opening of the cervix. Opening of the cervix itself fairly objectively characterizes the course of labor. When opening by 5 cm, 90% of women in labor are in the active phase, when opening less than 4 cm, 25% of women in labor are still in the latent phase of labor. It is advisable to diagnose disorders of the active phase when the cervix opens by 5 cm.

Some authors [Johnston, Greer, Kelly, Calder] believe that normal and pathological labor can be determined by the level of prostaglandins of the F and E series and their metabolites. Spontaneous labor is associated with an increase in prostaglandin metabolites in the mother's blood plasma, and PGF ₂ is an important stimulator of uterine activity, and its relative deficiency leads to dysfunction of labor. At present, attention has been increased to the role of the pelvic cavity in the advancement of the fetal head when interpreting physiological labor. Hydrostatic pressure in the uterine cavity is of particular importance. The contraction forces of the abdominal wall muscles and the uterine walls affect the hydrostatic pressure in the pelvic cavity, stimulating the advancement of the fetal head.

In recent years, a number of issues have been clarified regarding the relationship between the motor function of the uterus and blood flow in the uterus during pregnancy. Increased blood filling of the uterus reduces the activity of the myometrium, and this coincides with the studies of Russian scientists. According to Brotanek, amniotomy is always accompanied by a decrease in blood flow, and an increase in the tone of the uterus begins only after the blood flow level stabilizes at a lower level than before the opening of the amniotic sac. In the active labor phase, each contraction of the myometrium is preceded by a decrease in blood flow in the uterus for 30 seconds. With the onset of contraction, its level begins to level out, but again sharply drops as soon as the strength of uterine contractions begins to exceed 30 mm Hg with a peak decrease in blood flow at the height (acme) of the contraction.

Studying the relationship between the contractile activity of the uterus and blood flow, it is noted that during a contraction, the blood flow decreases, and during hypertonicity of the uterus, it decreases to a greater extent. During a strong contraction, the minimum blood flow in the uterus falls on the descending part of the contraction curve. Doctors called this the "phenomenon of slowing down of the uterine blood flow." The latter is 20-40 sec. The possible connection of this phenomenon with the development of late decelerations of the "deep 2" type is emphasized.

Our observations of the nature of uterine contractile activity based on the data of two-channel internal hysterography show that diastole (the descending part of the uterine contraction curve) does not change with the opening of the uterine os during weak labor, which may be one of the moments of disruption of the uterine self-regulation and thus lead to a slowdown in uterine blood flow precisely at the moment of determining the descending part of the contraction curve. It is possible that this may also be due to changes in the shape of the uterus itself at the moment of contraction and in the pause between contractions, as was shown by echographic studies. It was revealed that during transverse scanning during contraction the uterus has a round shape, and in the pause between contractions it takes a horizontal ovoid shape. Theoretically, it can be assumed that the increasing intrauterine pressure gives the uterus a spherical shape, which is confirmed by this study. In addition, ultrasound revealed a characteristic swelling of the lower posterior wall of the uterus (body) towards the sacrum.

It is believed that in the process of evolution in the hemodynamic system of the human uterus a mechanism of blood deposition in the internal vascular reservoirs of the uterus appeared, which ultimately became an instrument for the active formation of the size of the hydrodynamic extraovular volume expulsed from the cavity of the body of the uterus into the cylindrical part of the cavity of the lower segment, and the return of most of this volume back in the passive part of the labor contraction, which determines the biomechanics of the opening of the cervix in the first stage of labor in humans.

Main parameters of the motor function of the uterus during labor. From a brief review of modern data on the contractile activity of the uterus, it is evident that the same phenomena (parameters) of the motor function of the uterus are interpreted differently in different studies. This difference most often cannot be considered in the spirit of the sacramental formula: some studies give a true picture of the nature of the contractile activity of the uterus, while others give a distorted picture. This obviously happens because the processes in the mechanisms of self-regulation of the uterus have many different, as yet unknown sides and facets.

The most widely used method of assessing the progress of labor is the cervical dilation. Graphic representation of cervical dilation during labor was introduced in 1954 by E. A. Friedman. However, it should be recognized that the use of this method does not always provide a clear correlation between the dynamics of uterine activity and cervical dilation. This has given some authors reason to assert that slow cervical dilation depends mainly on low, rather than optimal, uterine activity.

Special computer programs have been developed and implemented to predict labor based on hysterographic data, as well as clinical signs. The main difficulty is to determine the most informative indicators that will allow a correct diagnosis to be quickly established at the onset of labor.

Attempts at mathematical analysis of the most informative features based on five-channel external hysterography data were undertaken. Significant unevenness of qualitative and quantitative indicators of uterine contractile activity during labor was revealed, combined with significant individual variability in the dynamics and duration of the main phases of labor, which significantly complicates the generalized partographic and tocographic characteristics of labor as a whole. This substantiates the advisability of the practical use of stage-dynamic analysis of labor by its phases based on systematic partographic and tocographic monitoring taking into account the state of the cervix and systematic comparison of the amplitude-time parameters of the uterine cycle with indicators typical of a normal uncomplicated course of labor.

In foreign literature, the most widely used method for measuring intrauterine pressure during labor is the assessment of uterine contractility in Montevideo units, where the average value of intrauterine pressure (the amplitude of contraction above the basal line) is multiplied by the multiple number of uterine contractions in 10 minutes.

The Alexandrian unit is also used, which includes, in addition to the Montevideo unit, the average duration of contraction per minute.

There is also an "active planimetric unit" - the area under the continuous intrauterine pressure curve for 10 min, and a "total planimetric unit" - the area above the active pressure curve for 10 min. However, these methods are very labor-intensive and require a lot of time to analyze the hysterograms.

The total area under the intrauterine pressure curve can be used most rationally, because, according to Miller, the tone of the uterus and the amplitude of contractions can indicate more fully the degree of progress of the cervical dilation. In this case, uterine activity is measured in Torr-minutes (i.e., in mm Hg/min). This method provides a high dependence between the values of uterine activity and the dilation of the cervix, which cannot be achieved by other methods.

In domestic works there are also attempts at quantitative analysis of hysterograms.

Great importance is attached to the frequency of contractions, believing that the more frequent the rhythm becomes and the shorter the intervals, the more significantly the tone of the uterus increases between contractions, up to the development of complexes of its uncoordinated contractions. It turned out that the tone changes very slowly during normal labor, increasing by approximately 1 mm Hg every hour of labor. An increase in tone is always accompanied by an increase in the frequency of contractions. Doctors suggest that the tone and frequency of contractions are interdependent, and their nature is the same and depends on the degree of excitability of the uterine muscles. It should be emphasized that, according to research, a significant increase in the tone of the uterus without a corresponding increase in the frequency of contractions has never been noted. Based on this, they come to the conclusion that of all the indicators used to assess uterine contractility during labor, changes in tone are the least demonstrative in quantitative terms according to internal hysterography, not to mention external hysterography, and to a lesser extent than other indicators - coordination, strength, duration, frequency and rhythm of contractions, which can be directly assessed. Therefore, the authors doubt the practical expediency of using changes in tone as the main indicator determining various anomalies of labor. Thus, the authors question the expediency of using those classifications of labor anomalies where uterine tone is used as a basis.

The famous German scientist H. Jung in his clinical and experimental studies adheres to the opposite point of view. Our studies also confirm it. The author introduced the concept of "tonic and phasic double principle of uterine contraction". Considering the issue of the tonic and phasic system of the uterus, the author points out that the contraction is a purely tetanic contraction, and the strength of the contraction is primarily regulated by the excitation frequency. Studies conducted with the removal of potentials from a separate fiber show that the uterus of animals and humans responds to an increase in the extracellular concentration of potassium by decreasing the membrane potential with a simultaneous increase in mechanical frequency and resting tone. If the potential decreases to a certain value, the sodium transporter is inactivated, the muscle reacts only tonically with additional depolarization. Based on these results, it is impossible to simply explain the oxytocin-induced increase in tone by a shortened relaxation time as a result of a strong increase in frequency.

As our studies have shown, with weak labor activity, there is a deepening of the severity of metabolic acidosis, a decrease in the total content of nucleic acids, potassium and calcium along with an increase in the activity of oxytocinase and inhibition of creatine phosphokinase. The introduction of oxytocin in a buffer solution containing tris, KCl, CaCl ₂ in certain proportions normalizes labor activity, as was shown in the experimental studies of H. Jung. Moreover, the author, during a critical examination of hysterograms, noted that even in clinical conditions after the introduction of oxytocin to the woman in labor, the tone does not return to the original even when the time interval between contractions is accidentally prolonged at least once. An increase in frequency and tone after the administration of oxytocin gives a picture similar to that after potassium depolarization. The dependence is explained by the depolarizing, i.e. membrane potential-lowering, action of oxytocin, first described by H. Jung in 1957. The frequency and increase in tone, as well as the increase in excitability, are associated with a decrease in the threshold caused by depolarization. This mechanism was confirmed by A. Csapo in 1961 and by other authors.

Important biochemical mechanisms of oxytocin action on the uterus include increased phosphoinositide metabolism and inhibition of adenylate cyclase activity. It has been shown that the effect of forskolin (an adenylate cyclase activator), as well as other substances that increase the level of cyclic adenosine monophosphate in the cell, indicates the participation of the adenylate cyclase system in myometrium contraction, especially in maintaining tone.

Thus, scientists from the modern positions of uterine biochemistry confirm earlier observations that, obviously, the adenylate cyclase system is responsible for the tonic component, and the phosphoinositide system is responsible for the phase component of human myometrium contraction. Therefore, the control of these processes through oxytocin receptors, as well as through the influence on the intracellular processes of the implementation of the phase and tonic components of contraction is very promising for the implementation of regulation of labor. Synthesis of oxytocin analogues that block or excite different subtypes of oxytocin receptors will make it possible to activate or reduce selectively the tonic or phase component of uterine contraction.

This proves the functionally independent principle of tone in the uterus and a relationship was found between tone and membrane potential.

It has been shown that the development of dominant contractile activity in a certain area of the myometrium depends on the intensity of the stimulus, the degree of excitability, and the conductivity of the myometrium. The existence of centers that cause uterine contraction with their constant location is subject to criticism due to:

• absence of any local morphological features;
• a richer distribution of nerve fibers in the lower segments of the uterus;
• known experimental studies indicating the possibility of the appearance of action potentials in any part of the myometrium.

The so-called “phasic (rhythmic) and tonic contraction systems” operate functionally separately from each other, although a close functional correlation can be found both at normal and at average values of membrane potential.

The increase in tone, however, cannot be explained solely by the secondary high frequency of contractions. In support of this position, Jung cites clinical observations with precise analysis of numerous hysterograms with high tone and high frequency of contractions, with the observation of individual longer pauses between contractions, and the tone in these cases did not fall further.

These studies show that it is currently premature in clinical terms to abandon those classifications where changes in tone are assumed to be the main indicator determining various anomalies of labor. There is considerable evidence that normal labor can only be observed when there is optimal labor with an amplitude of 50-70 mm Hg and a contraction frequency of at least 3 contractions per 10 min.

Weakness of labor activity according to the dynamics of intrauterine pressure is characterized by the amplitude of uterine contractions equal to 25-30 mm Hg or an abnormally low frequency of contractions - less than 3 contractions per 10 minutes. If the activity of the uterus is less than 100 Montevideo units, then the progression of labor will be slower than normal. At the same time, if uterine contractions have an average intensity of 50 mm Hg and the frequency of contractions is maintained between 4 and 5 contractions per 10 minutes, then the duration of the first period will be between 3 and 6 hours.

It is important to note that changes in the acid-base balance of the fetal blood begin to be observed with frequent contractions of the uterus, exceeding 5 in 10 minutes, or the basal (residual) tone of the uterus exceeds 12 mm Hg. This leads to a decrease in the pH value, i.e. an increase in uterine activity above the optimal contractile activity leads to an increase in the frequency of fetal hypoxia, since uterine contractions are a repeated stress for the fetus during labor.

The intensity of contractions increases from 30 mm Hg at the beginning of labor to 50 mm Hg at the end of the first stage of labor. The frequency of contractions increases from 3 to 5 contractions per 10 minutes and the basal tone of the uterus from 8 to 12 mm Hg. In primiparous women, the intensity of uterine contractions is greater than in multiparous women.

Domestic clinicians have long noted the fact that labor intensifies when the mother is in the side position, corresponding to the position of the fetus.

Caldeyro-Barcia (1960) formulated the "law of position" when the woman in labor lies on her side (right or left) - uterine contractions increase with a simultaneous decrease in the frequency of contractions compared to the position of the woman in labor on her back. Practical recommendations follow from this - in the presence of so-called tachysystole (frequent contractions) and hypertonicity of the uterus, as well as in the presence of uncoordinated uterine contractions during spontaneous labor and a small opening of the uterine os (by 1 cm), on the one hand, a decrease in basal tone and a decrease in the frequency of contractions and an increase in the intensity of uterine contractions are noted. On the other hand, uterine contractions on the side become coordinated, but the mechanism of this action is unknown. The law of position is noted in 90% of women in labor during spontaneous labor and in 76% during labor induced by oxytocin. The difference in average values when changing position is 7.6 mm Hg in contraction intensity and 0.7 contractions per 10 min in contraction frequency. Interestingly, no differences were noted in the prenatal period and in the dilation period.

Thus, in the presence of frequent contractions, combined with hypertonicity of the uterus, the woman in labor must be placed on her side. Some scientists, for example Pinto, believe that the mechanical concept of the relationship between uterine activity and cervical dilation exists only at the end of the second period (the period of expulsion) and in the afterbirth period, but not in the period of dilation.

The main indicators of uterine contractility are tone and excitability. Uterine tone can be assessed by palpation through the abdominal wall or using a tonometer.

It is noted that the most important feature of the contractile activity of the uterus during the normal course of labor is the presence of regular and coordinated contractions of the uterus, which, as labor progresses, increase in strength and duration and decrease from the fundus to the body and then to the lower segment of the uterus.

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