Role of hemodynamic system of uterus in labor process

Persistent meeting the trophic needs of fetus and uterus itself is one of the most important biological tasks of hemodynamic system of uterus. The necessity to minimize risks for fetus resulting from the influence of the transition from intrauterine to extrauterine existence has resulted in a discreet nature of labor process, allowing to alterate the stressful contractions with “rest” that gave rise to a uterine cycle called “contraction-pause”. There is no doubt that the necessity to minimize risks of fetus brain damaging that is sensitive to hypoxemia both during pregnancy and labor gave rise to the development of a special from of relations between the mother and fetus which is a unique phenomenon – uteropacental bloodcircuit. Surely, the tough dependence of uteroplacental hemodynamics on the myometric phactor promoted the evolutional development of this kinf of interaction of these systems both in the sphere of mutual regulation and combining their biomechanic possibilities. Besides, in labor both the functions (simultaneous participation in meeting trophic needs of fetus, in biomechanics of cervical dilatation and advancement of fetus through the parturient canal) are combined as much as possible.

The role of hemodynamic system in biomechanics of uterine contraction is double. By using one and the same phenomenon - forced blood depositing into special venous reservoirs (lacunary transformed vessels of the internal vascular layer of muometrium of the uterus body, dividing venous sinus, and lower segment and cervix), hemodynamic system of uterus directly participates in formimg the so-called “hydraulic wedge” and the process of the dilatation of the uterus orifice. In other words, two subsytems of the hemodynamic system are formed in uterus during labor. These subsytems have different tasks by using the same principle. The hemodynamic system of uterus, in particular, dividing venous sinus, venous depot of the internal muscular layer and intervillous space generate labor forces together with the contracting myometrium. The forces attempt to push a part of fetal-amniotic complex outside the contraction ring into the cavity of the exiting segment of uterus. Despite the seeming simplicity of the process of the hydraulic wedge formation, it is quite complex as it has a lot of components participating in the process as well as because of the principles of their self- and mutual regualtion. All the forced processes in the uterus body during physiological uterine contraction are carried out under the “shell” of the muscular layer which retains the volume of the internal invironment of uterus (dividing venous sinus, internal muscular layer and intervillous space and parts of fetal-amniotic complex higher than the contraction ring) during the entire systole of contraction due to its isometric contraction type. It is important that this mode of retaining the volume of the internal invironment of uterus is kept until a part of the volume (fetal-amniotic complex relocated ouside the contraction ring) starts descresing is defined. Three components of the hemodynamic system of uterus body can change their volume within the total volume of the internal environment of the uterus body. These are dividing venous sinus, lacunary transformed venous system of the internal muscular layer and intervillous space. Mechanoreceptoric stretching feedback mechanism also “stands guard” over keeping the volume that by using all its abilities to influence the hemocirculation in uterus body.

Untimely mazolysis and Couvelaire uterus may take place without this process. Consequently, the stable volume of the internal environment of uterus body during systole and part of contraction diastole is ensured by two myometrically dependant mechanisms which а) throw the blood suplus into vascular depots of the lower segment and cervix and b) regulate forced blood depositing in venous collectors and intervillous space.

The force becomes expulsional not because of shortening of the functional contracting modules, but as a result of appearing extraovularhydraulic volume in elements of hemodynamic system of uterus that relocates parts of fetal-amniotic complex outside the cavity of uterus body by using usual law of nature. Under the influence of this expulsion force the lower pole of the fetus (presenting part, fetal bladder) become the hydraulic wedge which, as has been proved by direct study, physically influence the lower third of the lower segment at the latent phase of labor, and the cervix from the end of the latent and till the end of the active dilatation phase. Effectiveness of the wedge influencing deformed parts of the exiting section of uterus at the first stage of labor depends on line tension of functional modules, but it is defined not only by intrauteral tension but also by extraovular hydrodynamic volume of blood pushed from uterus body into a vascular depot of the lower segment and cervix. It is the first “golden rule” of biomechanics of human uterine contraction.

Extraovular expulsional volume defining effectiveness of mechanic hydraulic wedge towards the deformed sections of the exiting segment of uterus is modulated by both functional capacity of the vascular depot of the lower segment and cervix and the ability of its tissue to deformation.

The actual effectiveness of myometric and hemodynamic systems of uterus body which is its central segment is defined by morphofunctional state of the lower segment and the cervix, in particular, full-value development of lacunar transformation of their venous link and tissue pliability to transformation (coefficient of elasticity). It is the second “golden rule” of biomechanics of human uterine contraction.

High level of resistence towards the deforming efforts of the tissue of the lower thirs of the lower segment and cervix shows the same high resistance towards the advancement of the hydraulic wedge, holding it from moving outside the contraction ring. Underdevelopment and decfective completion of the process of lacunar transformation of vessels of the lower segment and vesself of cervix dramatically lower the functional capacity of vascular depot of the exiting section of uterus thus limiting the transportation of suplus blood deposited into vascular reservoirs of uterus body. These two are the direct reasons for activation of myometric mechanisms that limit the deposited blood and, consequently, lower the expulsional hydrodynamic extraovular volume of blood. For today these issues have been throrughly studied both at the practical and theoretical levels.

How does the hemodynamic system of uterus during labor carry out its function at the level of the lower segment and, especially, cervix?

We have mentioned that, during the latent phase of the dilatation period the lower segment is subject to deformation – its geometry gets transformed from a spherical layer into a cylinder. It is well-known that the lower pole of fetus distructively and radially affects the low-third of the lower segment. This is hydraulic wedge that directly participates in deformation of the low-third of the lower segment. It is clear that the tissue of the low-third of the lower segment has a direct mechanic connection with that of cervix, the action of hydraulic wedge automatically broadcast these attempts to the adjoining tissue of cervix. As deformation of the low-third of the lower segment during the latent phase of the dilatation period takes place, there is also shortening and smoothing of cervix. Our study has shown that contractions of muscles of the lower segment not having a direct mechanic connection with a muscle ot uterus body start a few seconds later the same process in a muscle ot uterus body. This fact proves that contractions of muscles of the lower segment of uterus is induced bythe impuls of its tissue extrastretching, that is generated by the hydraulic wedge. Consequectly, myometium of uterus body, its lower segment and the hemodynamic system of uterus forming the hydraulic wedge during the physiological uterine contraction are all involved in the deformation of tissue of the low-third of the lower segment and the processes of shortening, smoothing and dilatation of cervix. While considering the events taking place in the exiting section of uterus during the physiological contraction we have managed to establish that a significant thickening of tissue of the lower segment is observed at the peak of blood depositing into vascular reservoirs of uterus body and, consequently, at the peak of rising the volume of the exiting section of uterus. Besides, this thickening of tissue is as significant and transient that it can be related only to the process of force boold depositing into its lacunary transformed vessels when an isometric contraction of the tissue of the lower segment located under the deforming influence of the hydraulic wedge (stretching effect). The process of force boold depositing into vascular depot of the lower segment increses the volume of vessels thus fininshing the effect of stretching on the reinforcing them functional contracting modules. This resuls in the rise of line tension of muscle through the extrastretching feedback mechanism. It is implemented in a higher speed of intracavitary tension in the functional hydrodynamic cavity of the lower segment. Dynamics of intracavitary tension in the lower segment additionally reinforces the effect of the hydraulic wedge also during the cervix deformation. After the process of the tissue deformation has been finished it is converted into the energy for cervical dilatation during the atie phase of the first stage of labor.

This biomechanically unique phenomenon simultaneously participating in cervix deformation and lowering resistance to this deformation, functions during physiological course of labor in mature cervix.

Besides, matured cervix is transformed into a cavernous body and its vascular depot becomes a “hydraulic continuation” of similar cascualr lumps of the lower segment and internal mascular layer of uterus boby. Arteriolar-venous shunts are developed in it by the moment of labor. This circumstance creates special conditions for depositing blood into lacunary transformed vessels of cervix. Forced hyperaemia of cervical tissue is observed even in the pause after a contraction ends, and it continues with a recerse deformation of orifiece, its tensity lowers. It means that when the effect of forced blood depositing does not influence the tissue of cervix, the temporary pause is much shorter than the corresponding pause that is observed during a physiological uterine cycle in the tissue of the lower segment and internal muscular layer of body of uterus, which stays under strict control of a myometric factor.

Cervix during labor has physiologic characteristics of a viscoelastic body. Force depositing of blood in its lacunary transformed vessels increases its volume thus stretching the tissue of its connective-tissue matrix and acffecting its viscous properties. During each of uterine cycle this stretching leaves residual transformation and lowers its ability to resist deformation during the next uterine cycle.

Thus, force blood depositing affects the tissue of connective-tissue matrix during the entire first stage of labor turning into a system of discreet preparation of the connective-tissue frame of cervix for the following uterine cycle. As the deformed part of cervix gains a full contact with the lower pole of fetus through which it aquires an incompressible and warp-free basis, force blood depositing generates a radically-directed force that directly participates in radial stretching of a certain part of uterus. After the cervix smoothing process is complete, this force is active during the whole tissue stratum of cervix joining the radially-directed force of deformation, that is generated by the hydraulic wedge. This process of efforts of nyometric and hemodynamic systems of uterus during labor ends at the moment of total dilatation of uterus wedge. It is the moment when uterus is transformed from a bearing organ into an expulsing one and placenta. The second stage of labor takes place. It has its own peculiarities of biomechanics that requires be separated studying and describing.

Conclusion

After existing for three decades the concept of biomechanics of human uterine contraction by Professor G. Savitsky has turned into a doctrine which not only relates to the theory of uterine action, but also directly linked to obstertrics practice. Like any doctrine it does not only exist but also has the basis for its further development and, unquestionably must have its critics. The elements of development and improvement are obvious, but there is an absence of serious and grounded criticism. During the three decades no work has been published in the press that can be considered as a denial of the main propositions of the concept. Our works are quoted quite often to understand that the basis proposiions of the concept are known at least to the part of the obstetric community who read not only their own memoirs. Our home literature publishes memoirs, and theses that to this or that extent discuss some aspects of the concept, some even confirm some of its propositions though without mentioning the existence of such a concept. The only pleasant exception was the spectacular doctoral thesis by F. Zabolaev, which not only supports the main propositons of the concept by Professor G. Savitsky, but also gives a grounded mening of the adaptively transformed venous sytem of uterus in biomechanic of uterine contraction at a serious methodical level. This situation is parnormal and such apophasis or scientifically unmotivated avoiding criticism and discussion does not promote development, first of all, native obstetrics. It is obvious fro the propositions of the concept that it is necessary to:

· Further develop all aspects of functional morphology of uterus during labor with the help of modern technologies of research.

· Multi-aspect and multi-component monitoring of nemodynamics problems of uterus during labor in all blood circuits.

· Study problems of functional morphology of cervix, in particular, those that are linked to the process of its cavernous transformation before delivery.

· Develop a scale of assessment of the contracting activity of uterus during labor, which would aloow for a continuous, adequate and safe monitoring of dynamics of changes in the intracavitary tension.

· Develop mechanism of control over dynamics of deformation processes of cervix during labor.

It is also necessary to finally overcome the stagnation period in studying problems of biomechanics of uterine contraction, that has lasted over half a century. The concept by Professor G. Savitsky has a logical and absolutely evidentiary scientific and practical basis. Today this concept can well be used as a basis for development of a modern view at the labor action as well as for forming a cybernetic model of human uterine contraction.

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