Fetal breathing is carried out by gas exchange through the utero-placental system through diffusion. There is an absorption of oxygen from the mother's blood and the release of metabolic products into it. Acidic and alkaline metabolic products are neutralized by the fetal blood buffer system. In the development of hypoxia, unoxidized metabolic products accumulate in the blood of the fetus, which in the first stages are neutralized by the fetal blood buffer system, and when the resources of the fetus are depleted, they cause a pronounced metabolic acidosis, which is manifested in a decrease in pH, an increase in base deficiency, a decrease in the number of buffer and standard bicarbonates, the partial pressure of CO 2.
Thus, the determination of the respiratory function of the fetus and the acid-base state (CBS) of its blood is the main and most reliable criterion for revealing the pathology of the fetus.
In the study of the acid-base state of fetal blood, Zaling's method, proposed in 1962, was widely used. The essence of the method consists in studying the acid-base state of blood microporations taken from the present fetal part into the previously heparinized capillary with sufficient opening of the uterine throat (not less than 4 cm). The study is conducted on a micro-Astrop device. At a pH of fetal blood of 7.25 and above there is no hypoxia, but the results can be considered reliable only in the next 15-30 minutes. Rapid reduction in the pH of fetal blood is a poor prognostic sign. When the signs of acidosis increase in fetal blood and in the placenta, enzymes are activated and the content of RNA and DNA decreases, which leads to a decrease in the activity of myofibrils. Consequently, with the weakness of labor, fetal hypoxia is always possible. Active management of labor with the application of rhodostimulation also leads to a decrease in the pH of fetal blood, and consequently, to its hypoxia.
Determination of the components of the acid-base state of fetal blood is the most reliable indicator of its condition. It should be emphasized, however, that the Zaling assay, along with high diagnostic value, also has a number of disadvantages that make its application difficult or even impossible in some cases. These disadvantages include the inability to conduct research with a whole fetal bladder, small openings of the uterine throat, low attachment of the placenta. In addition, there is a risk of infection of the fetus with frequent sample production and changes in the acid-base state when the test material is in contact with the ambient oxygen. The need to use sophisticated diagnostic equipment also applies to shortcomings.
The presence of deficiencies in the Zaling assay served as an incentive for the search for methods for determining the fetus's condition, correlated in importance and lacking these shortcomings. The presence of a single maternal-fetal blood circulation made it possible to presume the possibility of diagnosing the fetal status from the data of the acid-base state of the mother's blood. However, the study of blood COS of pregnant and maternity patients can not be considered a reliable criterion for determining the state of the fetus.
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