Diagnostic value of pH-metry of amniotic fluid and fetal head tissue
Last reviewed: 23.04.2024
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Much attention is paid to the study of amniotic fluid for evaluation of the fetus. The movement of the liquid goes in the direction of the mother-fetus-water-the mother, while about 1/3 of the volume of amniotic fluid is replaced every hour. Changes in the homeostasis of the fetus cause a rapid reaction of the amniotic fluid (7-9 min).
Amblerous waters have complex biochemical composition and multifaceted purpose. The chemical composition of amniotic fluid largely depends on the fetal metabolism, and when a fetus appears metabolic acidosis compensates for it due to its own buffer capacity. The exchange of ions between the fetal blood and the amniotic fluid occurs quickly, 7 days after the mother breathes 10% CO 2 in the amniotic fluid, acidosis increases.
The study of the acid-base state of amniotic fluid revealed its high dependence on fetal blood vessels, which served as the basis for the study of water for the purpose of diagnosing the fetal condition.
Various methods of obtaining amniotic fluid for research have been proposed. These include abdominal amniocentesis, collection of free flowing amniotic fluid, puncture of the lower pole of the fetal bladder, institution of the catheter for the fetal part with periodic collection of water portions.
All authors who studied the acid-base condition of amniotic fluid noted a high correlation between these parameters and fetal blood vessels and, consequently, according to the water study, it is possible to judge the state of the fetus.
A number of authors, conducting a transcorrelation analysis between the pH values of the blood of the mother, fetus, amniotic fluid and the Apgar score of the newborn, established a high correlation between these indices. The correlation of the mother's CBS with these parameters was either not detected at all, or was weakly expressed.
It has been established that the buffer capacity of amniotic fluid is half the buffer capacity of the fetal blood, so that the depletion of its resources is faster and with acidosis the fetus is expressed to a much greater extent. The dependence of the degree of fetal hypoxia on the pH of amniotic fluid is established. In the process of the generic act, taking into account the degree of opening of the uterine throat, their buffer capacity is gradually depleted with a gradual decrease in the acidity of the amniotic fluid. At the same time, J. Gaal and L. Lampe (1979) investigated the shifts in the pH of the amniotic fluid during the entire period of labor, but even under normal conditions (delivery without complications), large fluctuations in these parameters do not give an opportunity to judge the threatening state of the fetus. The authors recommend the use of a transcervical catheter for general use, especially in those maternity facilities that are poorly equipped with equipment for intensive birth surveillance. With its help, it is possible to continuously monitor changes in the CBS of amniotic fluid, the appearance of meconium in the waters (especially the so-called "rear" water with the pressed head) during the entire period of childbirth.
[Interrelation between indicators of metabolism of mother, fetus and amniotic fluid
Mother - placenta - fetus - amniotic fluid - a single system of fluid exchange. The existence of a connection between the metabolism of the mother and the fetus is proved. The presence of metabolic acidosis in the mother leads to acidosis in the fetus, the latter can not be regarded as a sign of intrauterine hypoxia. On the other hand, when the fetus develops hypoxic acidosis, the components of the acid-base state of the mother's blood are in physiological limits. Attempts were made to compensate for the metabolic acidosis of the fetus by prescribing the mother to alkaline therapy. Until now, the question is being discussed whether sodium bicarbonate should still be administered in lactic acidosis. The introduction of sodium bicarbonate in lactic acidosis seems logical, if we bear in mind the decrease in myocardial function, associated with a decrease in intracellular pH. In acute circulatory failure, increased ATS by CO 2 was demonstrated . Partial conversion of bicarbonate to CO 2 is fraught with an increase in this phenomenon and a subsequent decrease in intracellular pH. Alternative bicarbonate preparations - carbi-carb, ТНАМ and dichloroacetate are discussed in the literature.
The question of a positive correlation between the pH of fetal blood and amniotic fluid should now be considered resolved. Undoubtedly, the development of acidosis in the fetus leads to the development of acidosis in the amniotic fluid. Therefore, the pH-metry of amniotic fluid, reflecting the degree of acidosis in the fetus, can serve as a method for diagnosing its condition, and with a monitor study it is possible to identify the initial signs of fetal hypoxia, determine the effectiveness of the therapy and rationality of the tactics of labor and choose the optimal method of delivery.
The study of amniotic fluid is carried out by means of a monitor - by the establishment of the sensor of the domestic apparatus "Express" for the present part of the fetus with the calculation of the indices for every 5 min of the study, as well as the determination for each hour of the study of the intrachasic index value of the amniotic fluid, .
Diagnostic value of the determination of pH of the fetal head tissue
The probable parallelism between the pH of the blood and the pH of the tissues served as a theoretical prerequisite for the development of a new diagnostic method based on the monitored pH-metry of the tissue of the presenting part of the fetus. The first attempts to use a tissue electrode were made in 1974, when this method of research was predicted to have a great future in obstetric practice, but the technical difficulties of using a tissue pH electrode have not been overcome to date, despite a significant amount of research in this field.
The main unsolved problems of tissue pH-metry concern the reliability of the technical performance of the method and the correlation of its parameters with the pH of fetal blood. The questions of correlation of tissue pH values and fetal blood pH have not been solved to date. In addition, there is no evidence of a reliable technique for diagnosing the fetal status according to the monitored pH-metry of the fetal head tissue.