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Diagnostic value of pH-metry of amniotic fluid and fetal head tissue
Last reviewed: 06.07.2025
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Much attention is paid to the study of amniotic fluid to assess the condition of the fetus. The fluid moves in the direction mother - fetus - water - mother, with about 1/3 of the amniotic fluid volume being replaced every hour. Changes in fetal homeostasis cause a rapid reaction of the amniotic fluid (7-9 min).
Amniotic fluid has a complex biochemical composition and a multifaceted purpose. The chemical composition of amniotic fluid largely depends on the metabolism of the fetus, and when metabolic acidosis occurs in the fetus, it compensates for it due to its own buffer capacity. The exchange of ions between the fetal blood and amniotic fluid occurs quickly; acidosis increases in the amniotic fluid within 7 minutes after the mother inhales 10% CO2.
A study of the acid-base balance of amniotic fluid revealed a high dependence on the acid-base balance of fetal blood, which served as the basis for studying the fluid in order to diagnose the condition of the fetus.
Various methods have been proposed for obtaining amniotic fluid for examination. These include abdominal amniocentesis, collection of freely flowing amniotic fluid, puncture of the lower pole of the fetal bladder, insertion of a catheter behind the presenting part of the fetus with periodic collection of portions of the fluid.
All authors who studied the acid-base balance of amniotic fluid noted a high correlation between these indicators and the acid-base balance of fetal blood and, therefore, based on the data from the study of the fluid, one can judge the condition of the fetus.
A number of authors, conducting a transcorrelation analysis between the pH values of the mother's blood, fetus, amniotic fluid and the newborn's assessment according to the Apgar scale, have established a high dependence between these indicators. The correlation of the acid-base balance of the mother's body with these indicators was either not revealed at all or was weakly expressed.
It has been established that the buffer capacity of the amniotic fluid is half the buffer capacity of the fetal blood, due to which the depletion of its resources is faster and in case of fetal hypoxia, acidosis is expressed to a much greater extent. The dependence of the degree of fetal hypoxia on the pH of the amniotic fluid has been established. During the labor act, taking into account the degree of dilation of the cervix, a gradual depletion of their buffer capacity with a gradual decrease in the acidity of the amniotic fluid is noted. At the same time, J. Gaal, L. Lampe (1979) studied shifts in the pH of the amniotic fluid during the entire period of labor, but even under normal conditions (labor without complications), large fluctuations in these parameters do not allow us to judge the threatening condition of the fetus. The authors recommend the use of a transcervical catheter for widespread use, especially in maternity institutions that are poorly equipped with equipment for intensive monitoring of labor. With its help, it is possible to continuously monitor changes in the acid-base balance of amniotic fluid, the appearance of meconium in the waters (especially the so-called “back” waters with the head pressed down) throughout the entire period of labor.
[ Relationship between maternal, fetal and amniotic fluid metabolic parameters
Mother - placenta - fetus - amniotic fluid - a single system of fluid exchange. The existence of a connection between the metabolism of the mother and fetus has been proven. The presence of metabolic acidosis in the mother leads to acidosis in the fetus, the latter cannot be considered a sign of intrauterine hypoxia. On the other hand, with the development of hypoxic acidosis in the fetus, the components of the acid-base state of the mother's blood are within physiological limits. Attempts have been made to compensate for fetal metabolic acidosis by prescribing alkalizing therapy to the mother. The question of whether sodium bicarbonate should still be prescribed for lactic acidosis is still under discussion. The administration of sodium bicarbonate in lactic acidosis seems logical if we keep in mind the decrease in myocardial function associated with a decrease in intracellular pH. An increase in the AVR for CO 2 has been demonstrated in acute circulatory failure. Partial conversion of bicarbonate to CO 2 is fraught with an increase in this phenomenon and a subsequent decrease in intracellular pH. Alternative drugs to bicarbonate are discussed in the literature - carbicarb, TNAM and dichloroacetate.
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, 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 monitoring study, it is possible to identify the initial signs of fetal hypoxia, determine the effectiveness of the therapy and the rationality of labor management tactics, and choose the optimal method of delivery.
We conduct the study of amniotic fluid using monitoring - by inserting the sensor of the domestic device "Express" behind the presenting part of the fetus with the calculation of indicators for every 5 minutes of the study, as well as determining for each hour of the study the indicator of intra-hour indicators of the pH value of amniotic fluid proposed by us, taking into account the degree of dilation of the cervix.
Diagnostic value of determining the pH of the fetal head tissue
The probable parallelism between blood pH and tissue pH served as a theoretical premise for the development of a new diagnostic method based on monitoring 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 research method 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 this day, despite a significant amount of research in this area.
The main unresolved issues of tissue pH-metry concern the reliability of the technical implementation of the method and the correlation of its indicators with the pH of fetal blood. The issues of correlation of tissue pH values and fetal blood pH have not been resolved to date. In addition, there is no data on the availability of a reliable method for diagnosing the condition of the fetus based on the data of monitoring pH-metry of the fetal head tissue.