Aspiration of meconium and amniotic fluid

Meconium aspiration syndrome (MAS) is a respiratory distress disorder in the newborn caused by the presence of meconium in the tracheobronchial airways. Fetal aspiration of meconium-stained amniotic fluid may occur antepartum or intrapartum and may result in airway obstruction, impaired alveolar gas exchange, chemical pneumonitis, and surfactant dysfunction. These pulmonary effects result in severe ventilation-perfusion mismatch. To complicate matters further, many infants with meconium aspiration have primary or secondary persistent pulmonary hypertension of the newborn as a result of chronic intrauterine stress and pulmonary vascular thickening. Although meconium is sterile, its presence in the airways may predispose the infant to pulmonary infection. Meconium aspiration is essentially a clinical diagnosis and should always be suspected in a baby with respiratory distress and meconium-stained amniotic fluid at birth.

The passage of meconium in cephalic presentations has long attracted the attention of obstetricians. However, to date, the role of meconium as a sign of fetal distress has not been definitively established; the causes and mechanism of its passage, as well as the significance of the time of meconium passage for the outcome of labor, have not been fully clarified.

The frequency of meconium passage fluctuates between 4.5 and 20% and on average accounts for 10% of births with cephalic presentation of the fetus even with optimal management of the pregnant woman. The discrepancy in the frequency of meconium detection is explained by the different contingent of pregnant women and women in labor examined. A number of authors indicate that the presence of meconium in the amniotic fluid does not indicate hypoxia either at the time of the study or establish the period of its development, and therefore cannot serve as an absolute criterion for assessing the condition of the fetus during labor.

Other researchers associate this fact with a reflex reaction of the fetal intestine to some irritations that could have been noted long before the study.

[ 1 ]

Meconium aspiration is more common in postterm infants. Its incidence varies with gestational age. One study reported meconium aspiration in 5.1%, 16.5%, and 27.1% of preterm, term, and postterm infants, respectively.[ 2 ]

It is believed that the passage of meconium indicates a threatening condition of the fetus.

Most researchers indicate that the presence of meconium in the amniotic fluid increases the incidence of fetal hypoxia, perinatal mortality, and morbidity in newborns. In cases where the amniotic fluid is transparent at the onset of labor, perinatal mortality is low, while with meconium-stained fluid, the rate increases to 6%. In the presence of meconium in the amniotic fluid, a severe complication of the neonatal period is meconium aspiration syndrome, which leads to high mortality in newborns. However, only 50% of newborns whose amniotic fluid was stained with meconium at birth had primary feces in the trachea; in the latter group, if measures were taken, respiratory disorders (respiratory distress) developed in % of cases. Thus, the average incidence of symptomatic meconium aspiration syndrome is 1-2%. Aspiration syndrome is observed in post-term infants, those born at term but in a state of hypoxia, and in children with intrauterine growth retardation. Meconium aspiration syndrome rarely occurs with normal fetal development if birth occurs before the 34th week of pregnancy.

It was found that the intrauterine fetus with the presence of meconium in the amniotic fluid has a lower oxygen tension in the umbilical vein than with clear waters.

Some authors associated the passage of meconium with random defecation of a normal fetus with an overstretched intestine, sometimes associated it with the action of various drugs. However, in many cases, the coloration of amniotic fluid with meconium signals a threatening condition of the fetus, as indicated by monitoring data and biochemical changes in the blood.

Therefore, at present, most authors tend to regard the presence of meconium in the amniotic fluid as a sign of the onset of fetal hypoxia.

How does meconium aspiration develop?

Fetal hypoxia can cause mesenteric vascular spasm, intestinal peristalsis, relaxation of the anal sphincter, and passage of meconium. Compression of the umbilical cord stimulates a vagal response leading to passage of meconium even in a normal fetal condition. Convulsive respiratory movements both intrauterine (as a result of fetal hypoxia) and immediately after birth contribute to the aspiration of meconium into the trachea. The movement of meconium into small-caliber respiratory tracts occurs quickly, within 1 hour after birth.

The consequence of meconium aspiration is early mechanical obstruction of the airways with gradual development of chemical pneumonitis after 48 hours. Complete obstruction of the small airways leads to subsegmental atelectasis. They are adjoined by zones of increased aeration, arising due to the valve effect ("ball valve") during partial obstruction and the formation of "air traps". As a result, the ventilation-perfusion ratio and lung compliance decrease, their diffusion capacity decreases, intrapulmonary shunting and airway resistance increase. Against the background of increased breathing and uneven ventilation, alveoli may rupture, leading to air leakage from the lungs.

Vasospasm and impaired microcirculation in the lungs determine long-term pulmonary hypertension and the development of extrapulmonary shunts.

Amnioscopy can detect meconium in the amniotic fluid before or during labor. Detecting amniotic fluid coloration and determining its optical density can be a valuable method for diagnosing fetal distress. There are isolated reports of the possibility of detecting meconium in the fluid using echography.

Meconium is a green-black viscous substance that fills the large intestine of the fetus. Its chemical composition, morphological and ultrastructural data are well studied.

It has been established that meconium particles measuring 5-30 µm are a type of glucoprotein containing sialomucopolysaccharide; when assessed spectrophotometrically, meconium has the highest adsorption at 400-450 µm. Studies have shown that an increase in the serotonin level in waters by more than 2 times obviously leads to increased intestinal peristalsis. Predisposing factors are:

• hypertension;
• diabetes mellitus;
• isoimmunization;
• late toxicosis of pregnancy;
• Rhesus conflict;
• mother's age;
• number of births and abortions;
• history of stillbirth;
• collisions with the umbilical cord.

In case of cord entanglement, meconium discharge during labor is observed in 74%. It has been established that labor ends more quickly after the rupture of the fetal bladder and the discharge of green amniotic fluid, which may be associated with the high content of oxytocin in meconium. In case of weak labor, meconium discharge is detected in every fifth woman in labor. The significance of fetal factors influencing the discharge of meconium into the amniotic fluid has not been sufficiently studied. These include:

• hyaline membranes;
• pneumonia;
• chorioamnionitis;
• erythroblastosis.

The passage of meconium is more often observed when the fetus weighs more than 3500 g, and in children weighing less than 2000 g, meconium is passed extremely rarely, which may be due to its insignificant accumulation in the intestines of the fetus during premature birth or the reduced sensitivity of premature babies to a hypoxic state.

During labor, the fetus may aspirate amniotic fluid, both pure and containing microorganisms (even pus) and blood. This can cause transient tachypnea or persistent pulmonary hypertension. If the fluid is purulent, antibiotics are administered to prevent pneumonia.

[ 3 ], [ 4 ], [ 5 ]

The tactics of pregnancy and childbirth management in the presence of meconium in the waters have not been finally resolved. There are isolated reports on the importance of the time of meconium discharge and the degree of its coloration on the outcome of childbirth for the fetus and newborn.

It is noted that the coloring of the amniotic fluid after the discharge of meconium first appears in the bottom of the uterus in cephalic presentations of the fetus. Then the entire mass of amniotic fluid, including the anterior, is colored. The coloring of the nails and skin of the fetus with meconium pigments, as well as the flakes of caseous grease, is directly dependent on the time of meconium discharge: the coloring of the nails of the fetus occurs after 4-6 hours, and the flakes of grease - after 12-15 hours.

It is also suggested that meconium may appear in the second trimester of pregnancy and remain there until the onset of urgent labor, during which it is interpreted as a sign of a violation of the vital functions of the fetus. There is also evidence that the appearance of meconium in the waters is a sign of fetal death in the second trimester of pregnancy.

During labor, early meconium in the amniotic fluid is observed in 78.8%, and later - in 21.2%. Early minor meconium entry into the amniotic fluid, observed in 50% of pregnant women with meconium-stained water, was not accompanied by an increase in the morbidity or mortality of fetuses and newborns. Massive meconium entry was accompanied by increased morbidity and mortality of newborns in complicated pregnancy.

There are conflicting opinions regarding the diagnostic significance of the nature of meconium found in the amniotic fluid. Some authors believe that uniform meconium staining of the amniotic fluid indicates prolonged fetal distress, while suspended lumps and flakes indicate a short-term fetal reaction. An increase in meconium content is an unfavorable prognostic sign.

Some authors characterize light green meconium as "old, liquid, weak" and more dangerous for the fetus, and dark green as "fresh, recent, thick" and less dangerous, since its connection with perinatal mortality has not been established. In contrast, Fenton, Steer (1962) indicated that with a fetal heart rate of 110 beats/min and the presence of thick meconium, perinatal mortality was 21.4%, with weakly colored waters - 3.5%, with clear waters - 1.2%. It was also established that with the presence of thick meconium in the waters and the opening of the cervix by 2-4 cm, a decrease in the pH of the fetal blood occurs.

Moreover, a correlation has been established between the nature of meconium, the pH of fetal blood, and the condition of newborns according to the Apgar scale. Thus, according to research data, with thick meconium staining of the waters at the beginning of labor, the pH of fetal blood was below 7.25 in 64%, and the Apgar score in 100% was 6 points or lower. At the same time, the presence of meconium in the amniotic fluid without other symptoms (acidosis, fetal heart rate decelerations) cannot be regarded as evidence of deterioration in the condition of the fetus and, in this regard, there is no need to force delivery. At the same time, whenever fetal heartbeat abnormalities appear, in the presence of meconium in the waters, the risk to the fetus increases compared to clear waters.

In order to reduce the risk of complications for the fetus and newborn associated with asphyxia, in the presence of meconium in the waters, it is recommended to resort to operative delivery at a pH of 7.20 and below. If there are abnormalities in the fetal heart rate according to cardiotocography, then delivery is indicated in case of preacidosis (pH 7.24-7.20).

In this regard, in labor, when the water is stained with meconium, most researchers roll out the advisability of monitoring the condition of the fetus. When conducting a comprehensive assessment of the condition of the fetus during labor, it is possible to reduce perinatal mortality in the presence of meconium in the water to 0.46%.

The frequency of surgical interventions in the presence of meconium in the waters is 25.2% versus 10.9% in clear waters.

It is important to note that during a cesarean section, meconium may enter the abdominal cavity, which may result in a granulomatous reaction to a foreign body, which may result in adhesions and abdominal pain.

One of the severe complications of the neonatal period with the presence of meconium in the water is meconium aspiration syndrome, the incidence of which ranges from 1 to 3%. It is more often found in fetuses with early and abundant meconium than with its easy and late passage. With thick meconium staining of the amniotic fluid in the initial period of labor, its aspiration occurs in 6.7%. It is noted that with the passage of meconium in the amniotic fluid, 10-30% of newborns develop respiratory disorders of varying degrees. Meconium aspiration syndrome is more often observed in full-term and post-term infants with acute hypoxia. Hypoxic stress leads to an increase in fetal respiratory movements, and the meconium-stained amniotic fluid is aspirated. Meconium particles penetrate deep into the alveoli, causing chemical and morphological changes in the lung tissue. In some cases, meconium aspiration may occur in a more chronic form, which may contribute to the development of acute intrauterine pneumonia.

Meconium aspiration is an important cause of neonatal mortality, with rates, although lower than those of hyaline membrane disease, still representing a high percentage - 19-34%. Therefore, meconium aspiration syndrome is an important clinical problem faced by neonatologists in the intensive care unit.

To prevent the development of respiratory pathology in newborns, most authors point to the need to reduce aspiration to a minimum during labor. Aspirated meconium should be sucked out with a catheter for 2-3 hours. The need for careful management of labor and immediate suction of meconium from the upper respiratory tract is an important preventive measure to prevent neonatal mortality.

Thus, the data available in the literature indicate that the diagnostic and prognostic value of meconium in amniotic fluid has not been definitively established. However, most authors regard the presence of meconium in amniotic fluid as a sign of fetal distress.

Monitoring observation during labor using modern diagnostic methods (cardiotocography, amnioscopy, determination of the acid-base balance of fetal blood, pH-metry of amniotic fluid) in women in labor with the presence of meconium in the water allows us to clarify the condition of the fetus during labor and determine further labor tactics.

At the end of physiological pregnancy, in the absence of any abnormalities in the fetus's condition, the characteristic amnioscopic picture is a moderate amount of transparent (less often "milky") waters with a moderately high content of easily mobile flakes of caseous grease. The detection of meconium in the waters is regarded as a sign of fetal distress. Meconium pigments color the waters green. This coloration persists for a long time and can be detected after several hours and days. E. Zaling's calculations showed that with a living fetus, at least 4-6 days are required to eliminate meconium from the amniotic cavity. Consequently, it is impossible not to notice meconium when monitoring every 2 days. It has been noted that asphyxia of newborns is observed 1.5-2.4 times more often in the presence of meconium in the waters than in clear waters.

In order to improve the diagnostics of the fetus condition during labor in the presence of meconium in the amniotic fluid, a comprehensive assessment of the fetus condition was carried out, including cardiotocography, amnioscopy, determination of the acid-base state of the blood of the fetus and the mother, and monitoring pH-metry of the amniotic fluid. A clinical analysis of the course of labor was carried out in 700 women in labor, including 300 with the presence of meconium in the amniotic fluid; in 400 women in labor (control group) - 150 women in labor with timely discharge of water and 250 women in labor with untimely discharge of water. Clinical and physiological research was conducted in 236 women in labor.

The obtained information array of 148 features was statistically processed on an ES-1060 computer using an American package of applied statistical programs.

The conducted studies established that the number of abortions and miscarriages in the anamnesis was 2-2.5 times higher in the group with meconium in the water. Among women giving birth again, 50% of women had complicated previous births (surgical interventions, intrapartum death of the fetus), which was not observed in the control group of women in labor. Almost every second woman in labor in the main group had complicated pregnancy. It should be emphasized that only women in labor in the main group suffered from nephropathy. Edema and anemia of pregnancy were twice as common in women in labor with meconium in the water.

Older primiparous women also predominated in the main group, which confirms the opinion of the above-mentioned authors about the importance of the age of the mother in the passage of meconium.

Obviously, in case of severe concomitant diseases of the mother and complications of pregnancy, the conditions of nutrition and gas exchange of the fetus change first of all, caused by the disruption of uteroplacental blood circulation, which can lead to the passage of meconium into the amniotic fluid.

A certain dependence was revealed between the clinical course of pregnancy and labor and the condition of the fetus and newborn. Thus, a high dependence was revealed between nephropathy both during pregnancy and labor, weakness of labor, abnormalities of head insertion, umbilical cord entanglement around the fetus's neck and low Apgar scores of newborns. Every third mother in labor suffering from nephropathy (35.3%) and weakness of labor (36.1%) had newborns with an Apgar score of 6 points or less. Studies have shown that with nephropathy, the fetus experiences hypoxia only during the passage of meconium; asphyxia of newborns increases by 2.5 times compared to the control. It should be noted that the passage of meconium depends not so much on the degree of toxicosis as on its duration.

In women in labor with the presence of meconium in the amniotic fluid, a longer duration of labor was observed (13.6 ± 0.47 h) compared to the control group (11.26 ± 0.61 h).

Every second newborn born in asphyxia had the umbilical cord wrapped around the fetus's neck (50%), and every fifth (19.4%) had anomalies in the insertion of the head.

Complications of childbirth determined the high percentage of operative deliveries (14.33%), in the structure of which cesarean section accounted for 7.66%, obstetric forceps and vacuum extraction of the fetus - 6.67%.

Despite the fact that the literature contains reports of a low correlation (22.3%) between surgical interventions and meconium staining of amniotic fluid, a high dependence between the method of delivery and low Apgar scores has been revealed. Thus, asphyxia of newborns during the application of abdominal obstetric forceps was observed in 83.3%, during vacuum extraction of the fetus - in 40%, and cesarean section - in 34.7%.

Acceleration of the birth of the fetus by activating labor (quinine, oxytocin), as well as the use of obstetric forceps and a vacuum extractor, aggravates the pathological condition of the fetus, which is on the verge of failure of compensatory capabilities. In the presence of meconium in the waters and the phenomena of metabolic acidosis in the fetus, even a physiologically proceeding labor act can be such a load that at any moment it can lead to a failure of the compensatory mechanisms of the fetus.

Asphyxia of newborns, observed in 12% with the presence of meconium in the water, was the cause of a severe complication of the neonatal period - meconium aspiration syndrome (16.65%). Hypoxic stress leads to an increase in fetal respiratory movements and aspiration of amniotic fluid. Meconium aspiration syndrome is an important cause of neonatal mortality. According to our observations, meconium aspiration syndrome in asphyxia of newborns led to a fatal outcome in 5.5%, which is consistent with the literature data indicating an increase in perinatal mortality in this pathology to 7.5%.

Thus, the data convincingly show that the admixture of meconium in the waters should be regarded as a sign of fetal distress. The clinical and physiological study showed that in the presence of meconium in the waters, the acid-base balance indices of the fetal blood differ significantly from those in the control group. A significant decrease in the blood pH (7.26 ± 0.004) and base deficit (-6.75 ± 0.46) already at the beginning of labor in the presence of meconium in the waters indicates the strain of the compensatory mechanisms of the fetus. Our observations indicate the depletion of the reserve capacities of the fetus in the presence of meconium in the waters, which made it possible to detect preacidosis in its blood (pH 7.24-7.21) at the beginning of labor in 45.7%, and at the end of the dilation period - twice as often (80%), which is consistent with the data of Starks (1980), in whose studies significant acidosis in the blood was noted in fetuses that had passed meconium.

In the group of newborns with an Apgar score of 6 points or less, the fetal blood acid-base balance (ABS) indices reflect pathological acidosis: at the beginning of labor, pH is 7.25 ± 0.07; BE is 7.22 ± 0.88; at the end of the dilation period, pH is 7.21 ± 0.006; BE is 11.26 ± 1.52; an increase in pCO2 _, especially in the second period of labor (54.70 ± 1.60), indicates the presence of respiratory acidosis.

The results of the studies revealed a relationship between the fetal blood acid-base balance indices and low Apgar scores of newborns in the presence of meconium in the amniotic fluid. The mother's blood acid-base balance indices in these cases do not differ from the unambiguous values in the control group and are within physiological limits. Delta pH does not provide additional diagnostic information, since this index changes almost exclusively due to the fetal component. These data contradict the reports of some authors indicating a change in the mother's blood acid-base balance associated with intrauterine fetal hypoxia.

A clear correlation was found between the pH of fetal blood and the pH of amniotic fluid. Lower pH values of meconium-stained amniotic fluid (7.18 ± 0.08) at the beginning of labor and 6.86 ± 0.04 at the end of the dilation period fall within the "prepathological zone" - a high-risk zone for the fetus, and reflect the depletion of the compensatory resources of the intrauterine fetus.

In case of fetal hypoxia, the pH of the water decreases to 6.92, in case of mild asphyxia it is 6.93, in case of severe asphyxia - 6.66. In case of fetal hypoxia, the decrease in the pH of the water and fetal blood is caused by the release of a large amount of acidic metabolic products from the fetal body into the amniotic fluid. A decrease in the pH of the amniotic fluid (6.67 ± 0.11 at the beginning of labor and 6.48 ± 0.14 at the end of the second period of labor) in the group of newborns with low scores on the Apgar scale indicates severe acidosis, especially in the second period, when the reaction of the amniotic fluid shifts significantly to the acidic side, and the more significant it is, the more severe the condition of the fetus. 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. A decrease in the buffer capacity of the waters is manifested in fetal hypoxia and the presence of meconium is manifested in the form of an increase in the intra-hourly fluctuations in the pH of the waters to 0.04 ± 0.001 versus 0.02 ± 0.0007 in the control in the presence of light amniotic fluid. In addition, an increase in the intra-hourly fluctuations in the pH of the amniotic fluid can occur earlier than a decrease in the absolute value of their pH, which allows for the timely detection of the initial signs of fetal distress during labor.

Cardiotocography in the presence of meconium in the waters leads to a decrease in the amplitude of oscillations (6.22 ± 0.27) and myocardial reflex (10.52 ± 0.88), which indicates a decrease in the reserve capacity of the fetus and is consistent with the results of Krebs et al. (1980).

In the presence of meconium in the waters, pathological decelerations were registered four times more often (35.4 ± 4.69) than in clear waters (8.33 ± 3.56), indicating a violation of the vital functions of the fetus. However, in our observations, false-positive and false-negative results were noted. Thus, with normal indicators of the acid-base balance of the fetus's blood, pathological decelerations were registered in 24% of cases, while in the presence of acidosis in its blood, normal cardiotocography indicators were registered in 60%.

The appearance of meconium with normal CTG values and normal pH of fetal blood may be a temporarily compensated stage of the disturbance of its vital functions; however, whenever disturbances of the fetal heartbeat appear in the presence of meconium in the waters, the risk for it is greater than with clear waters.

To determine the diagnostic significance of various methods for assessing the condition of the fetus in the presence of meconium in the waters, we conducted a correlation analysis for the first time, allowing us to establish a connection between various signs. Correlation matrices were compiled for each group separately and for each stage of the birth act.

In the presence of meconium in the amniotic fluid, the pH of fetal blood correlated highly with the pH of the fluid and its intrahourly fluctuations, late decelerations; the pH of the fluid stained with meconium entered into a correlation with the myocardial reflex, oscillation amplitude, and decelerations. The average frequency correlated with decelerations.

A high correlation with the Apgar score was found for the pH of fetal blood, pH of amniotic fluid, intra-hourly fluctuations in pH of amniotic fluid, late decelerations, and pCO2 _of fetal blood. No correlation was found between the pH of fetal blood and that of the mother.

The conducted study allowed us to develop a method for comprehensive assessment of the condition of the fetus during labor in the presence of meconium in the amniotic fluid:

• During labor, all women in labor undergo cardiotocography to determine the average fetal heart rate, oscillation amplitude, myocardial reflex value, and pathological decelerations. Regardless of the CTG readings, amnioscopy is performed;
• If meconium is detected in the waters, the amniotic sac is opened and the acid-base balance of the fetal blood is examined using the Zaling method;
• if the fetal blood acid-base balance indicates intrauterine distress, an emergency delivery is performed;
• If the pH of the waters is consistently favorable, the condition of the fetus is further monitored until the end of labor; if acidosis in the amniotic fluid increases, the Zaling test is repeated.

The main complications of pregnancy in the presence of meconium in the water are late toxicosis (28.9%) and anemia of pregnant women (12%), which occur in them twice as often as in the control group.

In women giving birth with the presence of meconium in the waters, the main complications of the birth act are abnormalities of labor (31.3%), nephropathy (19.3%), umbilical cord entanglement around the fetus’s neck (21%), and abnormalities of head insertion (4.6%), observed twice as often as in the control group.

In the presence of meconium in the water, a high frequency of surgical interventions is noted (14.33%), in the structure of which caesarean section accounts for 7%, obstetric forceps application - 2% (abdominal), abdominal vacuum extractor - 1.67%.

In the presence of meconium in the water, neonatal asphyxia occurs 6 times more often than in the comparison group. A severe complication of the neonatal period - meconium aspiration syndrome - is the cause of death in 5.5% of newborns.

Multivariate discriminant analysis made it possible to predict operative delivery in the interests of the fetus in 84% of women in labor with the presence of meconium in the waters, and the condition of the newborn in 76%.

The high frequency of complications during pregnancy, childbirth, surgical interventions, as well as comprehensive monitoring of the condition of the fetus, allows us to classify women in labor with the presence of meconium in the amniotic fluid into a high-risk group requiring intensive monitoring during childbirth.