Hemolysis is the rupture of red blood cells in the blood and the escape of hemoglobin. Red blood cells are important transport vehicles in our body. Without red blood cells, oxygen cannot be delivered anywhere in the body. It can be said that once hemolysis occurs, it is very easy to endanger life.
We all know that a child does not necessarily have the same blood type as the mother. Then the question arises: the blood vessels are connected, why did hemolysis not occur?
A very critical anatomical structure is involved here: the placental barrier. The so-called placental barrier is the barrier between the placental villous tissue and the uterine blood sinus. The chorion contains branches of umbilical blood vessels, and many villi of different sizes are issued from the chorion. These villi are dispersed in the maternal blood, absorb oxygen and nutrients in the maternal blood, and excrete metabolites.
A brief summary of the role of the placental barrier: good things, pass; bad things, stay. Because the fetus is very fragile, the important role of the placental barrier is to protect the fetus. Even the blood of the mother is not allowed to cross half a step.
Here comes the answer: Because of the separation of the placental barrier, the blood of the fetus does not communicate directly with the blood of the mother. Therefore, hemolysis generally does not occur.
A careful friend will find the word “general” in this answer. That is to say, there are “unusual” cases.
So, what is an unusual situation?
Here, we must mention a special case, hemolytic disease of the newborn, also known as hemolytic disease of the newborn. Hemolytic disease of the newborn refers to the immune hemolysis of the newborn caused by the incompatibility between the mother and the fetus. During pregnancy or childbirth, a number of fetal red blood cells can enter the mother. If they are incompatible with each other, and the mother lacks the antigens of fetal red blood cells, the mother will produce corresponding antibodies. The immune antibodies can enter the fetus through the placenta and cause hemolysis.
Seeing this, many friends are very afraid!
Don’t worry, this doesn’t happen to everyone who is pregnant. Below, we briefly introduce:
ABO system is the most common hemolytic disease of newborns, followed by RH system.
About the hemolysis of the ABO system, give an example: after a woman with blood type O marries a man with type A, B or AB, the fetus obtained after pregnancy can be divided into type A and type B. , O type. The blood group antigen inherited by the fetus from the father is missing from the mother. This antigen enters the mother through the placenta, stimulates the mother to produce corresponding immune antibodies, and the antibodies enter the fetus. The combination of antigens and antibodies causes fetal cells to agglutinate and destroy, and then hemolysis occurs.
About RH system hemolysis, give an example: if the mother is Rh-negative and the fetus is Rh-positive, generally the first child will not develop the disease, but the second child will develop the disease; but if the Rh-negative mother Having received Rh-positive blood transfusion before the first child, the first child can also develop the disease.
As anaesthetists, we are more concerned with which anaesthetics tend to cross the placenta. Unfortunately, it has been found from the current research that almost all commonly used clinical anesthetics can pass through the placental barrier and have varying degrees of influence on neonatal neurodevelopment. Therefore, every anesthesiologist should strive for excellence in the selection of anesthesia drugs and anesthesia control, and strive to minimize the damage.
In conclusion, anesthetics with high concentration, small molecular weight, low degree of ionization, high lipid solubility and low protein binding rate can promote placental transport. A high degree of lipolysis, low lipid solubility, and high protein binding hinder the passage of anesthetics through the placenta.
In addition, many other factors affect maternal and fetal plasma concentrations through drug absorption, distribution, metabolism, and excretion:
1. Maternal factors: maternal blood concentration, route of administration, concomitant medication, pregnancy and other related diseases.
2. Placental factors: maturity, metabolic capacity, hemodynamics.
3. Fetal factors: fetal liver maturity, degree of drug uptake, body distribution, metabolism and excretion capacity.
The following content is extremely important, please keep it safe with friends of anesthesiologists!
1. Propofol easily crosses the placental barrier. But the good news is, it’s not completely useless. Available studies have shown that clinical use is safe with an induction dose of less than 2.5 mg/kg and a maintenance dose of less than 6 mg/kg of body weight per hour.
2. Ketamine crosses the placenta quickly. However, due to the sympathomimetic effect of ketamine on cardiovascular and its short half-life, it is generally believed that the advantages outweigh the disadvantages.
3. Sodium thiopental is out of history, but we need to know about it too. After the injection of thiopental sodium, a peak appeared in the fetal blood about 1 minute, and the elimination was slow.
4. Most inhalation anesthetics readily pass through the placental barrier.
5. After intravenous administration of fentanyl, the peak value is reached in 5 minutes and maintained for 30 to 60 minutes. Unless large doses of fentanyl (greater than 1 microgram per kilogram) are given intravenously immediately before delivery, it will generally not have a significant effect on the neonate, but be prepared to deal with neonatal respiratory depression.
6. Remifentanil easily crosses the placental barrier. But the good news is that the fetus also has the ability to metabolize remifentanil, and remifentanil can be rapidly metabolized or redistributed in the fetus.
7. Muscle relaxants Muscle relaxants are mostly drugs with large molecular weight, low fat solubility and high degree of dissociation, and the commonly used clinical doses are not found to pass the placental barrier. At least that dose is safe.
8. Various local anesthetics are able to cross the placenta, and their major effects are affected by pKa, maternal and fetal pH, and protein binding rates. Existing studies have shown that compared with bupivacaine and ropivacaine, lidocaine has a lower pKa and lower protein binding rate in humans, and these biochemical properties make it easier to cross the placenta.
9. Most anesthesia adjuvant drugs can easily pass through the placental barrier, and ephedrine, phenylephrine, atropine, and antihistamines can be transported to the fetus. Therefore, these drugs should be used with caution. But use it when you need it.
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