User:BKgunner2003/Fetal scalp blood testing

Fetal scalp blood testing is a technique used in obstetrics during active labor to confirm whether a fetus is receiving enough oxygen. This is a second-line monitoring assessment used to determine if fetal acidemia has occurred following fetal cardiac distress.

Two components that are commonly tested by this method are pH and lactate, both being indicators of acid base homeostasis. A low pH and high level of lactate indicate that there is acidosis, which in turn is associated with hypoxia. In one clinical study, the frequency of discordance between the scalp pH and lactates was 29.4% and the discordance often showed that the scalp lactate measurements were less favorable.

Scalp pH and lactate testing appear to have the similar sensitivity in predicting umbilical artery acidemia.Analysis of pH requires a relatively large amount of blood (30–50 μl), and sampling failure rates of 11–20% have been reported. Analysis of lactate only requires 5 μl of blood.

During pregnancy, placental gas-exchange is primarily responsible for fetus well-being. Placental dysfunction can results in fetal risks such as acidosis, hypoxia, and stillbirth. The normal arterial pH of the fetus is approximately 7.35 before labor. Furthermore, there is a declined in pH which has shown that the mean umbilical arterial pH at birth predicts a pH > 7.25. An abnormal decreases in pH, on the other hand, is shown that there may be a potential risk of acidosis in fetus if the pH is below a threshold of 7.21.

Fetal scalp blood sampling for lactate became well-known in the 1990s. One study has shown that there is a correlation between both umbilical cord pH and lactate measurement in fetus arterial blood. In a physiological standpoint, lactate levels in tissues earlier increase before pH decreases in oxygenation deficiency. The advantages of lactate measurement is that it is required 5 μl of fetal blood for analysis and provided immediate results compared to pH scalp testing. One of the observational studies suggested that a threshold of 4.8 mmol/L of lactate scalp measurement was chosen to prevent acidemia in newborns, which is corresponding to fetal scalp pH of 7.21.

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Procedure:

In an active labour, fetal scalp blood sampling technique requires at least 3 to 4 cm cervical dilation to visualize the fetal scalp. The procedure is performed by creating a shallow cut by a transvaginally inserted amnioscope. A small incision on a fetal scalp is made and then blood is collected using capillary tube.

Several operation-related factors and technical issues including inadequate incision, insufficient sample volume, possible contamination of blood sample before analyzing are encountered for downfalls of the procedure. Moreover, this process may be invasive, time-consuming, and painful for mother during labour. Despite of approximately 20% of failure rates and contraindications, fetal scalp blood testing should be avoided when there is potential risk of infections (e.g. HIV, Hepatitis B, Hepatitis C, etc) or any suspicions of rare bleeding disorders in newborns, known as hemophilia.

Preparation

Mechanism, how the test or procedure works

Legal issues, such as whether special counseling is mandated, if any

History of the test

The use of fetal scalp blood testing originated in Germany in 1961 and required 0.25 mL of blood drawn from the fetus. As one of the first methods of monitoring fetal wellbeing during labor, there were many disadvantages including the need for at least 3 cm dilation of the mother and extreme precision from the physician performing the procedure. Now, fetal scalp blood testing requires a considerable less amount of blood depending if testing pH or lactate.

Society and culture

Research

Veterinary use

Fetal blood sampling is a procedure used to diagnose, treat, or monitor different fetal problems. By using a small needle, different specialists are able to remove tiny amounts of blood from the fetus during pregnancy. The purpose of this procedure is to determine blood type, diagnose different genetic abnormalities, determine fetal infections, identify fetal anemia, and identify a low platelet count. This procedure is performed with the help of continuous ultrasound guidance to place a needle through the maternal abdomen into a tiny fetal blood vessel. The fetal blood sample then gets sent for testing.

Article about pH and lactate: One study was assessing the discordance between scalp pH and lactate performed from the same sample during labor. For more than a quarter of the samples, results were discordant between scalp pH and lactates, especially when cervix was full dilated and when the amniotic fluid was meconium-stained. An RCT comparing the relevance of each parameter according to obstetrical situation would be necessary. Using lactates value rather than pH or broth combined could result in an increase in obstetric interventions without decreasing severe acidosis. Study included all women with a singleton pregnancy whose fetus was in cephalic presentation at delivery and who had at least one FBS taken during labor. Conclusion: In more than a quarter of the cases in the study, their results were discordant. An RCT would be required.

Fetal scalp blood sampling for pH and lactate measurement: The use of fetal scalp blood sampling for fetal monitoring seems logical. This is because neonatal acidosis is one major criteria for birth asphyxia (when a baby's brain and organs do not get enough oxygen or nutrients during birth). FBS is invasive, but fetal scalp lactates measures require a much smaller volume of blood but a wider assessment is required.