Fetal scalp blood testing

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Fetal scalp blood testing
Purposeassess fetal oxygenation

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 supplementary procedure used to determine if fetal acidemia has occurred following fetal cardiac distress. [1] While continuous fetal heart rate monitoring is the primary method for assessing fetal wellbeing during labor, a change in fetal heart rate is not indicative of fetal acidemia. [1] [2] Some of the signs and symptoms of oxygen deprivation are pH in the umbilical cord, abnormal fetal heartbeat and abnormal coloration of amniotic fluid. [3] [4] This correlation can only be concluded by sampling fetal scalp blood and measuring acid status. [1] Therefore, fetal scalp blood testing could be used to reduce the number of unnecessary emergency caesarean sections made on the decision of fetal heart rate alone. [1]

Contents

Fetal blood sampling is a useful procedure that can diagnose, treat, or monitor different fetal problems. With the help of a small needle, various providers and specialists can remove small amounts of blood from the fetus. This procedure is used to help determine blood type, diagnose genetic abnormalities, identify fetal infections, fetal anemia, and low platelet count. With the help of continuous ultrasound guidance to place the needle in the abdomen into the tiny fetal blood vessel, the blood sample gets obtained and sent for testing. [5] Therefore, due to limitation in adding value, fetal scalp testing can serve as second line test to provide and strengthen the information on fetal acidosis and hypoxia status obtained from cardiocotography (CTG). [6] [7] Cardiocotography is the primary means of monitorning a fetus during pregnancy as well as labour. [4]

Fetal blood sampling is a complex procedure and performed when other tests are not possible. It can be done for pregnancies that are 18 weeks or later. The procedure has many benefits such as providing specific information about the baby's health and the ability to treat babies with any severe blood diseases before birth. Potential risks include bleeding from the fetal blood sampling site, infection, changes to heart rate, and death. [8]

History

The use of fetal scalp blood testing originated in Germany in 1961 and required 0.25 mL of blood drawn from the fetus. [1] 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. [9] Now, fetal scalp blood testing requires a considerable less amount of blood depending if testing pH or lactate.

One safer alternative to fetal scalp blood testing is what is called fetal scalp stimulation. It is a diagnostic test that helps detect metabolic academia. Fetal scalp stimulation is a second-line test that helps provide specialists reassurance that the labor process can safely continue. [10] In complicated pregnancies, it is recommended that there is continuous monitoring of the baby's heart rate using an electronic recorder, or CTG. During labor, babies can show abnormal readings on the CTG and, in some cases, this warrants an emergency caesarean section. To avoid this emergency C-section, the baby's scalp is stimulated vaginally to cause an increase in heart rate. This response is indicative that the baby is healthy and receiving enough oxygen and this test also provides a safe alternative as opposed to taking a small blood sample from the baby's scalp and testing the acid-base levels in the blood. This test is beneficial because it reduces the need to have to perform an emergency C-section when it may not be needed. [11]

Procedure

During a fetal scalp blood testing procedure, a small incision on the fetal scalp is made and blood is collected using a capillary tube. [9] [12] An amnioscope with a light source is used to expose the scalp of the fetus, and the procedure requires at least 3 to 4 cm of cervical dilation to visualize the fetal scalp. [9] [13] After blood collection, pH and/or lactate levels are tested requiring up to 25 minutes per sample. [9]

Fetal scalp blood testing is associated with a relatively high failure rate of up to 20% due to variables such as sample contamination with air or amniotic fluid, or inadequate sample volume. [9] [13] Moreover, this process may be invasive, time-consuming, and painful for mother during labour. [9] Rare complications have also been reported including infection, excessive bleeding, and leakage of cerebral spinal fluid. [9] Fetal scalp blood testing should be avoided when there is a potential risk of infection (e.g. HIV, Hepatitis B, Hepatitis C) or any suspicions of rare bleeding disorders in newborns, known as hemophilia. [9] [13] [14]

The procedure is contraindicated in the case of pregnancies less than 34 weeks, abnormal fetal heart rate, abnormal fetal blood status, and maternal infection. [15]

Fetal Monitoring Parameters

Two components that are commonly tested using this method are pH [16] 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. The two different types of fetal acidosis are respiratory acidosis or metabolic acidosis. Respiratory acidosis occurs when carbon dioxide accumulates due to decreased placental elimination. This is caused by an increase in pressure on the umbilical cord, but is quickly corrected upon childbirth as carbon dioxide levels return to normal once the baby begins breathing. [17] Metabolic acidosis is caused by anaerobic cell metabolism due to hypoxia. Anaerobic metabolism results in the production of lactate, which breaks down and lowers blood pH. [18] Even after correction, metabolic acidosis can persist for several hours before correction after childbirth. [17]

Uterine contractions during labor and delivery decrease placental blood flow, therefore the fetus is at an increased risk of hypoxia. [19] This is considered as one of the causes of acute fetal hypoxia among the likes of umbilical cord compression and myometrial contractures. [20]

Changes in fetal blood acidity can result in a shift in blood flow away from critical organs, particularly the brain. [18] Conditions which have been reported in newborns who suffered fetal acidemia include hypoxic-ischemic encephalopathy and periventricular leukomalacia. [18]

Scalp pH and lactate testing appear to have similar sensitivity in predicting umbilical artery acidemia. [21] Analysis of pH requires a relatively large amount of blood (30–50 μl), and sampling failure rates of 11–20% have been reported. [21] Analysis of lactate only requires 5 μl of blood and can more accurately identify the cause of acidosis if metabolically induced. [21] [17] Based on clinical literature, there has been discordance between scalp pH and lactate measurements, especially when the cervix was fully dilated. Using lactate values rather than pH or both could result in an increase in obstetric interventions without decreasing severe acidosis. Fetal scalp lactate measurements also require a much smaller volume of blood whereas fetal blood sampling is more invasive. A smaller volume of blood is required for testing of fetal blood sampling for lactate estimation. This test can help determine an abnormal fetal heart rate pattern. Also, this test requires a smaller volume of blood compared to a pH estimation. [22]

Reference ranges [18]
pHLactate
Normal≥ 7.25≤ 4.1 mmol/L
Pre-acidemia7.21–7.244.2–4.8 mmol/L
Acidemia≤ 7.20≥ 4.9 mmol/L

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. [9] [13] Furthermore, there is a declined in pH which has shown that the mean umbilical arterial pH at birth predicts a pH > 7.25. [23] 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. [9] [13]

Fetal scalp blood testing for lactate became well-known in the 1990s. [13] One study has shown that there is a correlation between both umbilical cord pH and lactate measurement in fetus arterial blood. [13] From 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. [13] [21] 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. [9] [13]

Using scalp lactate measurements compared to pH helps provide an easier and more affordable method than continuous electronic fetal monitoring. Nevertheless, literature has shown that both methods has no significant difference in measuring oxygen deprivation as well as in fetal outcome after delivery. [24]

Advancements

Improvements to the fetal blood scalp test procedure have been attempted to decrease invasiveness. Researchers have created a portable oximeter device which attaches to the hand of a physician and allows for the measurement of fetal oxygenation status. This method does not require blood draw from the scalp of the fetus, but instead acts as a indirect measurement of fetal blood pH status by measuring oxygen saturation. [1] Furthermore, studies are being done to evaluate and compare between the fetal scalp blood test and digital fetal scalp blood test, an alternative to fetal scalp blood test that is less invasive, as a second-line test for fetal hypoxia and acid-base status. [25]

See also

Related Research Articles

Obstetrics is the field of study concentrated on pregnancy, childbirth and the postpartum period. As a medical specialty, obstetrics is combined with gynecology under the discipline known as obstetrics and gynecology (OB/GYN), which is a surgical field.

<span class="mw-page-title-main">Placenta</span> Organ that connects the fetus to the uterine wall

The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate maternal and fetal circulations, and is an important endocrine organ, producing hormones that regulate both maternal and fetal physiology during pregnancy. The placenta connects to the fetus via the umbilical cord, and on the opposite aspect to the maternal uterus in a species-dependent manner. In humans, a thin layer of maternal decidual (endometrial) tissue comes away with the placenta when it is expelled from the uterus following birth. Placentas are a defining characteristic of placental mammals, but are also found in marsupials and some non-mammals with varying levels of development.

<span class="mw-page-title-main">Amniocentesis</span> Sampling of amniotic fluid done mainly to detect fetal chromosomal abnormalities

Amniocentesis is a medical procedure used primarily in the prenatal diagnosis of genetic conditions. It has other uses such as in the assessment of infection and fetal lung maturity. Prenatal diagnostic testing, which includes amniocentesis, is necessary to conclusively diagnose the majority of genetic disorders, with amniocentesis being the gold-standard procedure after 15 weeks' gestation.

A blood gas test or blood gas analysis tests blood to measure blood gas tension values, it also measures blood pH, and the level and base excess of bicarbonate. The source of the blood is reflected in the name of each test; arterial blood gases come from arteries, venous blood gases come from veins and capillary blood gases come from capillaries. The blood gas tension levels of partial pressures can be used as indicators of ventilation, respiration and oxygenation. Analysis of paired arterial and venous specimens can give insights into the aetiology of acidosis in the newborn.

Acidosis is a process causing increased acidity in the blood and other body tissues. If not further qualified, it usually refers to acidity of the blood plasma.

Fetal distress, also known as non-reassuring fetal status, is a condition during pregnancy or labor in which the fetus shows signs of inadequate oxygenation. Due to its imprecision, the term "fetal distress" has fallen out of use in American obstetrics. The term "non-reassuring fetal status" has largely replaced it. It is characterized by changes in fetal movement, growth, heart rate, and presence of meconium stained fluid.

<span class="mw-page-title-main">Cardiotocography</span> Technical means of recording the fetal heartbeat and the uterine contractions during pregnancy

Cardiotocography (CTG) is a technique used to monitor the fetal heartbeat and uterine contractions during pregnancy and labour. The machine used to perform the monitoring is called a cardiotocograph.

<span class="mw-page-title-main">Prenatal testing</span> Testing for diseases or conditions in a fetus

Prenatal testing consists of prenatal screening and prenatal diagnosis, which are aspects of prenatal care that focus on detecting problems with the pregnancy as early as possible. These may be anatomic and physiologic problems with the health of the zygote, embryo, or fetus, either before gestation even starts or as early in gestation as practicable. Screening can detect problems such as neural tube defects, chromosome abnormalities, and gene mutations that would lead to genetic disorders and birth defects, such as spina bifida, cleft palate, Down syndrome, Tay–Sachs disease, sickle cell anemia, thalassemia, cystic fibrosis, muscular dystrophy, and fragile X syndrome. Some tests are designed to discover problems which primarily affect the health of the mother, such as PAPP-A to detect pre-eclampsia or glucose tolerance tests to diagnose gestational diabetes. Screening can also detect anatomical defects such as hydrocephalus, anencephaly, heart defects, and amniotic band syndrome.

Oligohydramnios is a medical condition in pregnancy characterized by a deficiency of amniotic fluid, the fluid that surrounds the fetus in the abdomen, in the amniotic sac. It is typically diagnosed by ultrasound when the amniotic fluid index (AFI) measures less than 5 cm or when the single deepest pocket (SDP) of amniotic fluid measures less than 2 cm. Amniotic fluid is necessary to allow for normal fetal movement, lung development, and cushioning from uterine compression. Low amniotic fluid can be attributed to a maternal, fetal, placental or idiopathic cause and can result in poor fetal outcomes including death. The prognosis of the fetus is dependent on the etiology, gestational age at diagnosis, and the severity of the oligohydramnios.

A nonstress test (NST) is a screening test used in pregnancy to assess fetal status by means of the fetal heart rate and its responsiveness. A cardiotocograph is used to monitor the fetal heart rate and presence or absence of uterine contractions. The test is typically termed "reactive" or "nonreactive".

<span class="mw-page-title-main">Intrauterine hypoxia</span> Medical condition when the fetus is deprived of sufficient oxygen

Intrauterine hypoxia occurs when the fetus is deprived of an adequate supply of oxygen. It may be due to a variety of reasons such as prolapse or occlusion of the umbilical cord, placental infarction, maternal diabetes and maternal smoking. Intrauterine growth restriction may cause or be the result of hypoxia. Intrauterine hypoxia can cause cellular damage that occurs within the central nervous system. This results in an increased mortality rate, including an increased risk of sudden infant death syndrome (SIDS). Oxygen deprivation in the fetus and neonate have been implicated as either a primary or as a contributing risk factor in numerous neurological and neuropsychiatric disorders such as epilepsy, attention deficit hyperactivity disorder, eating disorders and cerebral palsy.

<span class="mw-page-title-main">Nuchal scan</span> Routine ultrasound done between 11 and 14 weeks pregnancy

A nuchal scan or nuchal translucency (NT) scan/procedure is a sonographic prenatal screening scan (ultrasound) to detect chromosomal abnormalities in a fetus, though altered extracellular matrix composition and limited lymphatic drainage can also be detected.

Placental insufficiency or utero-placental insufficiency is the failure of the placenta to deliver sufficient nutrients to the fetus during pregnancy, and is often a result of insufficient blood flow to the placenta. The term is also sometimes used to designate late decelerations of fetal heart rate as measured by cardiotocography or an NST, even if there is no other evidence of reduced blood flow to the placenta, normal uterine blood flow rate being 600mL/min.

<span class="mw-page-title-main">Percutaneous umbilical cord blood sampling</span>

Percutaneous umbilical cord blood sampling (PUBS), also called cordocentesis, fetal blood sampling, or umbilical vein sampling is a diagnostic genetic test that examines blood from the fetal umbilical cord to detect fetal abnormalities. Fetal and maternal blood supply are typically connected in utero with one vein and two arteries to the fetus. The umbilical vein is responsible for delivering oxygen rich blood to the fetus from the mother; the umbilical arteries are responsible for removing oxygen poor blood from the fetus. This allows for the fetus’ tissues to properly perfuse. PUBS provides a means of rapid chromosome analysis and is useful when information cannot be obtained through amniocentesis, chorionic villus sampling, or ultrasound ; this test carries a significant risk of complication and is typically reserved for pregnancies determined to be at high risk for genetic defect. It has been used with mothers with immune thrombocytopenic purpura.

<span class="mw-page-title-main">Velamentous cord insertion</span> Velamentous placenta

Velamentous cord insertion is a complication of pregnancy where the umbilical cord is inserted in the fetal membranes. It is a major cause of antepartum hemorrhage that leads to loss of fetal blood and associated with high perinatal mortality. In normal pregnancies, the umbilical cord inserts into the middle of the placental mass and is completely encased by the amniotic sac. The vessels are hence normally protected by Wharton's jelly, which prevents rupture during pregnancy and labor. In velamentous cord insertion, the vessels of the umbilical cord are improperly inserted in the chorioamniotic membrane, and hence the vessels traverse between the amnion and the chorion towards the placenta. Without Wharton's jelly protecting the vessels, the exposed vessels are susceptible to compression and rupture.

Amnioinfusion is a method in which isotonic fluid is instilled into the uterine cavity.

Uterine Tachysystole is a condition of excessively frequent uterine contractions during pregnancy. It is most often seen in induced or augmented labor, though it can also occur during spontaneous labor, and this may result in fetal hypoxia and acidosis. This may have serious effects on both the mother and the fetus including hemorrhaging and death. There are still major gaps in understanding treatment as well as clinical outcomes of this condition. Uterine tachysystole is defined as more than 5 contractions in 10 minutes, averaged over a 30-minute period.

Fetal scalp stimulation test is a diagnostic test used to detect fetal metabolic acidemia. It can be used as a non-invasive alternative to fetal scalp blood testing.

Rh factor testing, also known as Rhesus factor testing, is the procedure of determining the rhesus D status of an individual.

An Intrauterine transfusion (IUT) is a procedure that provides blood to a fetus, most commonly through the umbilical cord. It is used in cases of severe fetal anemia, such as when fetal red blood cells are being destroyed by maternal antibodies. IUTs are performed by perinatologists at hospitals or specialized centers.

References

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  10. ↑ Murphy DJ, Devane D, Molloy E, Shahabuddin Y, et al. (Cochrane Pregnancy and Childbirth Group) (January 2023). "Fetal scalp stimulation for assessing fetal well-being during labour". The Cochrane Database of Systematic Reviews. 1 (1): CD013808. doi:10.1002/14651858.CD013808.pub2. PMC  9831024. PMID   36625680.
  11. ↑ Murphy, D. J.; Devane, D.; Molloy, E.; Shahabuddin, Y. (2023). "Stimulating the baby's scalp as a test of the baby's well-being in labour". The Cochrane Database of Systematic Reviews. 1 (1): CD013808. doi:10.1002/14651858.CD013808.pub2. PMC  9831024. PMID   36625680 . Retrieved 2023-07-30.
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  14. ↑ Murphy DJ, Devane D, Molloy E, Shahabuddin Y, et al. (Cochrane Pregnancy and Childbirth Group) (January 2023). "Fetal scalp stimulation for assessing fetal well-being during labour". The Cochrane Database of Systematic Reviews. 1 (1): CD013808. doi:10.1002/14651858.CD013808.pub2. PMC  9831024. PMID   36625680.
  15. ↑ Young C, Ryce A (2018). Fetal Scalp Lactate Testing During Intrapartum Pregnancy with Abnormal Fetal Heart Rate: A Review of Clinical Effectiveness, Cost-Effectiveness, and Guidelines. CADTH Rapid Response Reports. Ottawa (ON): Canadian Agency for Drugs and Technologies in Health. PMID   30329245.
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  22. ↑ East CE, Leader LR, Sheehan P, Henshall NE, Colditz PB, Lau R (May 2015). Cochrane Pregnancy and Childbirth Group (ed.). "Intrapartum fetal scalp lactate sampling for fetal assessment in the presence of a non-reassuring fetal heart rate trace". The Cochrane Database of Systematic Reviews (5): CD006174. doi: 10.1002/14651858.CD006174.pub3 . PMID   25929461.
  23. ↑ Baschat AA, Galan HL, Lee W, DeVore GR, Mari G, Hobbins J, et al. (April 2022). "The role of the fetal biophysical profile in the management of fetal growth restriction". American Journal of Obstetrics and Gynecology. 226 (4): 475–486. doi:10.1016/j.ajog.2022.01.020. PMID   35369904. S2CID   247911334.
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  25. ↑ Murphy DJ, Shahabuddin Y, Yambasu S, O'Donoghue K, Devane D, Cotter A, et al. (October 2022). "Digital fetal scalp stimulation (dFSS) versus fetal blood sampling (FBS) to assess fetal wellbeing in labour-a multi-centre randomised controlled trial: Fetal Intrapartum Randomised Scalp Stimulation Trial (FIRSST NCT05306756)". Trials. 23 (1): 848. doi: 10.1186/s13063-022-06794-9 . PMC   9531493 . PMID   36195894.
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