Fetal scalp blood sampling: clinical theory and practical applications

Bretscher and Saling1 first introduced fetal scalp blood sampling to assess fetal well-being in the 1960’s, and the technique has evolved parallel to fetal heart rate monitoring (Tuffnel et al 20062).

Clinical Theory
Measuring fetal heart rate allows a clinician to identify when a fetus is in stress, however the cause of stress can not be identified fully through the interpretation of fetal heart rate alone. Although some Cardiotocograph machines have been shown to reduce the need for a fetal blood sample (Vayssière et al 20073), they still do not provide the most accurate information required to make a decision regarding the form of delivery.

Glucose is the main substrate for fetal energy production. In anaerobic conditions, glycolysis occurs where the fetus has full access to oxygen via the placenta. The glucose is turned into pyruvate and placed into the Citric Acid Cycle where ATP is generated and Carbon Dioxide (CO2) and water (H2O) are displaced as waste products. CO2 and H2O are then excreted along the placenta and disposed by the mother. (Figure 1)(Nordström 20044).

Figure 1: Schematic Representation of anaerobic and aerobic metabolism in the fetus. Diagram taken from Nordström 2004

However, when the fetus has no access to oxygen from the placenta, then anaerobic respiration takes place. The pyruvate does not enter the citric acid cycle but converts into lactate (Figure 1).

A build up of lactate and carbon dioxide causes lactacidema (metabolic acidosis) to occur, and over time become more severe. In the first instance lactacidemia can impair cellular function and later energy depletion can compromise cellular integrity. The depletion of the cells can therefore lead to fetal mortality; hence corrective action in a short time scale for fetal delivery is critical (Nordström 2004).

The build up of lactate and CO2 alters the acid-base balance of the blood, which can be identified by the fetal blood pH. It has been found that there is a good correlation between lactate concentration measured subcutaneously, and the levels within the brain during hypoxic experiments on sheep models (Nordström 20044). This correlation therefore supports the supposition that sampling of fetal blood from the capillaries in the scalp, the subcutus compartment (epidermis), is one of the best options to identify fetal well being.

Saline and Bretscher devised the ‘Gold Standard’ of fetal pH measurement based on a study of 80 cases they performed in 1962 (Nordström 20044). (Table 1).

Table 1: Cut off values of fetal blood pH and the actions to take.

These fetal blood pH levels have become the determining factors in deciding the best and most efficient procedure to deliver the baby. In the UK , the National Institute for Clinical Excellence (NICE) guidelines on Caesarean-section suggest that when a C-Section is contemplated due to a non-reassuring fetal heart rate or in cases of fetal comprise (such as lactemidia), a fetal blood sample should be taken provided there is no technical difficulty or contraindications (Annappa et al 2008v).

Fetal Scalp Blood Sampling is one of the most important procedures available that can aid in the increase of the number of normal deliveries around the world. Caesarean sections are high risk, abdominal surgery for the mother and are also high risk to the fetus. To ensure that the correct and most effective procedure is followed for the sake of the fetus and mother, the pH of the fetal blood is critical in aiding in the decision making process.

Therefore, the ease of taking a fetal blood sample is crucial in providing reliable pH results, as they are to be based on the ‘Gold Standard’ cut off values in Table 1.

There have only been two studies to date that have looked at the median time to take a fetal blood sample from the moment of decision to the result. Annappa et al (2008) found that the median time to take a fetal blood sample from decision to result was 17 minutes, Tuffnel et al (2006) also found a very similar time of 18 minutes to take a fetal blood sample. This length of time is crucial as the maximum decision to delivery time is 30 minutes for a caesarean section. The length of time to take the fetal blood sample must be taken into account for the decision to delivery time process.

Annappa and Tuffnel both found that the median time to prepare for a fetal blood sample was 8 minutes and 5 minutes respectfully. Therefore one area in the process of fetal blood sampling where time can be reduced is the preparation of the tools to take a fetal blood sample.

Practical Application

Clinical Innovations (Europe) now offers all options for fetal blood sampling to meet each individual’s requirements in a simple and effective way.

The Complete Fetal Scalp Blood Sampling (FSBS) Kit is made up of three main components:

  • ELA- an electronically illuminating Amnioscope
  • Lina™ Blood Sampler
  • The FSBS Kit- a tray that holds all items required for taking a fetal scalp blood sample.

The Kit contains the Lina™ Blood Sampler with accessorises in a tray and a resting place for ELA. The FSBS tray contains the blood sampler as well as:

  • Heparinised capillary tubes
  • Mixing Wires
  • End caps for all capillary tubes
  • Universal Blood Gas Analyser Adapter with Clot Buster
  • Long Cotton Tipped Buds

The Complete FSBS Kit is designed to provide you with all the essential tools for blood sampling. With heavily heparinised capillary tubes and mixing wires you have a relatively long time before the blood will start to clot, ensuring that the result from the blood gas analyser (which may be a few minutes walk down the hall) will be accurate.

The capillary tube adapter provides connection to the vast majority of blood gas samplers worldwide.

The Lina™ Blood Sampler is a single handed device for sampling fetal blood. The sampler contains a pre mounted capillary tube that is heavily sodium heparinised and a retractable pre-mounted angled blade providing incision depth of 1.7mm.

In order to collect fetal scalp blood, make a single incision to penetrate the fetal head, then retract the blade and rotate the blood sampler around to collect the blood in the pre-mounted capillary tube.

The benefits of a pre-mounted blade, include reducing the time require to prepare the sampler as well as reduced the risk of “needle stick” injury to the user. The unique sampler means that the use of petroleum jelly is not required to take an adequate sample. Petroleum jelly may interfere with the accurate analyses of blood or may clog the analyser.

The FSBS Kit is one of the world’s first procedure packs for fetal blood sampling where no petroleum jelly is required.

ELA – electronically illuminating Amnioscope

ELA is a self contained, self illuminating Amnioscope that is fully sterile and disposable.

The removal of the obturator from the ELA housing simply switches the light source on. Replacing it switches the light source off. This provides a light source right in the palm of your hands.

No additional light source or power is required with ELA. The Amnioscope is “single patient use” and can be used on the same patient multiple times.

This is a new direction in fetal blood monitoring containing the essential pieces of equipment in a new innovative design.

*Available Separately in the UK only

+ Adaptor should fit the majority of Blood gas analysers, however this is not guaranteed and it is the responsibility of the end user to check.

  1. Bretscher J, Saling E. pH values in the human fetus during labor. Am J Obstet Gynecol. 1967 Apr 1;97(7):906–911. [PubMed]
  2. Tuffnell D, Haw W, Wilkinson K. How long does a fetal scalp blood sample take? BJOG 2006; 113:332-334.
  3. Vayssière, C., David, E., Meyer. N. et al. A French Randomized controlled trial of ST-segment analysis in a population with abnormal cardiotocograms during labour. AmJ Obstete Gynecol 2007; 197:299.e1-299.e6.
  4. Nordström, L. Fetal Scalp and cord blood lactate. Best Practice & Research Clinical Obstetrics and Gyanceology 2004; 18 (3):467-476.
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