Since its first description, the techniques for the detection and interpretation of the Fetal Heart Rate have technically evolved. Even though it is now relatively easy to electronically continuously monitor the fetal heart, the interpretation can often be ambiguous in its nature. Much has been written on FHR pattern interpretation (Arulkumaran, Gibb, et al) and equivocal FHR patterns may cause uncertainty as to what the next action should be.
In 1964, Saling was reported to be using fetal scalp blood sampling to help assess fetal well-being. Therefore parallel to the evolution of fetal heart rate monitoring, was that of fetal blood pH assessment. During labour a decrease in perfusion from the placenta or umbilical cord to the fetus causes a lack of oxygen supply and in turn a build up of carbon dioxide. The eventual build up of carbon dioxide may lead to metabolic acidosis and hypoxia which in turn may lead to fetal death. Stress upon a fetus can be identified from abnormal heart rate patterns; however the causes of the stress cannot always be identified fully through interpretation of the fetal heart rate alone.
Saling and Bretscher devised what is now known as the ‘Gold Standard’ of fetal blood pH measurement. The cut-offs that they produced were as follows: A pH value greater than 7.25, the fetus is normal. A pH of between 7.2 – 7.25 the fetus is pre-acidotic and if the pH is less than 7.2 the fetus is acidotic.
With differing levels of stress which the fetus is under may determine the choice of delivery method, either naturally through the cervical womb, or with instrumental aids such as forceps or vacuum pumps, or a caesarean section. The choice of delivery can be dependent on a variety of clinical factors as cervical dilation, the rate of progress of labour, parity, fetal blood pH and the skill of the operator. It is now part of the UK’s National Institute of Clinical Excellence (NICE) guideline on contemplation of a caesarean section, due to abnormal heart rate, that a fetal blood sample should be offered if possible before a final decision is made. The satisfactory result of a Fetal Scalp Blood Sample (FSBS) may enhance the clinical decision and avoid unnecessary surgical intervention with the fetus being delivered vaginally.
How the Test is performed
There is no standard method for the collection of fetal scalp blood; most UK practitioners collect individual drops of fetal capillary blood in a heperanised tube following scalp skin puncture with a small blade.
Traditional equipment needed for Fetal Blood Sample:
- A sustainable light source and an Amnioscope with obturator, for viewing the operating area
- Ethyl Chloride for cleaning the fetal scalp and increasing capillary blood flow
- Long handled cotton buds
- A blade & holder, for making the incision
- A heperanised capillary tube
- Petroleum Jelly
- Access to a Blood gas analyser
Fetal Scalp Blood Sampling usually takes about 5 minutes:
- The fetal scalp is first visualised with an Amnioscope inserted vaginally and the scalp is cleaned using the long handled cotton buds.
- Ethyl Chloride is sprayed on the fetal scalp
- The sample is taken from a small cut made in the fetal scalp
- The fetal blood sample is then collected in an heparinised capillary tube (micro tube)
- The blood sample should be immediately analyzed for pH and base excess
Classifications of results
The results of the pH may or may not require a course of action to be taken. The following guidelines can be used:
- = > 7.25 FBS should be repeated if the FHR abnormality persists
- = 7.20-7.25 Repeat FBS within 30 minutes or consider delivery if rapid fall since last sample
- = < 7.20 Deliver immediately by safest and most appropriate method
* All scalp pH estimations should be interpreted taking into account the initial pH measurement, the rate of progress in labour and the clinical features of the mother and baby.
Current Problems and Future Solutions
As with most medical equipment there are short comings which need to be addressed. One of the most prominent problems is that more than one FSBS sample may need to be taken. NICE (National Institute for Clinical Excellence) recommend that if time permits a sample should be taken every 30 minutes or less if readings show pre-acidosis (pH 7.2-7.25). A minimum of three samples is recommended.
Once a sample is taken, it is analysed by a blood gas analyser. Delays in analysis may be affected by blood clotting and changes in the blood composition; this in turn may affect the accuracy of the results. If the volume of blood collected is too low, analyses may not be possible. Although the capillary tube is internally coated with sodium heparin, blood in the middle of the tube remains untouched and the introduction of “mixing wires” into the sample is recommended.
The ease at which the physician can take the sample will reduce the time it takes for an analysis to be made. To assist this, the Complete Fetal Scalp Blood Sampling Kit contains all that is necessary to complete a sample with speed and dexterity.
The blood sampling tray is wrapped in a sterile surgical drape, which comes inside a sterile bag. The tray contains a “Lina™” single handed FSBS, capillary tubes, cotton swabs, a place to rest the ELA™ and a set of analyser adapters (radiometers).
To ensure that enough samples are taken, four sodium heparinised capillary tubes are provided in total. There is a selection of mixing wires, end caps (for easy transportation of the sample to the analyser), adapters, cotton buds, etc.
Each blood gas analyser (radiometer) has slightly differing methods of aspirating the sample, a number of adapters are made available to help with this problem.
Other equipment to allow for ease of sampling, may include the use of the ELA™ Amnioscope, which provides an enclosed light source sufficient to highlight the sample area.
The fetal blood sampling tray is ready to use and can be placed at any location for easy access for the fetal scalp blood sample (FSBS) to be performed quickly and with ease.
Tuffnell D, Haw WL, Wilkinson K. How long does a fetal scalp blood sample take? BJOG. 2006. 113:332-334