25 Peripheral Tests of Fetal Well-being. Handbook CTG

 25

Peripheral Tests of Fetal Well-being

◈

Charis Mills and Edwin Chandraharan

Handbook of CTG Interpretation: From Patterns to Physiology, ed. Edwin Chandraharan.

Published by Cambridge University Press. © Cambridge University Press 2017.

Key Facts

Labour is a stressful process for the fetus, and the vast majority of fetuses mount

a successful compensatory response to mechanical or hypoxic stresses without

sustaining any neurological injury.

Intrapartum fetal hypoxia leading to fetal decompensation may be associated

with adverse perinatal outcomes and long-term neurological sequelae such as

cerebral palsy.

Due to the high false-positive rate of CTG (60–90 per cent), peripheral tests of

fetal well-being (fetal scalp blood sampling [FBS], fetal scalp lactate analysis

and fetal pulse oximetry) were developed to reduce intrapartum operative

interventions.

Peripheral tests of fetal well-being are not to be used in isolation but are

adjuncts to continuous fetal monitoring using CTG.

This chapter evaluates the current scientific evidence to support the use of these

additional tests of fetal well-being in improving neonatal outcomes.Fetal Scalp Blood Sampling

FBS was first introduced in 1962 by Erich Saling to estimate fetal acid–base

status as an indicator of hypoxia prior to the introduction of CTG into clinical

practice.1 FBS involves the introduction of an amnioscope into the woman’s

vagina. The fetal scalp is then visualized and a sample of capillary blood is

taken via a small puncture wound in the fetal scalp and is analysed for pH (or

lactate).

Unfortunately, the value of estimating capillary blood pH from a peripheral

tissue is often inaccurate as the site of fetal scalp sampling is not representative

of true acid–base balance. Moreover, the fetal scalp may be compressed during

labour and the presence of caput succedaneum may lead to inaccurate results.2

This may result in unnecessary interventions in fetuses that are not truly hypoxic.

A Cochrane Systematic Review of 13 studies on fetal heart monitoring found no

evidence from the studies that FBS reduces caesarean section rates or neonatal

seizures.3 In addition, there was no evidence of improvement in long-term

neurological outcomes.

However, the Intrapartum Guidelines of the National Institute for Health and

Clinical Excellence (NICE) in 2014 still recommends the use of FBS for a

pathological CTG, even though the Guideline Development Group has concluded

that FBS actually increases the number of caesarean sections and instrumental

vaginal deliveries.4 There is no robust scientific evidence from randomized

controlled trials (RCTs) to support reduction in neonatal acidosis.

FBS does not distinguish between respiratory and metabolic acidaemia.

Metabolic acidaemia is associated with neonatal morbidity. FBS is

contraindicated in active maternal infection including HIV and herpes and in

known or suspected fetal blood disorders.

Published data in the United Kingdom suggests that it takes 18 minutes to get

FBS results, and therefore, in cases of subacute hypoxia, the use of FBS may

delay delivery and worsen perinatal outcomes.Table 25.1 NICE classification of fetal blood sample results4

Scalp pH Scalp lactate Interpretation

≥7.25 ≤4.1 Normal

>7.21–7.24 4.2–4.8 Borderline

≤7.20 ≥4.9 Abnormal

Fetal Scalp Lactate Analysis

Scientific evidence suggests that the presence of meconium staining of amniotic

fluid as well as contamination with normal amniotic fluid itself may result in

erroneous values.5

Lactate is a metabolite in anaerobic metabolism and reflects tissue hypoxia.

Fetal scalp lactate can be obtained via the same method as FBS for pH, but

lactate is analysed rather than pH. Table 25.1 shows the NICE classification of

fetal blood sample results for both pH and lactate analysis.4 Repeat sampling

and intervention is based on the interpretation of the result, CTG trace, rate of

progress of labour and any clinical maternal or fetal indications.

A large multicentre Swedish RCT including 2,992 women compared lactate

analysis versus pH analysis of fetal scalp blood samples to determine the

effectiveness for each analysis and management of intrapartum fetal distress and

prevention of acidaemia at birth. This study found no significant differences in

rates of metabolic acidaemia at birth, Apgar score <7 at 5 minutes of life and

rates of operative deliveries for fetal distress between lactate and pH analysis.

Failure rates of achieving a sample were higher in the pH group.6 In other

studies, sampling and analysis is found to be more successful and quicker when

using fetal lactate analysis. Fetal lactate requires a smaller amount of blood (5

μL) than fetal pH (30–50 μL).

A recent Cochrane Systematic Review has also confirmed that although fetal

scalp lactate was easier to perform and required a smaller scalp blood sampleFetal Pulse Oximetry

Recommended Management

compared to FBS, there was no difference in operative interventions and

perinatal outcomes.7

Fetal pulse oximetry is an intrapartum test of fetal well-being that involves

attachment of a probe to the fetal head to measure oxygen saturation in fetal

blood. Fetal pulse oximetry relies on differential rates of absorption of infrared

beam of light by oxygenated and deoxygenated haemoglobin at two different

wavelengths during an arterial pulsation cycle. Fetal acidaemia is rare when

arterial oxygen saturation is >30 per cent; therefore, a SpO2 >30 per cent is

associated with good fetal outcome.

Technical difficulties include positioning of the probe against fetal cheek and

incorrect reading due to the presence of meconium and blood in amniotic fluid.

A recent Cochrane Review of seven RCTs, involving a total of 8,013 women,

showed that fetal pulse oximetry in conjunction with CTG does not improve

caesarean section rates compared with the use of CTG alone. One trial

comparing oximetry and CTG with CTG and fetal ECG showed an increase in

caesarean section rates in the fetal pulse oximetry group.8

Understand fetal physiology and employ regular CTG review and training to

avoid misinterpretation of CTG traces leading to unnecessary intervention in

fetuses that are not subject to hypoxia.

Where possible, if a CTG trace is nonreassuring, seek senior obstetric and

midwifery advice and use other additional tests that determine the oxygenation of

a central organ such as fetal ECG (STAN) rather than a peripheral test (FBS and

pulse oximetry).Consequences of Mismanagement

References

1. Bretscher J, Saling E. pH values in the human fetus during labour. American Journal of

Obstetrics and Gynaecology 1967;97:906–11.

2. Chandraharan E. Fetal scalp blood sampling during labour: is it a useful diagnostic test or a

historical test that no longer has a place in modern clinical obstetrics? BJOG 2014. Aug;

121(9):1056–60.

3. Alfirevic Z, Devane D, Gyte GML. Continuous cardiotocography (CTG) as a form of

electronic fetal monitoring (EFM) for fetal assessment during labour. Cochrane Database of

Systematic Reviews 2013;5:CD006066.

4. National Institute for Health and Clinical Excellence (NICE) 2014 Guidelines.

5. Losch A, Kainz C, Kohlberger P, et al. Influence on fetal blood pH when adding amniotic

fluid: an in vitro model. BJOG 2003;110:453–6.

Multiple and failed attempts at FBS for pH and lactate analysis can cause delay

in delivery. Recent scientific evidence suggests that it may double the caesarean

section rate.9

Failure to understand the pathophysiology of intrapartum fetal hypoxia may result

in a delay in delivery due to attempting FBS and thereby worsening perinatal

outcomes.

Rare complications of FBS (fetal scalp pH and lactate) include haemorrhage,

sepsis and leakage of cerebrospinal fluid.10

Peripheral tests of fetal well-being can lead to unnecessary maternal and fetal

interventions to fetuses that are not subjected to a hypoxic insult. A recent

Commentary in 2016 has questioned the ethical and moral issues arising due to

some national guidelines continuing to recommend FBS in routine clinical

practice without any scientific evidence of benefit but with potential harm.116. Wiberg-Itzel E, Lipponer C, Normaln M, et al. Determination of pH or lactate in fetal scalp

blood in management of intrapartum fetal distress: randomised controlled multicentre trial.

BMJ 2008; June 7 336(7656):1284–7.

7. East CE, Leader LR, Sheehan P, et al. Intrapartum fetal scalp lactate sampling for fetal

assessment in the presence of a non-reassuring fetal heart rate trace. Cochrane Database

Systematic Review 2010;CD006174.

8. East CE, Begg L, Colditz PB, et al. Fetal pulse oximetry for fetal assessment in labor.

Cochrane Database of Systematic Reviews 2014;10;CD004075.

9. Holzmann M, Wretler S, Cnattingius S, et al. Neonatal outcomes and delivery mode in

lab

ours with repetitive fetal scalp blood sampling. European Journal of Obstetric

Gynaecology Reproductive Biology 2015;184:97–102.

10. Schaap TP, Moormann KA, Becker JH, et al. Cerebrospinal fluid leakage, an uncommon

complication of fetal blood sampling: a case report and review of the literature. Obstetric

Gynecology Survey 2011;66:42–6.

11. Chandraharan E. Should national guidelines continue to recommend fetal scalp blood

sampling during labor? Journal of Maternal-Fetal & Neonatal Medicine 2016 Feb 24:1–4.