Analgesia, Anaesthesia and Pregnancy. 4th Ed. Róisín Monteiro

Chapter 20. Intrapartum fetal monitoring

Fetal monitoring is an important part of intrapartum care since labour is a stressful event for the fetus, and the ability of the fetus to maintain oxygenation is tested with each uterine contraction. Fetal wellbeing can be monitored routinely using the following methods:

• Assessment of liquor colour

• Fetal heart auscultation

• Fetal heart cardiotocography (CTG)

• Fetal electrocardiography (ECG) waveform analysis

• Fetal blood sampling (FBS) and oximetry

Special considerations

A low-risk pregnancy uncomplicated by any obstetric or medical problems will need a low level of fetal monitoring during labour, whereas a high-risk pregnancy (e.g. one complicated by hypertensive disease of pregnancy or poor intrauterine growth) will need increased intrapartum monitoring (see Chapter 19, Normal labour). The labour itself can also be assessed as either low- or high-risk; it is generally recognised that a spontaneous labour is usually low-risk whereas an augmented labour or an induced labour is usually high-risk.

Liquor colour

When the membranes are ruptured, the liquor may be observed for the presence of meconium; this may indicate fetal hypoxia, which can cause the anal sphincter to relax allowing the fetus to pass meconium. Meconium may range from ‘light’ or grade-1 staining (in which a small amount of meconium diluted in a large volume of amniotic fluid gives the latter a greenish or yellowish discolouration), through ‘moderate’ or grade-2 staining (definite green/brown-coloured waters) to ‘heavy’ or grade-3 staining (thick, dark green/black-coloured fluid). Although such grading is commonly used, it has poor consistency between observers and poor predictive value for identifying degrees of fetal compromise. However, the appearance of thick new meconium is an indication for urgent delivery. If meconium is aspirated into the lungs of the neonate, severe lung damage may ensue; therefore, it should be aspirated from the neonate’s mouth before intermittent positive-pressure ventilation is started. A paediatrician should be present at the delivery if meconium is observed. Meconium is more commonly seen in post-term labour.

Fetal heart rate and CTG

Fetal heart monitoring is recommended as follows:

1. Low-risk (low-risk pregnancy and normal labour). Intermittent monitoring with a Pinard’s stethoscope is said to be as effective as CTG monitoring. Structured intermittent monitoring involves listening immediately after a contraction for a minimum of 60 seconds and repeating this every 15 minutes in the first stage of labour (every 5 minutes in the second stage). The maternal heart rate must also be recorded hourly to differentiate between maternal and fetal heart rates. It has been suggested that CTG monitoring in the low-risk group may lead to unnecessary intervention and increased anxiety.

2. High-risk (high-risk pregnancy and induced or augmented labour). Continuous CTG monitoring of the fetal heart rate is recommended. This records both the fetal heart rate and the uterine contractions (and thus the effect of the latter on heart rate can be observed). The monitor uses either an external transducer or a ‘clip’ applied to the fetal head. It is generally recommended that women with epidural analgesia have continuous fetal monitoring, although there is some evidence to suggest that women who have a mobile (low-dose) epidural may not need this. The need for continuous fetal monitoring during epidural analgesia is related to: (1) the cardiovascular instability that may follow administration of local anaesthetic solutions into the epidural space, (2) the transient fetal bradycardia that may follow administration of epidural or spinal opioids, and (3) any preexisting maternal or fetal risk factors that contributed to the decision to have an epidural (e.g. previous caesarean section).

There are four features of the fetal heart rate that are especially important:

• Baseline rate: normally 110-160 beats/minute.

• Variability: normally 5-25 beats/minute. Opioids or other sedative drugs may cause a flat trace with a loss of beat-to-beat variability, which makes interpretation difficult.

• Accelerations from baseline (> 15 beats/minute for 15 seconds): two in 20 minutes are normally present. The significance of absent accelerations on an otherwise normal CTG is uncertain and so does not form part of the National Institute for Health and Care Excellence (NICE) description of CTGs (Table 20.1).

• Decelerations from baseline: normally absent. Decelerations are classified as early, variable and late. Early decelerations are synchronous with the contraction; they are benign and may be associated with compression of the fetal head in the pelvis. They mirror the uterine contractions and should be associated with good beat-to-beat variability. Variable decelerations vary in their shape, size and occurrence. They may or may not indicate fetal hypoxia. Late decelerations continue after the contraction has finished and are more ominous, especially if associated with other abnormalities, such as reduced variability.

The NICE guidelines give three descriptions of the CTG trace (reassuring, non-reassuring and abnormal; Table 20.1) and, based on this, give four management categories:

• Normal: all features are reassuring.

• Suspicious: one of the features is non-reassuring. This should prompt correction of any potential precipitating causes (such as hypotension or uterine hyperstimulation) and a plan for further review.

• Pathological: two non-reassuring features or one ‘abnormal’ feature is present.

In addition to a review by a senior midwife and obstetrician and correction of any potential causes, fetal scalp stimulation, blood sampling and/or expedition of birth should be considered.

Table 20.1 Description of the CTG trace features

^a Concerning characteristics in variable decelerations include duration > 60 seconds; reduced variability within the deceleration; failure to return to baseline; biphasic shape; no shouldering.

Adapted from National Institute for Health and Care Excellence. Intrapartum Care for Healthy Women and Babies. ClinicalGuideline 190. London: NICE, 2014 (updated 2017). www.nice.org.uk/guidance/cg190.

• Need for urgent intervention: acute bradycardia, or a single prolonged deceleration for 3 minutes or more. Delivery should occur if a bradycardia lasts 9 minutes.

Fetal heart rate monitoring has low specificity and sensitivity. Any trace that causes concern, especially in a high-risk pregnancy, is an indication for a fetal blood sample to be taken, unless there is evidence of acute fetal compromise, in which case urgent delivery is indicated. The CTG trace should be kept for at least 25 years in case of a later medicolegal claim.

Use of the CTG may be combined with fetal ECG waveform analysis (see below).

Fetal ECG waveform analysis

This requires application of an electrode to the scalp of the fetus following rupture of the membranes. It is thought that ST changes in the fetal ECG are associated with hypoxaemia.

Although this technique has not reduced the caesarean section rate, it is associated with a decrease in interventions during labour. Problems associated with its use include poor signal quality, difficulties in interpretation, poor compliance with guidelines and deterioration in the clinical status of the fetus without any ECG alert. Recently, European guidelines have been agreed with regard to ST waveform analysis, but it is not currently included in the NICE guidance.

Fetal blood sampling

When the fetus becomes hypoxic, there is a build-up of lactic acid and a reduction in the fetal pH. Fetal blood sampling allows a more accurate assessment of fetal wellbeing than the CTG and is likely to be performed whenever there is anxiety about the CTG or when there is meconium in the liquor. It is thought that the use of FBS may help reduce intraoperative interventions. The sample should be taken with the mother in the left lateral position and should not be taken immediately after a prolonged deceleration. Recommended actions for values of fetal pH and lactate are as follows:

• pH ≥ 7.25; lactate ≤ 4.1 mmol/l - should be repeated if the fetal heart rate abnormality continues.

• pH 7.21-7.24; lactate 4.2-4.8 mmol/l - borderline; should be repeated within 30 minutes, or delivery considered if there has been a rapid fall since the last sample.

• pH ≤ 7.20; lactate ≥ 4.9 mmol/l - indicative of significant acidosis and a need for urgent delivery of the baby.

Fetal gas tension measurement and oximetry

Non-invasive continuous transcutaneous measurement of oxygen and carbon dioxide tensions has been developed. This method requires the application of a suction ring to the baby’s head; therefore the cervix needs to be dilated. Fetal oxygen saturation may also be measured by using special pulse oximeters; however, the technique is not yet reliable enough for routine use. Preliminary results suggest that fetuses normally have saturations of 30-60%.

These techniques may be influenced by skin contact, uterine contractions, caput on the baby’s head and fetal hair.

Key points

• Good communication between anaesthetist, midwife and obstetrician is important.

• Fetal monitoring includes assessment of liquor colour, auscultation, cardiotocography and fetal scalp blood pH measurement.

• Fetal heart rate monitoring has poor specificity and sensitivity but this may be improved by ST analysis.

Further reading

Hinshaw K, Ullal A. Peripartum and intrapartum assessment of the fetus. Anaesth Intensive Care Med 2010; 11: 324-32.

National Institute for Health and Care Excellence. Intrapartum Care for Healthy Women and Babies. Clinical Guideline 190. London: NICE, 2014 (updated 2017). www.nice.org.uk/guidance/cg190 (accessed December 2018).

Viswanatha RK, Talaulikar VS, Arulkumaran S. Intrapartum fetal surveillance. Obstet Gynaecol Reprod Med 2017; 27: 363-72.