11 Intrapartum Monitoring of a Preterm Fetus. Handbook CTG

Intrapartum Monitoring of a Preterm

Fetus

◈

Ana Piñas Carrillo 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

The CTG patterns reflect the development and maturity of cardiac centres in the

central nervous system and hence differ between preterm and term fetuses.

Preterm fetuses, because of immaturity and low weight, are more likely to

sustain intrapartum hypoxic injury. The physiological reserves are less than

those in a well-grown term fetus, and in the presence of hypoxia, the

compensatory response mounted by a preterm fetus is limited. The classical

features observed on a CTG trace in the case of a term fetus may not be

observed with the same amplitude in a preterm fetus. They have less capacity to

release catecholamines due to smaller adrenal glands, and mild uterine

contractions can easily cause variable decelerations due to a thinner umbilical

cord with less Wharton jelly.

The NICE guidelines, used in the UK, do not recommend intrapartum monitoring

in preterm fetuses <37 weeks. In fact, none of the guidelines used worldwideKey Features on the CTG Trace

Key Pathophysiology behind Patterns Seen on

the CTG Trace

(NICE, FIGO, ACOG) describe patterns of normality and CTG interpretation in

fetuses <37 weeks of gestational age.

Outcome between 24 and 28 weeks depends on maturity at birth and birth weight

and not on the mode of delivery. Intrapartum monitoring of these fetuses can

result on the misinterpretation of uncertain CTG patterns and unnecessary

interventions that do not improve fetal outcome.

Onset of preterm labour between 24 and 28 weeks is associated with infection in

two-thirds of the cases. This presents an alternative pathway of fetal brain

damage independent of hypoxia, and this inflammation-mediated neurological

damage may be missed by the CTG trace.

Fetal baseline heart rate is higher at an average of 155 bpm at 24 weeks, and it

decreases as pregnancy progresses, with fetuses having an average baseline

heart rate of 140 bpm at term and 120 bpm at 41–42 weeks.

The frequency and amplitude of accelerations are reduced before 30 weeks,

frequently lasting for no longer than 10 seconds and with an increase of 10 bpm

from baseline.

The variability may also be reduced due to the immaturity of autonomic nervous

system.

The presence of decelerations not related to contractions has also been

described; typically, these are variable decelerations with low depth and

duration. Up to 75 per cent of preterm fetuses may show intrapartum

decelerations.Recommended Management

The baseline heart rate is maintained by the interaction of parasympathetic and

sympathetic systems. Before 30 weeks, there is a predominance of the

sympathetic system, and this results in an average higher baseline rate in preterm

fetuses. As the parasympathetic system develops, it counteracts the activity of the

sympathetic system, and the baseline rate decreases progressively from 30

weeks of gestation.

Similarly, the baseline variability is maintained by the interaction between the

two components of the autonomic nervous system. The lack of maturity of the

parasympathetic system results in an apparent decrease in baseline variability,

which is also influenced by the presence of baseline tachycardia secondary to

unopposed sympathetic activity.

The presence of decelerations not related to uterine contractions and the higher

incidence of variable decelerations intrapartum (up to 75 per cent) in preterm

fetuses can be explained by a decreased amount of amniotic fluid, reduced

Wharton jelly in the cord and lack of maturity of the fetal myocardium and its

glycogen stores.

CTG monitoring in fetuses <28 weeks is not recommended as no patterns of

normality have been described. Monitoring these fetuses can result in

unnecessary interventions (emergency caesarean section, instrumental delivery)

that have been demonstrated not to improve the outcome of these extremely

premature babies but have the potential to increase maternal morbidity.

Beyond 32 weeks, survival rates vastly increase and hence fetal monitoring is

recommended considering the special features that these fetuses show. Caution

should be exercised between 28 and 32 weeks of gestation as no data is

available on normality of CTG traces and how long abnormal features such as

atypical or late decelerations could be allowed to persist prior to the onset of

fetal decompensation.Key Tips to Optimize Outcome

When interpreting CTG traces of fetuses between 28 and 36 + 6 weeks, it is

essential to know its specific features and fetal physiological response. This can

result in different management than when those same features are observed in a

term fetus. For example, a preterm fetus at 28 weeks of gestation presenting with

a baseline heart rate of 155 bpm in the absence of chorioamnionitis or maternal

dehydration can be regarded as normal and will not require further intervention,

whereas a fetus at 42 weeks of gestation presenting with the same baseline has to

be closely monitored as it is likely to be an early sign of chorioamnionitis or an

ongoing chronic hypoxia even in the absence of other clinical signs.

If oxytocin augmentation is needed (i.e. due to severe sepsis) on a preterm fetus,

the frequency of increase may need to be modified, as these fetuses are more

likely to rapidly develop intrapartum hypoxia due to immaturity of the fetal

compensatory response to stress.

In the presence of tachycardia or reduced baseline variability, one should

always exclude iatrogenic causes such as administration of drugs such as

tocolytics (terbutaline), pethidine, magnesium sulphate and steroids.

Fetuses 24–28 weeks of gestational age: Continuous monitoring is not

recommended. An isolated higher baseline FHR or reduced variability should

not be considered a priori as abnormal and operative interventions are not

indicated for such ‘abnormal changes’ on the CTG trace that merely reflect

physiological immaturity of the autonomic nervous system. Remember that the

physiological reserves to respond to hypoxia are not the same as in a term fetus,

especially in the presence of chorioamnionitis.

Fetuses 28–32 weeks of gestational age: Fetal monitoring may be considered as

survival and neurological outcome significantly improves during this gestation.

Fetal baseline heart rate and variability are often comparable to those of a termPitfalls

Consequences of Mismanagement

Exercises

fetus, but accelerations can still be of a smaller magnitude. Always consider the

clinical picture when interpreting the CTG trace (chorioamnionitis, drugs).

Fetuses 32–34 weeks: The CTG trace can be classified according to the

guidelines used on term fetuses. However, it is important to remember that the

reserves in these fetuses are less than at term, and hence the ability to withstand

a persistent hypoxic insult can be compromised (i.e. oxytocin augmentation).

Monitoring preterm fetuses between 24 and 28 weeks and acting on uncertain

CTG patterns, leading to unnecessary interventions such as emergency caesarean

section.

Managing and interpreting CTG traces beyond 28 weeks as in a term fetus,

unaware of the physiological reserves and the possibility of a growth-restricted

fetus that has impaired response to hypoxic stress.

Use of additional tests is not recommended for preterm fetuses. Fetal blood

sampling has not been validated in this group, and fetal ECG (ST-Analyser) is

unreliable because of changes in the myocardium composition (increased water

content and less glycogen) and a smaller epicardial–endocardial interphase.

Unnecessary operative deliveries

Delivery of an extreme premature infant due to an overreaction to CTG patterns

Stillbirth

Neonatal death1. A 24-year-old primigravida presents at 28 weeks with reduced fetal movements.

The CTG trace is shown in Figure 11.1.

Figure 11.1

a. Classify the CTG applying the ‘8Cs’ approach.

b. What are the specific features different from those of a term fetus?

c. What changes would you expect to see if you repeat the CTG in 4 weeks’ time?

d. How would you assess fetal well-being at this stage of pregnancy?

ObGyncases (Healthcare Touch)

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