Fetal heart rate monitoring – principles and interpretation of

This article was originally published as Anaesthesia Tutorial of the Week 294 (2013) Claire Todd*, Matthew Rucklidge, Tracey Kay *Correspondence email: [email protected]

CARDIOTOCOGRAPHY The CTG monitor records the fetal heart rate (FHR) CHARACTERISTICS OF THE CTG

either from a transducer placed on the woman’s A combination of several abnormalities increases the Clinical Overview Articles abdomen or an electrode placed on the fetal scalp. likelihood of . An additional transducer placed on the woman’s abdomen simultaneously records uterine muscle Suspicious or abnormal features include: Summary contraction. These variables are plotted graphically so • Baseline FHR outside normal range of 110 – 160 that variations in FHR can be viewed over time and beats per minute (bpm) Continuous electronic interpreted in the context of the contractile state of fetal monitoring is the (Figure 1). • Baseline variability <5 bpm commonly performed by cardiotocography • Reduced or absent accelerations INDICATIONS FOR CONTINUOUS CTG (CTG). The CTG monitor MONITORING • Presence of decelerations. records both fetal heart rate (cardio) and maternal Intermittent auscultation of the FHR is recommended Baseline rate uterine contractions for women considered at low risk of complications (toco). An understanding The normal baseline fetal heart rate is defined as 110 – during labour. The UK National Institute of of the principles of CTG 160 bpm. Fetal bradycardia is a baseline rate of <110 Health and Clinical Excellence (NICE) make monitoring and a systematic bpm. Fetal tachycardia is a baseline rate of >160 bpm. recommendations for continuous CTG monitoring approach to CTG analysis which include:1 may enable anaesthetists Many fetal baseline bradycardias have no identifiable to better appreciate why 1. Meconium staining of liquor cause but may occur as a result of: obstetricians make specific • Cord compression and acute fetal hypoxia clinical decisions. This 2. Maternal pyrexia – defined as 38.0°C or 37.5°C understanding may aid on two occasions two hours apart • Post-maturity (> 40 weeks gestation) communication and timely • Congenital heart abnormality. delivery especially when the 3. The use of for labour augmentation fetus is considered at high 4. Fresh bleeding developing in labour Fetal tachycardia is associated with: risk. • Excessive fetal movement or uterine stimulation 5. At the woman’s request • Maternal stress or anxiety 6. Abnormal FHR detected during intermittent auscultation: • Maternal pyrexia Claire Todd • FHR <110 beats per minute (bpm) • Fetal infection Specialist Trainee in Anaesthesia • FHR >160 bpm • Chronic hypoxia • Any decelerations after a contraction • Prematurity (<32 weeks gestation). Matthew Rucklidge 7. Women receiving regional anaesthesia/ Consultant Anaesthetist analgesia. Continuous electronic fetal monitoring Fetal heart rate variability Tracey Kay is recommended for at least 30 minutes during Variability refers to the normal beat to beat changes Consultant Obstetrician establishment of regional analgesia and after in FHR. Normal variability is between 5-15 bpm. Royal Devon and Exeter NHS administration of a further bolus of local Variability can be measured by analysing a 1-minute Foundation Trust anaesthetic agent. In most UK centres, continuous portion of the CTG trace and assessing the difference Barrack Road CTG monitoring is performed after the insertion between the highest and lowest rates during that Exeter EX2 5DW of a labour epidural. period. Variability can be defined as: UK

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B

Figure 1. Normal CTG with fetal heart rate uppermost (A) and the tocogram, showing uterine contractions below (B). The fetal heart rate is within the normal range and has normal baseline variability. The arrows demonstrate healthy fetal heart rate accelerations.

Normal 5-15 bpm accelerations are present, the CTG is said to be reactive. Accelerations are often associated with fetal activity and are considered an Increased >15 bpm indication that the fetus is healthy.

Decreased <5 bpm Decelerations Absent <2 bpm Decelerations are periodic, transient decreases in FHR, usually associated with uterine contractions. They can be subdivided into Fetal hypoxia may cause absent, increased or decreased variability. four main types by their shape and timing in relation to uterine Other causes of decreased variability include normal fetal sleep- contractions. Uterine contractions must be monitored adequately in wake pattern, prematurity and following maternal administration of order for a deceleration to be correctly classified. certain drugs including opioids. Decelerations may be: Accelerations (Figure 2) • Early Accelerations are periodic, transient increases in FHR, defined as an increase in FHR >15 bpm for more than 15 seconds. When • Late

Figure 2. CTG demonstrating fetal heart rate accelerations

page 6 Update in Anaesthesia | www.wfsahq.org/resources/update-in-anaesthesia • Variable posture and are a normal finding in the second stage of labour. They • Prolonged. are not associated with a poor fetal outcome. Early decelerations (Figure 3) Late decelerations (Figure 4) Early decelerations tend to occur with each contraction and are Late decelerations are uniform in shape on the CTG, but unlike uniform in shape. Early FHR decelerations appear as a mirror image early decelerations start after the peak of the uterine contraction. of the uterine contraction trace. The onset of the deceleration occurs A deceleration in which the lowest point occurs more than 15 at the onset of the contraction and the baseline FHR recovers by the seconds after the peak of the uterine contraction is defined as a end of the contraction. The FHR usually does not fall by more than late deceleration. They are often associated with a decrease in the 40 bpm during an early deceleration. variability of the baseline FHR.

Early decelerations are caused by compression of the fetal head Late decelerations are associated with decreased uterine blood flow during a contraction. They are often relieved by changing maternal and can occur as a result of:

Figure 3. CTG demonstrating early decelerations. Note the onset of the deceleration occurs with the onset of the uterine contraction

Figure 4. CTG demonstrating late decelerations resulting from cord compression

Update in Anaesthesia | www.wfsahq.org/resources/update-in-anaesthesia page 7 • Hypoxia Prolonged decelerations/ bradycardia (Figure 6) • Placental abruption A deceleration with a reduction in FHR of greater than 30 bpm that • Cord compression / prolapse lasts for at least 2 minutes is termed a prolonged deceleration They are caused by a decrease in oxygen transfer to the fetus, so can arise as • Excessive uterine activity a consequence of a wide variety of disorders including: • Maternal hypotension / hypovolaemia. • Maternal hypotension Variable decelerations (Figure 5) Variable decelerations describe FHR decelerations that are both • Umbilical cord compression variable in timing and size. They may be accompanied by increased • Uterine hypertonia. variability of the FHR. They are caused by compression of the umbilical cord and may reflect fetal hypoxia.

Figure 5. CTG demonstrating variable decelerations A

Figure 6. Prolonged deceleration/bradycardia (A - bracketed) representing fetal distress

page 8 Update in Anaesthesia | www.wfsahq.org/resources/update-in-anaesthesia Table 1. Categorisation of CTG

Reassuring Non-reassuring Abnormal Baseline rate (bpm) 110 - 160 100 - 109 <100 Comments:- 161 - 180 >180 Variability (bpm) 5bpm or more <5 for 40 min or more but <5 for 90min or more Comments:- < 90 min Accelerations Present None Comments:-

Decelerations None Early Atypical variable Comments:- Variable Late Single prolonged Single prolonged deceleration up to 3 min deceleration > 3 min Opinion Normal CTG Suspicious CTG Pathological CTG (All four features (One non-reassuring (two or more non-reassuring or one or more abnormal features) reassuring) feature) Dilatation Comments:- Contractions ...... :10

Action Date Signature

Time Status

CTG CATEGORISATION Table 2. Classification of fetal blood sampling (FBS) results1 Following CTG analysis, the findings can be used to categorise the FBS result (pH) Classification Recommended CTG as: normal, suspicious or pathological. Many UK maternity subsequent action units have adapted recommendations from NICE to produce a sticker (below) that is placed in the records of the labouring woman ≥ 7.25 normal Repeat within 1 hour if FHR trace remains pathological or sooner if to assist her on-going management. further abnormalities FETAL SCALP ELECTRODE 7.21 – 7.24 borderline Repeat within 30 minutes if FHR A fetal scalp electrode is a device that is attached to the skin of the trace remains pathological or sooner if further abnormalities presenting part (most commonly the scalp) of the fetus to assess the fetal heart rate pattern when external monitoring cannot be used ≤ 7.2 abnormal Seek obstetric advice, delivery is or when the quality of external monitoring is poor (e.g. maternal indicated obesity). It provides a more accurate and consistent transmission of 2 the fetal heart rate than external monitoring because external factors to improve fetal oxygenation before delivery. These manoeuvres may such as movement do not affect it. be performed with continuous CTG monitoring and if successful may reduce the urgency to deliver allowing time to provide neuraxial FETAL BLOOD SAMPLING anaesthesia. Steps in intrauterine resuscitation can be remembered Fetal blood sampling (FBS) involves taking a small sample of blood by the acronym SPOILT: from the presenting part of the fetus during labour. FBS is advised • Syntocinon off in the presence of a pathological CTG unless there is clear evidence P of severe fetal compromise. If fetal compromise is evident, (e.g. • osition full left lateral prolonged deceleration of greater than 2 minutes duration), FBS • Oxygen should not be undertaken and the baby should be delivered urgently. • I.V. – infusion of crystalloid fluid Other contraindications to FBS include: • Low blood pressure – if present give i.v. vasopressor • Maternal infection (such as HIV, hepatitis, herpes simplex virus) • Tocolysis - terbutaline 250mcg sc (a β -agonist) or GTN (2 x • Fetal bleeding disorders 2 400mcg puffs sublingual) • Prematurity (<34/40). REFERENCES INTRAUTERINE FETAL RESUSCITATION 1. Guideline CG55, National Institute for Health and Clinical Excellence, September 2007. Intrapartum care: management and delivery of care to If CTG monitoring indicates serious fetal compromise, or FBS result women in labour. Available at: http://guidance.nice.org.uk/CG55 is abnormal, a decision to deliver, often by emergency caesarean 2. Thurlow JA, Kinsella SM. Intrauterine resuscitation: active management of section, should be made. A number of manoeuvres can be performed fetal distress. Int J Obstet Anesth 2002; 11: 105-16.

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