6 Avoiding Errors. Handbook CTG

 6

Avoiding Errors

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Maternal Heart Rate

Sophie Eleanor Kay 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 misinterpretation of maternal heart rate (MHR) artefact as fetal heart rate

(FHR) can potentially mask abnormal FHR trace, giving the appearance of a

falsely reassuring trace. This can lead to increased perinatal morbidity and

mortality due to the nonrecognition of intrapartum hypoxia or fetal demise in the

second stage.1–4

The misinterpretation of MHR artefact can potentially appear as an abnormal

trace, masking a normal FHR trace and resulting in unnecessary interventions

such as caesarean section.1,3

Studies have suggested that clinicians underdiagnose misinterpretation of MHR.

The risk factors for MHR misinterpretation include an active fetus, twin

gestation and obesity.5 It is felt to be related to increased maternal movement in

the second stage of labour.4

External FHR monitors and internal fetal scalp electrodes (FSEs) are both

susceptible to maternal artefact.2,4Key Features on the CTG Trace

Figure 6.1 FHR showing decelerations with contractions (upper tracing), whereas the

maternal heart rate shows accelerations with contractions (lower tracing).

During the second stage of labour, based on maternal physiology, one would

expect MHR accelerations during contractions or bearing-down efforts,

whereas, based on fetal physiology, one would expect the FHR to show

decelerations1–3,6 (Figure 6.1).

Unexpected low range of FHR: Maternal baseline tends to be 60–100 bpm,

whereas fetal baseline rate is 110–160 bpm.2,3

Higher mean variability1,2 with repetitive accelerations coincides with

contractions (Figure 6.2).

A sudden loss of FHR recording due to capturing the MHR periodically and then

returning to capture FHR.2,3

A sudden improvement of the CTG trace: disappearance of decelerations and

appearance of high-amplitude accelerations with or without a shift in baseline

FHR.

Continuation of CTG recording after delivery.1Figure 6.2 Increased baseline FHR variability with repetitive accelerations associated with

maternal heart rate recording.

Key Pathophysiology behind Patterns Seen on

CTG Trace

Recommended Management

The recording of MHR with external FHR monitors is due to the transducer

picking up sound waves reflected from large maternal vessels.1,2 During labour,

as the fetal head moves deeper into the pelvis, clinicians often move the

abdominal transducer further towards the pelvis so as to improve the ‘signal

quality’ of the CTG trace, which increases the likelihood of picking up MHR.2

FSEs detect and amplify FHR without the maternal signal. However, if there is

no detectable signal, such as in the case of fetal demise, the maternal signal is

amplified and displayed.1,4

Normally with FHR interpretation, decelerations are seen with uterine

contractions due to head compression, which activates the parasympathetic

nervous system, leading to decelerations of the FHR, or due to umbilical cord

compression or utero-placental insufficiency as a result of reduced placental

perfusion.2

However, if MHR is being interpreted, accelerations will be seen with

contractions and active pushing. This is due to increased cardiac output related

to increased cardiac stroke volume in labour, along with increased heart rate.1 It

is believed that haemodynamic changes occur due to the displacement of blood

from the choriodecidual space and an increase in venous return to the heart.1

Maternal anxiety and pain leads to catecholamine release during labour,

contractions and active pushing, further increasing MHR.1,2,4Key Tips to Optimize Outcome

Once signal ambiguity is suspected, evaluate by assessing maternal pulse in

comparison to auscultation of recorded FHR. If FHR and MHR are closely

approximated, then it suggests misinterpretation of MHR.3

Recording of MHR can be excluded by simultaneous recording of FHR and

MHR such as by using a pulse oximetry probe.1–4

Placement of an FSE is thought to reduce chances of erroneous recording of

MHR, but it is still susceptible to artefact.2

Fetal ECG signals can be analysed using a STAN (ST-Analyser) monitor with an

FSE and ‘reference electrode’ on the maternal thigh.2

If FHR is being recorded, then the ECG complex will show a p-wave. If MHR is

being recorded, the p-wave will be absent because the low-voltage maternal pwave does not have sufficient ‘signal strength’ to reach the maternal skin

electrode placed on the thigh.2

The knowledge of the physiological differences between fetal and maternal heart

rate ensures more prompt recognition of erroneous recording and appropriate

action to ensure that FHR is correctly recorded.2

Quick clarification and identification of concerns3 is essential if the recorded

pulse rate is similar to the recorded FHR (Figure 6.3).

Simultaneous use of maternal pulse oximetry with FHR monitoring or STAN

monitoring of fetal ECG waveform can help clinicians to exclude MHR

monitoring with confidence.1–3Figure 6.3 The maternal pulse (dots) is very similar to the recorded baseline FHR. In this

case, the mother had raised temperature, and it is clear that erroneous monitoring was

avoided.

Common Pitfalls

Consequences of Mismanagement

References

1. Sherman DJ, Frenkel E, Kurzweil Y, Padua A, Arieli S, Bahar M. Characteristics of

maternal heart rate patterns during labor and delivery. American College of Obstetricians and

Gynecologists. 2002;99:542–7.

Late/nonidentification of MHR as FHR.

Late/nonidentification of changes of heart rate from decelerations during

contractions (indicative of FHR) to accelerations during contractions (indicative

of MHR).

Poor neonatal outcome secondary to missed diagnosis of fetal hypoxia, fetal

bradycardia/tachycardia or fetal demise.2,7

Inappropriate intervention secondary to interpreting MHR as FHR, with the

background of a normal FHR.12. Nurani R, Chandraharan E, Lowe V, Ugwumadu A, Arulkumaran S. Misidentification of

maternal heart rate as fetal on cardiotocography during the second stage of labor: the role of

the fetal electrocardiograph. Acta Obstetricia et Gynecologica Scandinavica.

2012;91:1428–32.

3. Emereuwaonu I. Fetal heart rate misrepresented by maternal heart rate: a case of signal

ambiguity. Am J Clin Med. 2012;9(1):52–7.

4. Paquette S, Moretti F, O’Reilly K, Ferraro ZM, Oppenheimer L. The incidence of maternal

artefact during intrapartum fetal heart rate monitoring. J Obstet Gynaecol Canada.

2014;36(11):962–8.

5. Herbert WN, Stuart NN, Buter LS. Electronic fetal heart rate monitoring with intrauterine

fetal demise. J Obstet Gynecol Neonat Nurs. 1987;16(4):249–52.

6. Abdulhay EW, Oweis RJ, Alhaddad AM, Sublaban FN, Radwan MA, Almasaeed HM.

Non-invasive fetal heart rate monitoring techniques. Biomed Sci Eng. 2014;2(3):53–67.

7. Hanson L. Risk management in intrapartum fetal monitoring: accidental recording of the

maternal heart rate. J Perinat Neonatal Nurs 2010;24(1):7–9.