4 Understanding the CTG. Handbook CTG

 4

Understanding the CTG

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Technical Aspects

Harriet Stevenson and Edwin Chandraharan

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

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

Introduction

The CTG machine (Figure 4.1) allows recording of fetal heart rate (FHR) and a

representation of uterine activity over time. This allows an assessment of the integrity of

autonomic nervous system control of the FHR (baseline FHR and variability), the

integrity of somatic nervous system (accelerations), the sleep–activity cycle of the fetus

as well as the presence of ongoing mechanical or hypoxic stresses (i.e. decelerations)

Figure 4.1 CTG machine with the transducer and the ‘Toco’.Parts of the Machine

CARDIOtocograph – Records the Features of the FHR

Transabdominal Monitoring – Noninvasive Monitoring

The FHR is recorded both on the CTG paper and is displayed on the CTG

monitor (Figure 4.2). It is also heard as an audible signal (which can be turned

down or off as necessary).

This is measured using a Doppler ultrasound device; this works by propagating a

sound wave through the mother’s abdomen. The speed at which a sound wave

travels through a substance (or medium) is determined by the density, with sound

travelling roughly four times faster in water than air.

When two substances of different densities lie next to each other, the surface or

boundary at which they meet is called an interface. As a sound wave travels

through the first substance and comes to the interface with the other substance,

some of the sound will pass or propagate through the second substance and some

will be bounced back towards the source of the sound. It is this sound wave that

is bounced back and detected by the Doppler ultrasound transducer.

A Doppler ultrasound apparatus is placed on the maternal abdomen over the

fetus’ anterior shoulder (as determined by palpation) and repositioned until a

good signal is achieved. A water-based ultrasonic gel is placed between the

transducer and the woman’s abdominal wall to provide a good contact. This

ultrasound gel has a similar density to the woman’s abdomen allowing sound

waves to travel through it in a similar way, thus cutting down on interference.

The transducer exists to make and receive sound waves. The sound wave is

made by passing a high-frequency electrical current through a piezoelectric

crystal. When an electric current is passed through a piezoelectric crystal, the

crystal changes shape; this change in shape creates a sound wave, which

propagates through the woman’s abdomen. The piezoelectric effect is also such

that when a piezoelectric crystal is squeezed or released from pressure, it willBox 4.1 Explanation of Doppler Shift

In a CTG machine, the sound waves generated by the transducer normally ‘hit’

the fetal heart chambers, which are in constant motion, thereby creating a

Doppler shift.

When a defined frequency of sound waves is sent from the transducer, if the

surface on which they bounce off is stationary, the same frequency of wave

length will be reflected back. In contrast, if the sound waves ‘hit’ a moving

object (e.g. fetal cardiac chambers), then, the frequency of reflected sound wave

will be altered resulting in a ‘Doppler shift’.

Caution: Erroneous monitoring of maternal iliac vessels may occur. The

Doppler shift caused by moving blood within the vessels may result in the

maternal pulse to be monitored instead of the FHR.

Fetal Scalp Electrode - Invasive Monitoring

convert some of this energy into an electric current; this electric current from

reflected sound waves is used to determine the FHR.

How often the electric current to the piezoelectric crystal is turned on and off

determines the frequency of the sound waves. Frequency is measured in hertz

(with 1 hertz being 1 cycle per second).

Doppler ultrasound in this instance is not being used to create an image of the

fetus but rather to determine the frequency of the Doppler shift within the fetal

circulation changes. See Box 4.1 for an explanation of Doppler shift.

This is an invasive method of monitoring in which an electrode is attached to the

baby’s head. For this method to be suitable, the membranes either must have

already been ruptured or should be artificially ruptured to allow application of

the scalp clip. There must also be sufficient dilation of the cervix to allow the

electrode to pass through.CardioTOCOgraph – Measurement of Uterine Activity

Abdominal Transducer - Non Invasive Monitoring

Abdominal transducer is used to measure uterine activity (tocograph).

Measurement of FHR is achieved by measuring the time between R deflections

on the fetal ECG. This is referred to as the ‘R–R interval’.

One disadvantage is that this method of monitoring is not suitable for women

with an increased risk of vertical transmission, e.g. HIV, hepatitis B or C.

Rarely, the fetal scalp electrode (FSE) may cause injury to the fetal scalp.

One advantage of this method is a reduced chance of ‘loss of contact’ as the clip

is applied directly to the baby’s scalp.

The ‘toco’ is placed on the maternal anterior abdominal wall over the fundus of

the uterus, held in place by a stretchy elastic band to monitor the frequency and

length (i.e. duration) of uterine contractions. The amplitude of the tocograph is

related to the change in shape and tone of the anterior abdominal wall and does

not reflect the strength of the uterine contraction. As the uterus lies beneath the

anterior abdominal wall, it changes the shape and tone of the overlying

abdominal wall during uterine contractions. This creates a pressure wave that is

recorded by the tocograph (Figure 4.2).

However, other factors can also change the shape and tone of the abdominal wall

such as vomiting or pushing with the valsalva manoeuvre. Therefore, the

recording on the tocograph does not always represent uterine activity.

The strength of contractions is best assessed with how painful they are to the

woman, whether she is making good progress in labour and whether there are

ongoing changes (decelerations) on the CTG trace. Fewer contractions of a good

length and strength can be superior to frequent, weak, short-lived contractions in

achieving progress in labour.Figure 4.2 CTG display.

Internal Pressure Transducers - Invasive

Monitoring

CardiotocoGRAPH– Display of the CTG Trace

Paper Printout

This method of measuring the pressure generated by contractions uses direct

manometry or a pressure transducer on the tip of a flexible catheter, which is

threaded into the uterine cavity via the cervix. Though, in theory, they offer more

accurate measurement of strength and timings of contractions including the

‘resting tone’, they are rarely used outside of a research context in the United

Kingdom. One of the drawbacks of internal pressure monitoring is that the

uterine cavity is split into several compartments by the fetal parts. The pressure

in different compartments will vary leading to erroneous results.

All CTG traces should be identified with unique patient identifiers and correct

time and date as one would for any other documentation in a patient’s notes. It is

very important to check that paper has been loaded in correct orientation.

One should be aware of the ‘paper speed’, which refers to the speed at which

the CTG trace moves. In the United Kingdom, the paper speed is 1 cm per

minute, and in the United States, a paper speed of 3 cm per minute is used,

whereas in Scandinavian countries, a paper speed of 2 cm is used.Electronic Display and Storage

Pitfalls

Doubling of FHR

If the baseline FHR is <100 bpm, sometimes the CTG machine may double the heart rate

and, therefore, an erroneous recording may be obtained. This would result in a

‘bradycardia’ of 60 bpm being recorded as 120 bpm, leading to false reassurance. A

sudden shift in the previously recorded baseline heart rate, absence of accelerations and

a reduction in baseline variability may give a clue to doubling of FHR.

Halving of FHR

If the baseline FHR is >200 bpm, the CTG machine may halve the FHR to 100 bpm as it

tries to ‘autocorrelate’ the signals to ensure that the recording falls within the normal

Advantage: It can be inserted into the hand-held notes and travel between

centres with the mother. This allows any CTG traces done to be compared with

the fetus’ previous traces.

Disadvantage: Paper traces are recorded on ‘thermosensitive’ paper, which

degrades over time. This is a reason that traces are stored in dark-brown

envelopes to avoid fading when exposed to light. For risk management, the CTG

traces should be photocopied, which will avoid such fading and would enable

storage of traces for a longer period of time.

Electronic display allows the CTG to be displayed both in the room and on a

central monitor. This allows the labour ward coordinator or obstetrician to

monitor the CTG trace of more than one woman at a time without having to go

into the room or disturb the woman in labour. It allows the trace to be stored

electronically on a central system for a prolonged length of time.range. Therefore, in cases of fetal tachycardias, especially supraventricular

tachycardias, a lower heart rate may be erroneously monitored

Erroneous Monitoring of Maternal Heart Rate as FHR

If the fetal heart transducer is placed over the maternal iliac vessels, especially during

the second stage of labour when the fetal head (and the heart) is lower within the birth

canal, the transducer may pick up stronger signals from the pulsations of maternal iliac

vessels. This would lead to erroneous recording of maternal heart rate as FHR and

resultant false reassurance and poor perinatal outcomes. A sudden shift in baseline FHR,

accelerations coinciding with contractions and a sudden improvement in a decelerative

CTG trace may indicate erroneous monitoring of the maternal heart rate.

Loss of Contact or Poor Signal Quality

This may occur due to incorrect placement of the transducer or due to maternal obesity.

Internal monitoring using FSE should be considered, if there are no contraindications for

the same.

Interference

The use of a transcutaneous electrical nerve stimulation machine for pain relief during

labour may result in the interference of electrical signals, especially if fetal ECG signals

are obtained via the FSE.

Incorrect Placement of Thermosensitive Paper

The CTG trace may be recorded upside down (Figure 4.3), resulting in confusion or

errors in interpretation leading to poor perinatal outcomes.Figure 4.3 Recording of the CTG trace ‘upside down’ giving a false impression of

‘reduced baseline variability’. Note the date and time printed upside down at the bottom of

the CTG trace.

Further Reading

1. Chandraharan E, Arulkumaran S. Prevention of birth asphyxia: responding appropriately to

cardiotocograph (CTG) traces. Best Pract Res Clin Obstet Gynaecol. 2007; 21(4): 609–24.

2. Tolcher MC, Traynor KD. Understanding cardiotocography: technical aspects. Current

Women’s Health Reviews. 2013(9): 140–44.

3. Chandraharan E, Arulkumaran S. Electronic fetal heart rate monitoring in current and future

practice. J Obstet Gynecol India. 2008; 58(2): 121–30.