Dissatisfaction with the electronic recording of fetal heart rate and uterine contractions (the cardiotocogram
or CTG) has resulted in a search for new techniques of monitoring the fetus during labour. It is important
that each method has a sound physiological and pathophysiological basis, that a model for the interpretation
of changes is elucidated and that each method is thoroughly evaluated before introduction into clinical
practice. Analysis of the ST waveform of the fetal electrocardiogram (FECG) is the most advanced of the
new techniques under investigation. Experimental studies have shown that elevation of the ST waveform
occurs with a switch to myocardial anaerobic metabolism and a negative waveform occurs during direct
myocardial ischaemia. Human observational studies have suggested that a combination of ST waveform and
CTG analysis may improve the specificity of intrapartum monitoring and reduce unnecessary intervention.
A high quality FECG signal is necessary for waveform analysis. The FECG can be recorded from a scalp
electrode (FSE) during labour. The suitability of 5 commonly available FSEs for ECG waveform analysis
was compared. Single spiral FSEs had the most favourable physical and electrical properties and produced
the best quality signals in a randomised clinical trial of 50 fetuses in labour.
Intervention rates and neonatal outcome in labours monitored with CTG alone were compared with those
monitored with the combination of ST waveform analysis plus CTG (ST+CTG) in a randomised clinical
trial of 2434 high risk labours in a large district general hospital over an 18 month period. There was a 46%
reduction in operative intervention for fetal distress in the ST+CTG group (p<0.001, OR 1.96 [1.42-2.71]).
There was a trend to less neonatal metabolic acidemia (p = 0.09, OR 2.63 [0.93-7.39]) and fewer low five
minute Apgar scores (p = 0.12, OR 1.62 [0.92-2.85]) in the ST+CTG arm.
All recordings were reviewed retrospectively, blind to outcome and the CTG classified as normal,
intermediate or abnormal according to the trial protocol. There was no significant difference in the
proportion of recordings in each category between the trial arms. Operative intervention in the ST+CTG
arm was significantly reduced in recordings classified as normal and intermediate by the review (12/1043
ST+CTG arm versus 48/1066 CTG arm, p <0.001). Three patterns of ST+CTG change were identified. 1.
Normal CTG, persistent stable ST waveform elevation. These fetuses had good outcome and a
significantly higher mean pH (7.29) and lower base deficit (1.1 rnmol/1) at delivery. The raised ST
waveform may reflect sympathoadrenal stimulation from the general arousal of labour or a response to mild
but compensated hypoxaemia and is in keeping with experimental data. 2. CTG abnormal, progressive
elevation in ST waveform. All cases occurred towards the end of second stage. These fetuses had a
significantly lower mean pH (7.05) and higher base deficit (7.6 mmol/1) than all other groups. This
combination identified fetuses who were developing a metabolic acidosis as a result of significant hypoxia.
3. Abnormal CTG and a negative ST waveform. All cases with persistently negative waveforms were
depressed at birth, required resuscitation and had low arterial pHs (where available). This high risk group
probably had depleted myocardial glycogen reserves and suffered direct myocardial hypoxia, as seen in
animal studies. These findings indicate that ST waveform analysis can discriminate CTG change during
labour, the combination can result in a reduction in unnecessary intervention and has the potential to more
accurately identify fetuses at risk of neonatal morbidity.
The term 'monitoring' implies a degree of automatic surveillance but this is not the case as CTG and
ST+CTG records are subjectively interpreted, frequently by junior, inexperienced staff. The retrospective
review of cases in the trial revealed significant errors in the use of fetal blood sampling and the
interpretation of both CTG and ST+CTG recordings during the study. The feasibility of representing expert
clinical knowledge in a decision support tool to provide consistent, accurate interpretation of the CTG was
demonstrated in two clinical studies. The full potential of ST+CTG analysis may only be achieved with
some degree of automatic data processing and interpretation.
The randomised trial also demonstrated the lack of appropriate measures of neonatal outcome with which to
judge the effectiveness of fetal monitoring. Analysis of cord artery and vein blood gas status at delivery can
provide useful information about fetal oxygenation prior to delivery but currently the information is poorly
used, if at all. Use of erroneous data, inappropriate measures of 'acidemia', failure to distinguish between
respiratory and metabolic components and unphysiological expectations about relationships to other
measures of neonatal outcome were some of the problems highlighted. The use of generic terminology such
as 'birth asphyxia' or 'acidosis' which have varying definitions has caused much confusion and should be
avoided. There is unlikely to be one 'gold standard' measure of neonatal condition at delivery.
Date of Award | 1993 |
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Original language | English |
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Awarding Institution | |
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An evaluation of electronic fetal monitoring with clinical validation of ST waveform analysis during labour
Westgate, J. A. (Author). 1993
Student thesis: PhD