Fetal ECG during labour: a presentation of a microprocessor system

A second look at intrapartum fetal surveillance and future directions

Intrapartum fetal surveillance aims to predict significant fetal hypoxia and institute timely intervention to avoid fetal injury, and do so without unnecessary operative delivery of fetuses at no risk of intrapartum hypoxia. However, the configuration and application of current clinical guidelines inadvertently undermine these aims because of persistent failure to incorporate increased understanding of fetal cardiovascular physiology and adaptations to oxygen deprivation, advances in signal acquisition/processing, and related technologies. Consequently, the field on intrapartum fetal surveillance is stuck in rudimentary counts of the fetal R-R intervals and visual assessment of very common, but nonspecific fetal heart decelerations and fetal heart rate variability. The present authors argue that the time has come to move away from classifications of static morphological appearances of FHR decelerations, which do not assist the thinking clinician in understanding how the fetus defends itself and compensates for intrapartum hypoxic ischaemic insults or the patterns that suggest progressive loss of compensation. We also reappraise some of the controversial aspects of intrapartum fetal surveillance in modern obstetric practice, the current state of flux in training and certification, and contemplate the future of the field particularly in the context of the emerging role of artificial intelligence.

Invasive prenatal diagnostic techniques

Subspecialization in fetal medicine has expanded the scope for obstetricians to investigate the fetus and its surrounding environment in greater detail. Of the advances in technology that have made this possible, ultrasonography is the most important. A high degree of skill is required if it is to be applied to the performance of invasive diagnostic techniques. These should be performed only in specialized centres with appropriate facilities and an adequate referral base. However, it is important for obstetricians to know what is possible in the field of prenatal diagnosis and therapy in order to give patients the best advice.

The effect of maternal hypothermic cardiopulmonary bypass on fetal lamb temperature, hemodynamics, oxygenation, and acid-base balance

OBJECTIVE

To evaluate fetal-maternal temperature relationship and fetal cardiovascular and metabolic response during maternal hypothermic cardiopulmonary bypass in pregnant ewes.

METHODS

Cardiopulmonary bypass was instituted in 9 pregnant ewes, reaching 2 different levels of maternal hypothermia: 24 degrees C to 20 degrees C (deep hypothermia) in group A (5 cases) and less than 20 degrees C (very deep hypothermia) in group B (4 cases). Hypothermic levels were maintained for 20 minutes, then the rewarming phase was started. Fetal and maternal temperature, blood pressure, heart rate, electrocardiogram, blood gases, and acid-base balance were evaluated at different levels of hypothermia and during recovery.

RESULTS

Fetal survival was related to maternal hypothermia: all group A fetuses survived, while 2 of 4 fetuses of group B in which maternal temperature was lowered below 18 degrees C died in a very deep acidotic and hypoxic status. Maternal temperature was always lower than fetal temperature during cooling; during rewarming the gradient was inverted. The start of cardiopulmonary bypass and cooling was associated with transient fetal tachycardia and hypertension; then, both fetal heart rate and blood pressure progressively decreased. The reduction of fetal heart rate was of 7 beats per minute for each degree of fetal cooling. Deep maternal hypothermia was associated with fetal alkalosis and reduction of Po(2). Very deep hypothermia, in particular below 18 degrees C, caused irreversible fetal acidosis and hypoxia.

CONCLUSIONS

Deep maternal hypothermic cardiopulmonary bypass was associated with reversible modifications in fetal cardiovascular parameters, blood gases, and acid-base balance and therefore with fetal survival. On the contrary, fetuses did not survive to a very deep hypothermia below 18 degrees C.

Improving intrapartum surveillance: an individualised T/QRS ratio?

OBJECTIVES

To test the T/QRS ratio of the fetal electrocardiogram for normal distribution and assess the potential value of an individualised T/QRS ratio threshold to depict abnormality in the detection of fetal compromise during labour.

STUDY DESIGN

A retrospective analysis of twenty intrapartum fetal electrocardiogram recordings obtained on the labour ward of the Queen's Medical Centre, Nottingham.

RESULTS

In two of the twenty cases the T/QRS ratio was normally distributed. An increase in the T/QRS ratio over the 97.5th and 99.5th centile for 2 consecutive minutes, calculated on an individual basis, would appear to discriminate best between biochemically compromised and non-compromised fetuses. In no case was the T/QRS ratio seen to exceed 0.25 for periods previously described to be related to poor outcome.

CONCLUSION

T/QRS ratio changes with individually calculated criteria for abnormality may be of benefit in the detection of fetal compromise but the effect on the intervention rate remains to be established. The use of an absolute threshold for T/QRS ratio abnormality which is based on the assumption of a normal distribution needs to be viewed with caution.

The effect of epidural bupivacaine on the fetal electrocardiogram

Fetal electrocardiogram waveform analysis was used to assess the effect of epidural bupivacaine on the fetal myocardial conducting system by evaluating its effect on the PR interval, RR interval, T/QRS ratio and the PR-RR correlation coefficient. There were no significant changes in either the PR interval or the PR-RR correlation coefficient. There was a significant increase in the fetal heart rate and a significant fall in the T/QRS ratio. Epidural bupivacaine does not alter fetal myocardial conduction as measured by the PR interval and it does not induce ischaemic cardiac changes as assessed by the T/QRS ratio.

The fetal electrocardiogram

Advances in microprocessing technology have made fetal ECG analysis a feasible adjunct to fetal surveillance. Time interval and morphology changes of the FECG occur during fetal hypoxia. The use of these changes to detect a fetus at risk of intrapartum asphyxia awaits validation in terms of both future and ongoing clinical trials. Recognition of FECG changes during decelerations may improve the sensitivity of EFM. Antepartum FECG analysis has potential for the detection of a number of pathological fetal conditions, including intrauterine growth retardation, but remains hampered by low signal-to-noise ratios, rendering successful signal acquisition unreliable.

The effect of uterine contractions on the T/QRS ratio of the fetal electrocardiogram

The aim of this study was to establish whether the T/QRS ratio of the fetal electrocardiogram (ECG) was influenced by uterine contractions. Data was collected from 55 women during labour using a purpose built computer which also measured background noise. The T/QRS ratio was measured on individual complexes and the measurements averaged. Measurements made during periods of excessive noise were excluded. The average T/QRS ratio during contractions showed a small but significant increase of 1.02% (principally in the second half of the contraction). The significance of this small increase is uncertain and for most fetuses the ratio would remain in the normal range during contractions. The higher T/QRS ratios, recorded in the noisy records, draw attention to the importance of accounting for this problem when fetal ECG data are reported.

The use of the fetal electrocardiogram in labour

It is clear that considerably more work needs to be done before fetal ECG waveform analysis can be adopted for the purposes of routine fetal monitoring. It must be decided exactly what we want fetal ECG waveform analysis to achieve. If it is to reduce the number of operative deliveries performed in the presence of normal or intermediate CTG patterns then the fetal ECG shows promise, but its physiological significance in this context remains uncertain. If fetal ECG analysis is to be related to outcome, as opposed to events in labour, then several very large well conducted randomised prospective controlled trials will be needed. At present a large multicentre study (European Community Concerted Action Project 1989) is taking place in which a large number of intrapartum ECG records are being collected, blinded to the clinician and stored onto optical disc via a personal computer. A multicentre study in the United Kingdom using the Nottingham system is also being planned. We need await the outcome of these to verify if any of a variety of fetal ECG variables can be related to outcome. It behoves us to validate fetal ECG analysis against outcome measures that truly indicate whether the labour process has compromised or damaged the fetus. We must not fall into the trap of yet again wholeheartedly embracing a test that has not been properly validated.

Problems in the clinical use of intrapartum fetal ECG monitoring

Commercial machines are now available to monitor the fetal electrocardiograph in labour (Cinventa, Sweden). We report our experience of the first one hundred women we monitored by this method. They were divided into five groups dictated by the change in the fetal scalp electrode used for monitoring and the changes made in the computer software used for signal processing. There were progressive significant improvements in the ability of the system to produce a continuous heart rate trace such that it became acceptable for routine fetal monitoring. The quality of the electrocardiogram improved to 50% of the 'check ECG complexes' being printed accurately. Electrical signal distortion causing baseline wander is however the significant remaining problem. On-line analysis of the T/QRS ratio improved very significantly from 36% to 84% of the monitoring time. There were no significant improvements in the recordings made in the second stage, which remained poorer in quality and reliability in all groups. As well as changes in the computer software, the use of a single helix electrode and practical experience contributed to the improvements.

ECG waveform analysis in intrapartum fetal monitoring

Intrapartum cardiotocography (CTG) has a high false positive rate and, in the absence of facilities for fetal scalp blood sampling, is associated with increased operative deliveries for 'fetal distress'. It is presently possible to obtain a representative fetal ECG signal using the fetal scalp electrode. There is renewed interest in fetal ECG waveform analysis in the hope that it will improve the specificity of detecting intrapartum fetal hypoxia when there is an abnormal fetal heart rate pattern. Two portions of the fetal ECG are potentially useful: the ST waveform (T/QRS ratio) and the PR interval correlated with the RR interval. Animal experiments suggest that changes in the ST waveform, increase in the T wave amplitude in particular, reflect myocardial anaerobic metabolism. Clinical studies have shown poor correlation between the T wave changes and fetal heart rate changes. There is also concern that the sensitivity of the ST waveform changes for fetal acidaemia may be poor. More information is required before its use is incorporated into routine clinical practice.

Non-invasive optical monitoring of cerebral blood oxygenation in the foetus and newborn: preliminary investigation

Near infra-red spectroscopy was applied as a non-invasive and continuous technique for the in vivo monitoring of blood and tissue oxygenation in human neonates. Monitoring of cerebral blood oxygenation in the wavelength range 775-904 nm was carried out on preterm infants after inducing a transient mild hypoxic change; the measurements were performed either by the transmission or reflection (backscattering) mode of monitoring. The results of these investigations were used to assess the application of the technique to foetal monitoring. A series of foetal monitoring studies was performed to investigate the influence of maternal contractions on foetal cerebral blood oxygenation. Although only changes in haemoglobin concentration can be monitored at present, the results suggest that near infra-red monitoring could provide a non-invasive, real-time monitoring method in intensive neonatal and intrapartum care.

Clinical assessment of fetal electrocardiogram monitoring in labour

OBJECTIVE

To assess the potential clinical value of fetal electrocardiographic (ECG) monitoring in labour.

DESIGN

Descriptive study of the use of ECG waveform analysis during labour and its correlation with other indices of fetal and neonatal well-being.

SETTING

Teaching hospital in Oxford.

SUBJECTS

86 high risk pregnancies.

MAIN OUTCOME MEASURES

Suitability of ST Segment Analyser (Cinventa, Sweden) for clinical use; relation between the T/QRS ratio during labour (an index of ST segment and T wave elevation) and intrapartum cardiotocography, umbilical artery pH at birth and Apgar scores; T/QRS ratio trends in labour.

RESULTS

The system was robust and user-friendly. No statistically significant relation was found between T/QRS ratios in labour and FHR abnormalities in the cardiotocograph. There was a weak relation between T/QRS ratios and umbilical artery acidosis: at a cervical dilatation of 4 cm, the Spearman rank correlation of the mean T/QRS ratio with umbilical artery actual base deficit was r = -0.31, 0.05 greater than P greater than 0.01. The correlation of T/QRS with the umbilical artery pH was not statistically significant, although a trend was present (Spearman rank: r = -0.26, P greater than 0.05). Correlation of the T/QRS ratio with Apgar scores at 1 and at 5 min was not statistically significant. Only 3 of 16 infants with an Apgar score of less than 7 at 1 min had a mean T/QRS ratio above 0.25 at any time during labour.

CONCLUSION

Further research is necessary before a decision can be made whether this new method of fetal monitoring should be introduced into clinical practice.

Fetal acidaemia, the cardiotocograph and the T/QRS ratio of the fetal ECG in labour

OBJECTIVE

To relate the T/QRS ratio of the fetal electrocardiogram (ECG) to the cardiotocogram (CTG) and fetal pH during labour.

DESIGN

Prospective data collection from selected monitored labours.

SETTING

A postgraduate teaching hospital delivery suite.

SUBJECTS

113 women in labour at term.

MAIN OUTCOME MEASURES

Correlation of fetal T/QRS ratio values with pH values at the time of fetal blood sampling and at birth (umbilical artery blood). Comparison of the predictive values of raised T/QRS ratio and a pathological CTG for fetal acidemia.

RESULTS

Complete data sufficient for analysis was available for 51 fetal scalp blood samples and 93 umbilical artery pH samples. The median (range) of T/QRS ratio values before birth of 88 babies not requiring admission to the neonatal unit was 0.13 (0.00-0.32) with a 97.5th centile value of 0.28. T/QRS ratios did not correlate with fetal scalp pH values. Fetal scalp acidaemia (pH less than 7.20) was detected with rates of 50 and 13% respectively by a pathological CTG and by a T/QRS ratio above 0.28, the positive predictive values being 40% and 50%, respectively. There was a significant correlation between increasing T/QRS ratio and falling pH. Detection rates (sensitivities) for umbilical artery acidaemia (pH less than 7.12) were 76% and 29% whereas positive predictive values were 38% and 71% respectively for a pathological CTG and a T/QRS ratio above 0.28.

CONCLUSION

A raised T/QRS ratio (greater than 0.28) had a considerably lower detection rate for fetal acidaemia during labour than a pathological CTG.

Direct antenatal fetal electrocardiographic waveform analysis

OBJECTIVE

To establish a technique for continuous recording of fetal electrocardiograms (ECG) for waveform analysis in the antenatal period.

DESIGN

Prospective descriptive study.

SETTING

Fetal Medicine Unit, Queen Charlotte's and Chelsea Hospital, London, UK.

SUBJECTS

35 women undergoing antenatal fetal blood sampling.

INTERVENTIONS

One end of an insulated Cooner wire was attached to the sampling needle and the other to an automatic ECG-ST waveform analyser.

MAIN OUTCOME MEASURES

ECG signals were obtained with the needle in the fetal abdomen during intrahepatic umbilical vein sampling or aspiration of fetal urine but not when it was in the placental cord insertion.

RESULTS

Continuous recording of the T/QRS ratio was obtained for a total of 166 min (mean 8 min per fetus) from 20 fetuses (16-38 weeks). The T/QRS ratio had no correlation with gestational age and fetal heart rate and was similar to values described in term fetuses in labour.

CONCLUSIONS

The technique described can identify ST waveform changes and may be useful in the investigation of fetal cardiac arrhythmias, intrauterine growth retardation and in monitoring fetal transfusions.

Fetal ECG waveform analysis should improve fetal surveillance in labour

Fetal heart rate (FHR) and fetal electrocardiogram (ECG) recordings were obtained from a scalp electrode with maternal thigh as reference and used for ST waveform analysis in 201 patients in labour. Nearly 45% had suspicious or abnormal FHR traces whilst only 27% had T/QRS ratio greater than 0.25 (mean +/- 2 SD). A normal T/QRS ratio identified 99.3% of fetuses with normal buffering capacity in cord artery blood. Of 13 infants with a cord artery blood pH less than 7.15, standard bicarbonate was less than 15.0 mmol/l in five who had an average T/QRS ratio less than 0.25 throughout labour. Of the eight with respiratory acidosis, five had an increase in T/QRS ratio greater than 0.25 for longer than 20 minutes prior to delivery, in two the ratio increased during the last few minutes and one had no change (pH 7.14). Persistent elevation of T/QRS in the first stage of labour identified those with decrease in buffer capacity in cord arterial blood (sensitivity of 94.1%). Acute hypoxia was recognized by the rapid rise in T/QRS. The specificity of T/QRS to identify fetuses at risk increased by combining the ST waveform analysis with FHR changes.

Long-term ST waveform changes in the ovine fetal electrocardiogram: the relationship to spontaneous labour and intrauterine death

The experimental evaluation of the fetal ECG waveform as a method of fetal surveillance has been based largely on experimentally induced hypoxia. The present data, obtained from chronically instrumented fetal lambs, includes three fetuses monitored throughout spontaneous labour, six fetuses with spontaneously developed long-term ST waveform changes and another three fetuses which died in the post-operative period. Uterine contractions could by themselves cause an increase in T wave height (T/QRS ratio greater than 0.30). If the ST elevation was normalised between contractions the fetus seemed to compensate to a moderate degree of hypoxia. When oxygenation was further reduced the T wave remained elevated between contractions and a progressive increase occurred in the T/QRS ratio (greater than 0.60) during the final stages of labour, in parallel with increasing blood lactate levels. Long-term ST changes with T/QRS consistently exceeding 0.30 were found to be related to low haemoglobin levels and/or fetal hypotension. Subsequently, all these fetuses died during labour as compared with a survival rate of 40% in fetuses showing a normal ST waveform. In this group negative T waves are reported as a sign of failing myocardial response to hypoxia. Death in utero, whatever the cause (bleeding, infection or spontaneous hypoxia) was always preceded by marked ST waveform changes. It is concluded that ST elevation with high peaked T waves can appear on a long-term basis in fetuses with intrauterine complications.

STAN--the Gothenburg model for fetal surveillance during labour by ST analysis of the fetal electrocardiogram

Waveform analysis of the fetal ECG for fetal surveillance is regaining widespread interest. This paper presents our present knowledge on ST waveform analysis during human labour. A unipolar scalp lead with the maternal thigh as reference makes it possible to identify the T wave and at the same time decrease signal noise. The development of an ECG analyser (STAN) has enabled us to collect 201 ECG recordings during labour on which ST analysis could be undertaken. The data indicate that waveform analysis of the fetal ECG improves fetal surveillance during labour.

Electrocardiogram waveform in fetal sheep: a system for ECG analysis

Computerised ECG analysis is helpful for the analysis of fetal ECGs, recorded during animal experiments. It provides a tool for rapid measurement of characteristics of ECG signals. Such an ECG analysis system is described. The different steps of the signal analysis are discussed and preliminary results are presented. The current version of the program is implemented in Turbo-Pascal and runs on an IBM-compatible PC with an 8087 processor and an EGA colour display.

Electrocardiographic waveform changes and catecholamine responses during acute hypoxia in the immature and mature fetal lamb

The aim of the present study was to investigate the changes in electrocardiographic waveform and the release of catecholamines in the fetal lamb during nonacidemic fetal hypoxia. Chronically instrumented fetal lambs were subjected to reproducible hypoxia by reduction of the maternal placental blood flow. This was achieved by complete obstruction of the maternal aorta for 60 seconds. The fetuses were divided into an immature (119 to 126 days, n = 10) and a mature group (129 to 141 days, n = 6). Both groups of fetuses had a marked fall in oxygen tension (from 2.43 +/- 0.12 to 1.46 +/- 0.12 and 2.22 +/- 0.15 to 1.11 +/- 0.17 kilopascals [kPa] in the immature and mature groups, respectively) and in oxygen saturation (from 48% +/- 3% to 17% +/- 2% and 49% +/- 3% to 15% +/- 3%, respectively), but only modest changes occurred in pH and carbon dioxide tension. Basal fetal catecholamine concentrations did not differ between the groups but increased more significantly in the mature group with acute hypoxia. An increase in the T wave amplitude of the fetal electrocardiogram occurred in both groups during the latter part of occlusion with peak values shortly after removal of the occlusion. A linear correlation was found between the plasma epinephrine concentration and the T/QRS ratio in the mature group. Fetuses in both groups showed a marked bradycardia of similar magnitude during the occlusion but differed during the early phase of heart rate recovery by a slower acceleration of heart rate in the mature group. In connection with this marked bradycardia, the mature group showed a significant rise in mean arterial blood pressure at end of the occlusion. We suggest that fetal maturity has a significant influence no only on the release of catecholamines during nonacidemic hypoxia but also on the cardiovascular reaction pattern and changes in the ST waveform.

Fetal monitoring

This paper introduces fetal monitoring by describing the medical problems of the fetus in utero. The various methods currently used to monitor the fetus are reviewed with their respective advantages and shortcomings. New methods of surveillance of the fetus are highlighted as well as the mother's reaction to the technology.