ST waveform changes of the fetal electrocardiogram during labour--a clinical study

Intrapartum Fetal Electrocardiogram in Small- and Large-for-Gestational Age Fetuses

OBJECTIVE

This study aimed to evaluate whether intrapartum fetal electrocardiogram (ECG) tracings with ST-elevation or depression occur more frequently in each stage of labor in small-for-gestational age (SGA) or large-for-gestational age (LGA), as compared with appropriate-for-gestational age (AGA) fetuses.

STUDY DESIGN

We conducted a secondary analysis of a large, multicenter trial in which laboring patients underwent fetal ECG waveform-analysis. We excluded participants with diabetes mellitus and major fetal anomalies. Birth weight was categorized as SGA (<10th percentile), LGA (>90th percentile), or AGA (10-90th percentile) by using a gender and race/ethnicity specific nomogram. In adjusted analyses, the frequency of ECG tracings with ST-depression or ST-elevation without depression was compared according to birthweight categories and labor stage.

RESULTS

Our study included 4,971 laboring patients in the first stage and 4,074 in the second stage. During the first stage of labor, there were no differences in the frequency of ST-depression in SGA fetuses compared with AGA fetuses (6.7 vs. 5.5%; adjusted odds ratio [aOR]: 1.41, 95% confidence interval [CI]: 0.93-2.13), or in ST-elevation without depression (35.8 vs. 34.1%; aOR: 1.17, 95% CI: 0.94-1.46). During the second stage, there were no differences in the frequency of ST-depression in SGA fetuses compared with AGA fetuses (1.6 vs. 2.0%; aOR: 0.69, 95% CI: 0.27-1.73), or in ST-elevation without depression (16.2 vs. 18.1%; aOR: 0.90, 95% CI: 0.67-1.22). During the first stage of labor, there were no differences in the frequency of ST-depression in LGA fetuses compared with AGA fetuses (6.3 vs. 5.5%; aOR: 0.97, 95% CI: 0.60-1.57), or in ST-elevation without depression (33.1 vs. 34.1%; aOR: 0.80, 95% CI: 0.62-1.03); during the second stage of labor, the frequency of ST-depression in LGA compared with AGA fetuses (2.5 vs. 2.0%, aOR: 1.36, 95% CI: 0.61-3.03), and in ST-elevation without depression (15.5 vs. 18.1%; aOR: 0.83, 95% CI: 0.58-1.18) were similar as well.

CONCLUSION

The frequency of intrapartum fetal ECG tracings with ST-events is similar among SGA, AGA, and LGA fetuses.

KEY POINTS

· SGA and LGA neonates are at increased risk of cardiac dysfunction.. · Fetal ECG has been used to evaluate fetal response to hypoxia.. · Fetal ST-elevation and ST-depression occur during hypoxia.. · Frequency of intrapartum ST-events is similar among SGA, AGA and LGA fetuses..

Fetal electrocardiography ST-segment analysis for intrapartum monitoring: a critical appraisal of conflicting evidence and a way forward

BACKGROUND

In the past century, some areas of obstetric including intrapartum care have been slow to benefit from the dramatic advances in technology and medical care. Although fetal heart rate monitoring (cardiotocography) became available a half century ago, its interpretation often differs between institutions and countries, its diagnostic accuracy needs improvement, and a technology to help reduce the unnecessary obstetric interventions that have accompanied the cardiotocography is urgently needed.

STUDY DESIGN

During the second half of the 20th century, key findings in animal experiments captured the close relationship between myocardial glycogenolysis, myocardial workload, and ST changes, thus demonstrating that ST waveform analysis of the fetal electrocardiogram can provide information on oxygenation of the fetal myocardium and establishing the physiological basis for the use of electrocardiogram in intrapartum fetal surveillance.

RESULTS

Six randomized controlled trials, 10 meta-analyses, and more than 20 observational studies have evaluated the technology developed based on this principle. Nonetheless, despite this intensive assessment, differences in study protocols, inclusion criteria, enrollment rates, clinical guidelines, use of fetal blood sampling, and definitions of key outcome parameters, as well as inconsistencies in randomized controlled trial data handling and statistical methodology, have made this voluminous evidence difficult to interpret. Enormous resources spent on randomized controlled trials have failed to guarantee the generalizability of their results to other settings or their ability to reflect everyday clinical practice.

CONCLUSION

The latest meta-analysis used revised data from primary randomized controlled trials and data from the largest randomized controlled trials from the United States to demonstrate a significant reduction of metabolic acidosis rates by 36% (odds ratio, 0.64; 95% confidence interval, 0.46-0.88) and operative vaginal delivery rates by 8% (relative risk, 0.92; 95% confidence interval, 0.86-0.99), compared with cardiotocography alone.

Scientific and clinical evidence for the use of fetal ECG ST segment analysis (STAN)

Fetal electrocardiogram waveform analysis has been studied for many decades, but it is only in the last 20 years that computerization has made real-time analysis practical for clinical use. Changes in the ST segment have been shown to correlate with fetal condition, in particular with acid-base status. Meta-analysis of randomized trials (five in total, four using the computerized system) has shown that use of computerized ST segment analysis (STAN) reduces the need for fetal blood sampling by about 40%. However, although there are trends to lower rates of low Apgar scores and acidosis, the differences are not statistically significant. There is no effect on cesarean section rates. Disadvantages include the need for amniotic membranes to be ruptured so that a fetal scalp electrode can be applied, and the need for STAN values to be interpreted in conjunction with detailed fetal heart rate pattern analysis.

Intrapartum cardiotocography (CTG) and ST-analysis of labor in diabetic patients

AIM

To determine the prevalence and types of intrapartum cardiotocography (CTG) patterns and investigate their relationship to moderate acidemia in term fetuses of diabetic mothers. Also, to assess if the combination of fetal electrocardiogram (FECG) and those CTG patterns strengthens the association with moderate acidemia.

MATERIAL AND METHODS

The material for this study is obtained from the Swedish randomized control trial and the European Union ST-analysis trial. We developed an analytical model for CTG patterns based on the progress in CTG changes, in a longitudinal periodic manner. The model was then combined with information regarding changes in ST interval that indicate threatening asphyxia, and the findings were analyzed to determine correlation with the presence of moderate acidemia at birth.

RESULTS

This study involved data of 413 diabetic mothers. A preterminal CTG was more common in the diabetes mellitus (DM) group (6/70, 8.6%) than in the gestational diabetes (GD) group (3/307, 1.0%; P=0.003). For diabetic mothers (i.e., DM+GD) with a normal CTG at the start of monitoring, the presence of FECG data indicating asphyxia significantly increased the likelihood of an umbilical artery pH<7.15 at birth [odds ratio (OR)=3.65, 95% confidence interval (CI)=1.33-10.05]. Among labors where the CTG was non-reassuring at the start of monitoring, no significant association was found between pH<7.15 and indication to intervene according to FECG information (OR=1.51, 95% CI=0.33-7.0).

CONCLUSIONS

A preterminal CTG is more common in the fetuses of DM than GD mothers during labor. When CTG was normal at the start of recording, the addition of FECG information gave a significant add on information to predict moderate acidemia.

ST analysis of fetal electrocardiography in labor

Since its introduction more than 40 years ago, electronic fetal monitoring has become widely used for intrapartum surveillance to determine fetal wellbeing in labor. Although fetal hypoxia and acidosis are reflected in changes in fetal heart rate, there is no evidence that cardiotocography has been effective in reducing neonatal morbidity related to fetal distress occurring during labor. Indeed the specificity of this tool is poor and in many instances the incorporation of electronic fetal monitoring into intrapartum care has merely led to an increase in medical intervention rather than an improvement in neonatal outcome. Fetal electrocardiography (ECG) analysis provides an additional method for assessing the response of the fetus to hypoxia and in particular to the development of metabolic acidosis. ST changes in the fetal ECG can be quantified with computational analysis, reducing subjective interpretation that has been problematic with traditional electronic fetal monitoring. Formal algorithms indicating appropriate points for intervention in labor have been designed. The fetal ECG has been shown to be a useful adjunct to traditional electronic fetal monitoring in several randomized controlled trials with evidence of reduced rates of neonatal encephalopathy and reduced rates of obstetric intervention.

ST analysis as an adjunct to electronic fetal monitoring: an overview

Conventional electronic fetal monitoring in the United States has value as a screening tool but is extremely limited as a diagnostic tool. ST analysis was developed as an adjunctive technology, able to measure changes in the ST segment of the fetal electrocardiogram during periods of hypoxia, improving the identification of the fetus at risk for metabolic acidemia at birth. Currently used only in a handful of hospitals in the United States, studies abroad have demonstrated that an integrated approach utilizing electronic fetal monitoring, ST analysis, and standardized guidelines in a selected patient population can improve neonatal outcome, decrease acidemia at birth, and decrease obstetric operative delivery. Research is needed to determine whether similar results are possible in the US population.

Fetal cerebral energy metabolism and electrocardiogram during experimental umbilical cord occlusion and resuscitation

OBJECTIVE

The purpose of this experimental study was to elucidate alterations in fetal energy metabolism in relation to ECG changes during extreme fetal asphyxia, postnatal resuscitation and the immediate post-resuscitatory phase.

STUDY DESIGN

Five near-term fetal sheep were subjected to umbilical cord occlusion until cardiac arrest followed by delivery, resuscitation and postnatal pressure-controlled ventilation. Four sheep served as sham controls and were delivered immediately after ligation of the umbilical cord. Fetal ECG was analysed online for changes of the ST segment. Fetal metabolism was monitored by intracerebral and subcutaneous microdialysis catheters.

RESULTS

Fetal ECG reacted on cord occlusion with an increase in the T-wave height followed by changes in intracerebral levels of oxidative parameters. Cerebral lactate/pyruvate ratio and glutamate increased to median (range) of 240 (200-744) and 34.0 (22.6-60.5) mmol/l, respectively; both parameters returned to baseline after resuscitation. Cerebral glucose decreased to 0.1 (0.08-0.12) mmol/l after occlusion and increased above baseline upon resuscitation. In subcutaneous tissue as well as blood the increase in lactate occurred with a delay compared to cerebral levels.

CONCLUSION

The fetal ECG changes related to asphyxia preceded the increase in excitotoxicity as determined by increase in cerebral glutamate during asphyxia. Cerebral lactate increase was superior to subcutaneous lactate increase.

Acceptability of the fetal electrocardiographic (STAN) monitoring system by staff at a high risk maternity unit

AIMS

To assess staff acceptability of STAN monitoring system.

METHODS

This was a prospective study of health care professionals involved in the first 18 months of STAN use at a high risk maternity unit.

RESULTS

During the study period 75% of midwives and 80% of doctors questioned felt confident when interpreting STAN cases. A total of 88% of midwives and 90% of doctors felt adequately supported in their actual clinical use of the system. Of all the obstetricians who used the STAN, 95% felt it was an acceptable alternative to fetal blood sampling. One of the main complaints was poor signal quality which occurred in 16% of cases. As many as 97% of doctors and 96% of midwives considered STAN to be an acceptable way of monitoring high risk women after its introduction.

CONCLUSIONS

Introduction of a new monitoring system to a high risk labor ward is possible, and acceptable to its staff. Constant practical and theoretical support and adherence to strict, easily understood guidelines is required.

Maternal satisfaction and acceptability of foetal electrocardiographic (STAN®) monitoring system

OBJECTIVE

To assess the acceptability of the foetal electrocardiographic (STAN®) monitoring system by patients at a London Hospital, before its introduction into routine clinical care.

METHOD

This was a prospective questionnaire-based study of patients who were monitored in labour by foetal electrocardiographic (STAN®) monitoring system.

RESULTS

Ninety-three percent (CI 85, 98) of the patients felt that the midwife(s) responsible for their intrapartum care took time to explain why their babies were being monitored continuously in the first instance. Ninety-nine percent (CI 93, 99.9) of patients felt that the obstetricians explained why they were being monitored continuously in labour. After delivering and having been monitored with STAN® 95% (CI 87, 99) of women felt that it was an acceptable way of monitoring their babies in labour.

CONCLUSION

In resource-limited organisation, such as the NHS, holistic approach to intrapartum care may sometimes feel difficult to achieve. However, with emphasis on women centred care and information sharing women are able to understand and comment on intrapartum monitoring systems.

Intrapartum fetal ECG and diabetes

AIM

The objective of this study was to determine the prevalence and types of ST-segment changes of the fetal electrocardiogram (FECG) during labour in term fetuses born to mothers with diabetes mellitus (DM) or gestational diabetes.

METHODS

This was a retrospective case-control study involving populations from two multi centre trials: the Swedish Randomized Control Trial and the European Union ST-analysis (EU-STAN) trial. ST-segment changes were assessed in 104/309 cases and 207/468 controls from the Swedish and EU-STAN trials, respectively.

RESULTS

ST depression was present on the FECG in 22.1% of fetuses of mothers with DM compared to 12% of controls OR = 2.6, 95% CI = 1.4-4.7, p = 0.002 after adjusting for trial, birth weight, and nulliparity. ST elevation was present in 47.1% of DM patients and 41.2% of controls (OR = 1.4, 95% CI = 0.9-2.3, p = 0.18).

CONCLUSION

ST depression on the FECG was significantly more prevalent in the fetuses of mothers with DM, probably not indicating hypoxia but an altered ability of the myocardium to respond to the stress of labour. Further studies into the mechanism of fetal compromise during diabetic labour, are required.

Use of Fetal Electrocardiogram for Intrapartum Monitoring

Introduction: Intrapartum fetal monitoring is essential for the identification of fetal hypoxia to reduce perinatal morbidity and mortality. Cardiotocography is associated with low specificity for fetal acidosis and poor perinatal outcome leading to unnecessary operative deliveries. ST waveform analysis of the fetal electrocardiogram has been shown to be a promising adjunctive intrapartum assessment tool. We aim to present the pathophysiology, the role of intrapartum monitoring and the practical usage of this relatively new technology in our review. Methods: An electronic search of Medline and OVID was carried out, followed by a manual search of the references identified by the electronic search. Results: The incorporation of ST waveform analysis to cardiotocography has been shown to reduce the rates of neonatal metabolic acidosis, moderate and severe neonatal encephalopathy, thus improving perinatal outcome. The reduction in operative delivery rates due to fetal distress is also significant. The pathophysiology and practical usage of this technology were discussed. Conclusions: With more accurate identification of fetal hypoxia and reduction of unnecessary intervention rates, incorporation of ST waveform analysis of fetal electrocardiography into cardiotocography can improve the standard of intrapartum fetal monitoring. Key words: Cardiotocography, Fetal electocardiography, Fetal ST waveform analysis, Intrapartum, Perinatal outcomes

Prenatal diagnosis of QT prolongation by fetal magnetocardiogram--use of QRS and T-wave current-arrow maps

To determine the T wave of a fetal magnetocardiogram (FMCG), we have evaluated the T/QRS ratio and obtained current-arrow maps that indicate weak currents. We measured FMCG signals for 52 normal fetuses and two abnormal fetuses with prolonged QT waves by using three superconducting quantum interference device (SQUID) systems: a nine-channel system, a 12-channel vector system and a 64-channel system. The T/QRS ratio was calculated for all the normal fetuses from the maximum magnitudes of the QRS complex and the T wave. Current-arrow maps of the QRS complex (R wave) and T wave were obtained by using the 64-channel system, and the phase differences of the total-current vectors were calculated by using the current-arrow maps. The results showed that the T/QRS ratio had a wide variability of 0.35 for the normal fetuses. However, the magnitude of the prolonged T wave was as weak as the detection limit of the SQUID magnetometer. Although the T/QRS ratios for the fetuses with QT prolongation were within the normal range (< 0.35), the weak magnitude of the prolonged T wave could be evaluated. On the other hand, by comparing the current-arrow maps of the R and T waves for the normal fetuses, we found that the maximum-current arrows were indicated as either in the same direction or in opposite directions. These patterns could be identified clearly by the phase differences. Very weak prolonged T waves for the two abnormal fetuses could be determined by using these current-arrow maps and phase differences. Consequently, although the T/QRS ratios of FMCG signals have a wide distribution, we have concluded that the current-arrow map and phase difference can be used to determine the T wave of an FMCG signal.

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.

Electronic fetal monitoring: a brief summary of its development, problems and prospects

Electronic fetal monitoring (EFM) was introduced into clinical practice 30 years ago and spread very rapidly in most developed countries. The early optimism that EFM would lead to a marked reduction in fetal neurological injury has not been realized; however, it is now recognized that most such damage is unrelated to perinatal events. Clinical trials have shown that although EFM does reduce the incidence of intrapartum asphyxia, its use is also associated with an increase in cesarean sections. Abnormal fetal heart rate (FHR) patterns are poor predictors of fetal depression at birth when used without additional confirmatory information. An additional problem has been inconsistency in the interpretation of EFM tracings even among experts. This has reduced the clinical effectiveness of EFM and has also contributed to an increase in litigation in cases with adverse neonatal outcomes. Despite these shortcomings EFM continues to be used extensively on most obstetrical services, suggesting that obstetrical physicians and nurses find the technique helpful and will continue to use it until a better alternative comes along. The combination of relevant clinical data with EFM by means of intelligen computer systems may improve both the consistency and predictive value of intrapartum fetal assessment in the future.

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 T/QRS ratio of the electrocardiogram does not reliably reflect well-being in fetal lambs

OBJECTIVE

Our purpose was to determine the diagnostic power of the T/QRS ratio of the electrocardiogram to predict fetal well-being.

STUDY DESIGN

In 47 fetal lambs (3 to 5 days after surgery, gestational age 123.5 +/- 3.0 days) asphyxia was induced by restriction of uterine perfusion. Fetuses were either pretreated with an adenosine transport inhibitor (n = 16) or a calcium channel blocker (n = 12) or served as controls (n = 19). Arterial oxygen content > or = 1.5 mmol/L or pH > or = 7.15 were chosen as limits for fetal well-being.

RESULTS

Arterial oxygen content was reduced from 3.3 (+/- 1.0) to 1.3 (+/- 0.5) mmol/L, and pH decreased to 7.03 (+/- 0.10). Mortality was 53%. Both drugs did not affect well-being, survival, or the T/QRS ratio. Maximum T/QRS ratios were reached at the peak of asphyxia. Sensitivity and specificity of the T/QRS ratio were 24.0% and 42.6% to predict hypoxemia and 25.1% and 45.3% to predict acidemia. Pearson correlation coefficients for T/QRS ratio versus oxygen content and pH were 0.169 and 0.192, respectively.

CONCLUSIONS

(1) In fetal lambs the T/QRS ratio failed to predict hypoxemia or acidemia. (2) Fetal survival was not correlated with the height of the T/QRS ratio during or after asphyxia.

Antenatal fetal magnetocardiography: a new method for fetal surveillance?

OBJECTIVE

To establish the reliability of fetal magnetocardiography as a method of measuring the time intervals of the fetal heart during the antenatal period.

DESIGN

A prospective study.

SETTING

Wellcome Biomagnetism Unit, Southern General Hospital.

SUBJECTS

One hundred and six low risk pregnant women at 20 to 42 weeks gestation.

MAIN OUTCOME MEASURES

Success in obtaining QRS complexes, P waves and T waves. Correlation of time intervals with fetal outcome.

RESULTS

The technique was acceptable to pregnant women. A QRS complex was successfully demonstrated in 68 (67%) of the unaveraged traces. Using off-line averaging techniques on these 68 cases, P waves were obtained in 75% and T waves in 72%. Although good quality traces were obtained throughout the range of gestational ages, in general it was more difficult below 28 weeks. QRS duration (R2 = 7%, P = 0.02) demonstrated a positive linear correlation with increasing gestation. Of the 35 (51%) cases with umbilical vein pH analysis available, only one result was less than 7.2. No significant relation was found between measurements of the fetal waveforms and the pH results.

CONCLUSION

The technique of fetal magnetocardiography provides a significant advance in the technological field for the demonstration of QRS complexes and the full PQRST waveforms in gestations from 20 weeks onwards. With further technical improvements the clinical impact of this technique can be assessed more fully.

Improved equipment for abdominal fetal electrocardiogram recording: description and clinical evaluation

Reliable computer-based equipment for transabdominal or indirect recording of fetal electrocardiogram (FECG) is described. The proposed equipment allows a real-time displaying of the signals (fetal + maternal ECG) without averaging procedures and it does not require the employment of a shielded room; moreover, it is user-friendly to medical personnel. An elementary form of semi-automatic computation of the fetal heart rate (FHR) was also implemented. The equipment simultaneously acquires three signals from seven electrodes, six placed on the maternal abdomen following the three space axes, and one placed on the left leg as a indifferent electrode. The signals are magnified and analogically filtered before undergoing digital finite impulse response (FIR) filter. Then the signals are displayed on the screen of a personal computer (PC). The PC also provides the possibility of storing the acquired signals for further analysis of elaborations. The quality of the recordings allows the analysis of both the rapid and slow electrical phenomena of the fetal heart, and it is not significantly influenced by the occurrence at the same time of uterine contractions. The performance of this method was assessed in 140 pregnant women with gestation periods of 29-42 weeks. In 131 cases (93.6%) the fetal QRS complex was detected and the FHR was obtained. A reliable evaluation of P and QRS waves and of ST interval, in spite of the interference of the maternal complexes, was possible in 102 cases (72.8%).

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.

The fetal ECG: accuracy of measurements

Analysis of fetal heart rate is very dependent upon the accuracy with which fetal events can be timed. This depends upon signal quality and the type of processing used in signal analysis. Signal quality is affected by mode of recording and positioning of sensors. Signal loss can be kept to below 5% in routine clinical practice with appropriate attention to technique. Accuracy of beat-to-beat measurement needs to be 1 ms if short-term variability (epoch length 1/16 min) is to be assessed reliably.

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.

Change in electrocardiogram T-wave amplitude during umbilical cord compression is predictive of fetal condition in sheep

OBJECTIVE

The purpose of this study is to assess the usefulness of the dynamic change in T/QRS ratio in fetal electrocardiograms in predicting the fetal condition when repetitive variable decelerations are seen in intrapartum cardiotocograms.

STUDY DESIGN

We investigated the relationship, using linear regression and Wilcoxon's test, between T/QRS and blood gas values, catecholamine concentrations, and blood pressure during repetitive cord compression in five chronically instrumented lamb fetuses.

RESULTS

T/QRS during cord compression correlated significantly (p less than 0.01) with fetal arterial pH (r = -0.7711), norepinephrine concentration (r = 0.7551), and duration of elevated blood pressure during compression (r = -0.8619). Fetal arterial pH and base excess were lower, the duration of elevated blood pressure during compression was shorter, and carbon dioxide partial pressure and catecholamine concentrations were higher in the stage with higher (greater than 0.50) T/QRS during compression (p less than 0.005).

CONCLUSION

We can estimate the severity of fetal distress by measuring T/QRS near the bottom of the decelerations.

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.

Comparison of the T/QRS ratio of the fetal electrocardiogram and the fetal heart rate during labour and the relation of these variables to condition at delivery

The T/QRS ratio of the fetal electrocardiogram (ECG) was recorded to within 30 min of delivery from 105 women in labour. There were no significant differences in the mean T/QRS ratio in the last hour of record between those with normal and intermediate, or abnormal fetal heart rate (FHR) patterns. In 66 labours the mean T/QRS ratio in the first hour of record was compared with that of the last hour; the only significant change was a small decrease in the mean ratio from 11% to 7% in a group of 11 fetuses with an abnormal FHR pattern throughout the recording time. Eight babies were born with evidence of acidosis (umbilical artery pH less than or equal to 7.16), and another four were born in poor condition (1 min Apgar score less than 4) without evidence of acidosis; none had a mean last hour T/QRS ratio significantly different from the previously established normal range.

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.

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.

Microprocessor based waveform analysis of the fetal electrocardiogram during labor

An increase in T wave amplitude of the fetal ECG (FECG) has experimentally been correlated to elevated catecholamine levels and myocardial glycogenolysis. The FECG changes have also been described during human delivery. The present aim was to investigate whether these ECG changes could be reproduced in an easily handled real time microprocessor system, and to correlate them to biochemical and clinical data. During 40 deliveries the FECG signal was transferred to a microcomputer system for real time averaging of the FECG. There was a high capacity of the system to reproduce the ST waveform changes though the averaging procedure reduced the QRS magnitude by 10%. With a normal umbilical artery pH (greater than or equal to 7.25) the highest T/QRS ratios recorded during each delivery was 0.26 +/- 0.19 (mean +/- S.D.). With lowered pH (less than 7.25) the T/QRS increased to 0.33 +/- 0.10 (P less than 0.02). A similar difference between the two groups was seen when the T/QRS ratios from the last hour before birth were compared; 0.13 +/- 0.08 and 0.18 +/- 0.05, respectively (P less than 0.01). Changes in the ST waveform with T/QRS greater than or equal to 0.30, ST segment alterations, or negative T waves appeared during 40% of the deliveries, however, 30% were short standing changes (less than 30 min). Intermediate CTG changes during at least 30 min occurred in 41% and the pattern was classified as abnormal in 18%. Using the scalp electrode as signal source, the ECG analysis could add further information to the routine CTG recording on the fetal condition during delivery.

The fetal ECG throughout the second half of gestation

The results of a study in which multi-lead simultaneous recordings of the abdominal fetal electrocardiogram (FECG) have been made are presented. A homogeneous volume conduction model based on the actual recorded geometry was made. Using this model, fetal vectorcardiograms (FVCGS) were computed. Before 28 weeks of gestation the inter- and intra-individual variability of the observed signals could be removed by computing FVCGs in a fetal reference frame. This opens up the possibility of using the shape of the QRS complex for diagnostic purposes.

Variation in the T/QRS ratio of fetal electrocardiograms recorded during labour in normal subjects

The T/QRS ratio of the fetal ECG was obtained during labour from 25 women with normal pregnancies. The poor signal-to-noise ratio of the unprocessed signals, chiefly due to baseline wander, led to a wide variation between individual measurements. This problem was overcome by data averaging, the ratio being expressed as a mean over 1-min epochs. The average T/QRS ratio of each labour record ranged from 4% to 23% with a mean of 10% (for all 25). The average range (between 5th and 95th centile) of the 1-min T/QRS ratios was 13% and there were no significant changes as labour progressed. The effect of contractions on the T/QRS ratio was measured from eight subjects and found to be inconsistent.

Fetal ECG during labour: a presentation of a microprocessor system

Changes in the ST waveform of the fetal ECG have been detected in 47 term deliveries with vertex presentations using a specially developed microprocessor-based system for on-line recording of T wave amplitude. The T wave was quantified by the T/QRS ratio. The recording included one scalp electrode for exploration and a maternal skin electrode as reference. Signal quality allowed optimal ST waveform assessment in 89% of the cases. The degree of perinatal asphyxia was judged from cord artery acid-base status and the neonatal outcome. In completely normal fetuses at term the mean T/QRS ratio was 0.148 with a standard deviation of 0.048. With this basic information we can proceed in the investigation of the T/QRS ratio as a means for fetal surveillance.

Fetal ECG during labour: a suggested standard

Recording the fetal ECG as a means of fetal monitoring during labour has been investigated by many researchers. Different signal processing methods have been used, making comparisons difficult so that agreement on several topics has not been reached. We suggest the basis of a standard method of recording and processing the signal which we believe will be acceptable to the research and clinical communities.

The ECG waveform

Different portions of the ECG waveform are influenced differently. The QRS waveform is influenced principally by ventricular muscle mass but may be influenced by differences in ventricular filling which occur with cardiac failure or transiently with cord compression. The PR interval normally has a positive correlation with the R-R interval but this correlation becomes negative, probably as a result of catecholamines, when the fetus is stressed. This difference between PR/RR correlation may help distinguish vagally mediated decelerations from those associated with hypoxia. The ST waveform becomes elevated when myocardial oxygen delivery is insufficient to maintain aerobic metabolism to meet the energy demands of the myocardium, and anaerobic metabolism occurs with the production of lactate; this is a dynamic situation and may occur at different levels of oxygenation in different fetuses, depending on their ability to compensate and the workload on the myocardium. It is very likely that changes in PR/RR relationships and the ST waveform will be useful in intrapartum monitoring in the future, and this contention is supported by recent analysis of PR/RR and ST segment shifts (Figure 16). It seems clear that the detailed study of the ECG waveform provides more, and perhaps clinically more useful, information from the same signal and electrode as heart rate, and it is no longer reasonable to ignore it. However, as clinicians we have an extraordinary ability to move with disenchantment from one method of fetal monitoring to another of no proven value, with a firm but unreasoned conviction that it is better. ECG analysis is by no means ready for clinical use if we are to avoid the same pitfalls as continuous FHR monitoring. Further clinical assessment will not be easy. In modern obstetric practice asphyxiated infants are fortunately not common, so there are practical difficulties in monitoring sufficient numbers of patients to evaluate new indices of asphyxia, as well as the difficulties of defining an objective endpoint. Thorough clinical (probably multicentre) studies are needed with well defined endpoints and this is beginning. Further fetal lamb studies are also needed, particularly to examine the interrelationships between changes in heart rate, PR interval and ST waveform; it is hoped that this chapter has emphasized the importance of such animal studies in understanding the physiological and therefore clinical background to any change.(ABSTRACT TRUNCATED AT 400 WORDS)