The normal condition of the fetal electrocardiogram during labour

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.

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.

Surveillance during labour

The technology of intrapartum surveillance made rapid strides from the 1960s through the 1980s but then stagnated as increasing resort to caesarean section was made rather than improving measures of fetal condition and labour progress. However, despite caesarean section rates commonly over 30%, medicolegally expensive mistakes continue to be made because it is difficult to teach clinicians to make reliable use of existing technology. It may be that as with aircraft navigation, the safest solution is to replace human judgement with the obstetric equivalent of automatic pilots.

Multicentre study of fetal cardiac time intervals using magnetocardiography

OBJECTIVE

A database with reference values of the durations of the various waveforms in a magnetocardiogram of fetuses in uncomplicated pregnancies is assessed. This database will be of help to discriminate between pathologic and healthy fetuses. A fetal magnetocardiogram is a recording of the magnetic field in a location near the maternal abdomen and reflects the electric activity within the fetal heart. It is a non-invasive method, which can be used with nearly 100% reliability from the 20th week of gestation onward.

DESIGN

Durations of the waveforms were assembled from averaged magnetocardiograms and statistically processed.

SETTING

Fetal magnetocardiograms were measured with different magnetocardiographs. All measurements were carried out in magnetically shielded rooms.

SAMPLE

Fetal magnetocardiograms were obtained for 582 healthy patients.

METHOD

The durations of the waveforms were extracted from fetal magnetocardiograms measured at the cooperating centres. The variables collected included the duration of the P-wave, the PR interval, the PQ interval, the QRS complex, the QT interval and the T-wave and QTc value. The results were compared with values extracted from electrocardiograms of fetuses measured via electrodes attached to the maternal abdomen, from electrocardiograms measured during labour using a scalp electrode, and from electrocardiograms recorded in newborns, that were found in the literature.

MAIN OUTCOME MEASURES

Values of the durations are given as a function of gestational age including the regression line as well as the bounds marking the 90%, 95% and 98% prediction interval.

RESULTS

The durations of the P-wave, the PR interval, the QRS complex, the QT interval and QTc value increase linearly with gestational age. The durations of the PQ interval and the T-wave are independent of fetal age.

CONCLUSION

The values found agree with those found in the literature. The scatter of the data is wide due to the variation in normal physiology, the measuring system and signal processing and the subjectivity of the researcher. However, the system can define normal ranges and may be used in diagnosis.

Do fetal electrocardiogram PR-RR changes reflect progressive asphyxia after repeated umbilical cord occlusion in fetal sheep?

The aim of this study was to determine whether there is a relationship between changes in PR-RR correlation of the fetal ECG and progressive changes in fetal acid-base status and blood pressure (BP) during repeated umbilical occlusion. Chronically instrumented fetal sheep at 126.8+/-0.6 d (mean+/-SEM) were randomized to receive 1 min of total umbilical cord occlusion either every 5 min for 4 h (1:5 group; n=8), or every 2.5 min until BP fell <2.7 kPa (20 mm Hg) on two successive occlusions (1:2.5 group; n=8). The PR-RR correlation was determined in 5- or 2.5-min intervals. Umbilical cord occlusion caused variable decelerations with initial sustained hypertension. In the 1:5 group BP remained elevated throughout, and there was little change in acid-base status (pH=7.34+/-0.07, base deficit=1.3+/-3.9 after 4 h). In contrast, after the third occlusion the 1:2.5 group showed progressive hypotension during occlusions, and severe progressive metabolic acidemia (pH 6.92+/-0.1, base deficit 17.0+/-4.7 mmol/L after the last occlusion). In both groups, the PR-RR relationship switched from positive to negative with the onset of occlusions, then reverted to positive after a variable interval. In the 1:2.5 group later reversion of the PR-RR to positive was associated with earlier and more prolonged hypotension during the middle and end of the occlusion series (p < 0.001). We conclude that the initial switch to a negative PR-RR relationship during repetitive umbilical occlusion was due to a reflex-mediated response unrelated to fetal acidosis or hypotension. Both stable well compensated fetuses and severely hypoxic, hypotensive fetuses subsequently showed a positive PR-RR correlation.

European Community Multicentre Trial "Fetal ECG Analysis During Labour": the P-R interval

The aim of the European Community Multicentre Trial on Fetal ECG Analysis During Labour was to collect a clinical database of electrocardiograms on which the different patterns of ST waveform and time interval changes, so far identified, could be studied. The aim of this paper was to study the PR-RR relationship and the P wave patterns during normal labour. One thousand three hundred fifty accelerations and 350 decelerations were analysed in 618 labours, all with normal fetal outcome. All ECGs were recorded with the STAN recorder and linked to a PC system for data acquisition and analysis. A positive relationship between P-R and R-R intervals was identified in all reactive fetal heart rate traces. A P-R shortening was identified also with all decelerations of more than 40 bpm from the baseline, resulting in a negative PR-RR relationship. Experimental works have described a P-R shortening concurrent with the maximal R-R lengthening during acutely induced hypoxemia in fetal lambs. In our study normal cord acid-base status indicated that the majority of bradycardia episodes recorded were not related to acute hypoxemia: PR-RR relationship changes seem therefore to indicate an intact physiological fetal heart adaptive response to rapid change in the environment of either hypoxemic or haemodynamic origin. A negative PR-RR relationship by itself seems to be nothing more than an indicator of decelerations during labour and seems unable to discriminate between decelerations of different origin. Finally the P wave was no longer detectable during decelerations of more than 800 msec: in this situation the P-R interval can not be reliably measured.

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.

A comparative study of the fetal electrocardiogram recorded by the STAN an Nottingham systems

OBJECTIVE

To compare the T:QRS ratio recorded by the STAN and Nottingham fetal electrocardiogram (FECG) monitors.

DESIGN

Prospective observational study.

SETTING

London teaching hospital delivery suite and research unit.

METHODS AND SUBJECTS

The T:QRS ratios generated by the STAN and Nottingham FECG monitors were simultaneously recorded and compared using signals generated from a computer-produced ECG signal and signals from 11 term fetuses recorded during labour.

RESULTS

There was an acceptable level of agreement between the two systems with the computer-generated signals, but it was not clinically acceptable with the signals from the fetuses recorded during labour. Disagreements in the T:QRS values were probably due to differences in the reference points for the measurement of the S-T segment and T-wave height.

CONCLUSION

The different points of reference for measurement of S-T segment and T-wave height can explain poor agreement between the two methods of FECG waveform analysis. The suggested adopted points of reference are those corresponding to adult electrocardiographic methodology.

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.

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.

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.

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.

Interpretation of the fetal ECG during labor: the effect of uterine contractions

This study was performed in order to investigate the fetal electrocardiogram (FECG) during uterine contractions associated with normal labor. Twenty-five patients with low risk pregnancy between 38-41 weeks gestation were studied during the active stage of labor. Both FECG and intra-uterine pressure are obtained in a conventional manner and are continually sampled into the computer. The FECG is averaged point-to-point, synchronized to the peak of the R-wave. This is performed by a QRS detection algorithm which is based on a digital analysis of slope, amplitude and width. A digital band-pass filter composed of cascaded high-pass and low-pass filters reduces false detections and permits the use of auto-adjustable low thresholds. A separate averaging is performed on the T-wave in order to prevent attenuation due to variable R-T interval. The T wave is subsequently aligned in time and position to the rest of the QRS complex. A significant increase was observed in the T/QRS amplitude ratio during the first half of the uterine contraction. Such an increase was also observed in the short and long-term FHR variability. No significant changes were observed in the other components of the FECG. In conclusion, by implementing a computer based system it is possible to analyse the FECG during labor. Based on this and previous studies it may well prove to be a sensitive indicator of fetal condition.

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)

Technical progress in fetal electrocardiography--a review

The fetal electrocardiogram (FECG) was first demonstrated 80 years ago, but initial progress in research into the subject was slow and limited by the technical equipment available. As improved amplifiers became available, the detection of the waveform became easier, but observation of waveform morphology was still difficult because of background noise. The signal-to-noise ratio of the original FECG waveform was improved considerably with the use of directly applied fetal electrodes, and the development of digital computer techniques has allowed signal enhancement. It is now possible to monitor the FECG waveform continuously in labor and reliably produce waveforms with all of the components clearly defined.

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

Waveform changes in the fetal electrocardiogram (ECG) are under reassessment as a means of fetal surveillance. The purpose of the study was to identify ST waveform changes from the fetal scalp lead during labour and to compare any change with conventional signs of fetal distress. Two scalp electrodes provided a sufficiently stable signal from which ST waveform changes could be identified and measured as the T/QRS ratio. No infant was clinically hypoxic. Only 26% of the 46 patients studied had a continuously normal CTG whereas 67% had no ST waveform abnormalities. A relation was found between the two variables as the mean T/QRS ratio increased from 0.20 (SD 0.11) during epochs of normal CTG to 0.27 (SD 0.17) with CTG changes. A linear correlation (r = 0.58, P less than 0.01) was found between the T/QRS ratio before birth and the cord plasma lactate values.

Changes in the ST waveform of the fetal lamb electrocardiogram with hypoxemia

The ST waveform of the fetal electrocardiogram (ECG) was examined in 10 chronically instrumented fetal lambs from 115 days to term. Averaged ST waveforms were plotted at 5-minute intervals in six fetuses for 2 to 22 days. No diurnal or other rhythms were seen. To correct for changes in signal gain the amplitude of the T wave was measured relative to the amplitude of the QRS complex. The T/QRS ratio was normally less than 0.30. Persistently elevated ST waveforms with a T/QRS range 0.32 to 0.65 preceded fetal death by some days in three fetuses and were associated with anemia and/or hypotension in a further three. In these animals hypoxia produced a further rise in the ST waveform (mean T/QRS, from 0.48 to 0.81) and all died during labor. In lambs with a normal ST waveform there were differences in the response to hypoxia over 1 hour for a similar fall in PO2. In eight experiments the ST segment and T wave increased (mean T/QRS, from 0.17 to 0.59) and promptly reverted to normal with normoxia. There was a significant rise in mean arterial blood pressure, plasma lactate, and glucose and a fall in pH. In four experiments there was little change (mean T/QRS, from 0.19 to 0.25), with a small rise in plasma lactate suggesting that these lambs were able to maintain aerobic myocardial metabolism. Overall there was a strong correlation of the T/QRS ratio to the rate of rise of lactate. These findings complement previous results in the acute fetal lamb preparation and suggest that ST waveform elevation expressed as the T/QRS ratio identifies a change to anerobic myocardial metabolism.

The intraventricular conduction time of fetal heart in pregnancies complicated by rhesus haemolytic disease

The duration and shape of the fetal QRS complex were studied in 88 pregnancies complicated by rhesus isoimmunization. A clear correlation between ventricular depolarization time and haemoglobin levels at birth was observed. A single QRS value greater than 4 SD above the normal mean value, or a tendency to rapid increase was very suggestive of a bad prognosis, while a QRS duration below +4 SD or declining in sequential determinations reflects a temporary compensation of fetal conditions. From the QRS complex analysis a clear indication appeared of the constant and early involvement of the heart in terms of myocardial hypertrophy and/or cardiac enlargement. The sensitivity of QRS to anaemia and the feasibility of continuous non-invasive monitoring might allow, with the determination of bilirubin concentration in amniotic fluid, improvement in the management of rhesus haemolytic disease.