Analysis of heart rate and beat-to-beat variability: Interval difference index

The quality of intrapartum cardiotocography in preterm labour

OBJECTIVES

The aim of this study is to determine the quality of the foetal heart rate (FHR) recording, defined as signal loss, during preterm labour below 28 weeks gestational age (GA) and contribute to the discussion if cardiotocography (CTG) is of value for the extreme preterm foetus.

METHODS

From January 2010 to December 2019 a retrospective study was conducted with data of 95 FHR recordings of singletons born between 24 and 28 weeks GA at the Amsterdam University Medical Centre, location VUmc. FHR tracings had a duration of at least 30 min and were obtained via external ultrasound mode. Data of all recordings were divided in two groups according to gestation (24-26 weeks and 26-28 weeks). Signal loss was analysed. Statistical significance was calculated by non-parametric tests and chi-square tests. The median signal loss and the proportion of cases exceeding the International Federation of Gynaecology and Obstetrics Guidelines (FIGO) threshold of 20% signal loss were calculated.

RESULTS

One-third of the recordings exceeded the 20% FIGO-criterion for adequate signal quality during the first stage of labour. In the second stage, this was nearly 75%. Similarly, the median signal loss was 13% during the first and 30% during the second stage of labour (p<0.01).

CONCLUSIONS

The quality of FHR monitoring in the extreme preterm foetus is inadequate in a large proportion of the foetuses, especially during the second stage. FHR monitoring is therefore controversial and should be used with caution.

The Noninvasive Fetal Electrocardiogram During Labor: A Review of the Literature

Importance

The introduction of the cardiotocogram (CTG) during labor has not been found to improve neonatal outcome. The search for a more reliable, less invasive, and patient-friendly technique is ongoing. The noninvasive fetal electrocardiogram (NI-fECG) has been proposed as one such alternative.

Objectives

The aim of this study was to review the literature on the performance of NI-fECG for fetal monitoring during labor. Following the PRISMA guidelines, a systematic search in MEDLINE, EMBASE, and Cochrane Library was performed. Studies involving original research investigating the performance of NI-fECG during labor were included. Animal studies and articles in languages other than English, Dutch, or German were excluded. The QUADAS-2 checklist was used for quality assessment. A descriptive analysis of the results is provided.

Results

Eight articles were included. Pooled analysis of the results of the separate studies was not possible due to heterogeneity. All studies demonstrate that it is possible to apply NI-fECG during labor. Compared with Doppler ultrasound, NI-fECG performs equal or better in most studies.

Conclusions and Relevance

NI-fECG for fetal monitoring is a promising noninvasive and patient-friendly technique that provides accurate information. Future studies should focus on signal quality throughout labor, with the aim to further optimize technical development of NI-fECG.

New Method for Beat-to-Beat Fetal Heart Rate Measurement Using Doppler Ultrasound Signal

The most commonly used method of fetal monitoring is based on heart activity analysis. Computer-aided fetal monitoring system enables extraction of clinically important information hidden for visual interpretation—the instantaneous fetal heart rate (FHR) variability. Today’s fetal monitors are based on monitoring of mechanical activity of the fetal heart by means of Doppler ultrasound technique. The FHR is determined using autocorrelation methods, and thus it has a form of evenly spaced—every 250 ms—instantaneous measurements, where some of which are incorrect or duplicate. The parameters describing a beat-to-beat FHR variability calculated from such a signal show significant errors. The aim of our research was to develop new analysis methods that will both improve an accuracy of the FHR determination and provide FHR representation as time series of events. The study was carried out on simultaneously recorded (during labor) Doppler ultrasound signal and the reference direct fetal electrocardiogram Two subranges of Doppler bandwidths were separated to describe heart wall movements and valve motions. After reduction of signal complexity by determining the Doppler ultrasound envelope, the signal was analyzed to determine the FHR. The autocorrelation method supported by a trapezoidal prediction function was used. In the final stage, two different methods were developed to provide signal representation as time series of events: the first using correction of duplicate measurements and the second based on segmentation of instantaneous periodicity measurements. Thus, it ensured the mean heart interval measurement error of only 1.35 ms. In a case of beat-to-beat variability assessment the errors ranged from −1.9% to −10.1%. Comparing the obtained values to other published results clearly confirms that the new methods provides a higher accuracy of an interval measurement and a better reliability of the FHR variability estimation.

Use of automated fetal heart rate analysis to identify risk factors for umbilical cord acidosis at birth

OBJECTIVE

To identify clinical parameters and intrapartum fetal heart rate parameters associated with a risk of umbilical cord acidosis at birth, using an automated analysis method based on empirical mode decomposition.

METHODS

Our single-center study included 381 cases (arterial cord blood pH at birth pHa ≤7.15) and 1860 controls (pHa ≥7.25) extracted from a database comprising 8,383 full datasets for over-18 mothers after vaginal or caesarean non-twin, non-breech deliveries at term (>37 weeks of amenorrhea). The analysis of a 120-min period of the FHR recording (before maternal pushing or the decision to perform a caesarean section during labor) led to the extraction of morphological, frequency-related, and long- and short-term heart rate variability variables. After univariate analyses, sparse partial least square selection and logistic regression were applied.

RESULTS

Several clinical factors were predictive of fetal acidosis in a multivariate analysis: nulliparity (odds ratio (OR) 95% confidence interval (CI)]: 1.769 [1.362-2.300]), a male fetus (1.408 [1.097-1.811]), and the term of the pregnancy (1.333 [1.189-1.497]). The risk of acidosis increased with the time interval between the end of the FHR recording and the delivery (OR [95%CI] for a 1-min increment: 1.022 [1.012-1.031]). The risk factors related to the FHR signal were mainly the difference between the mean baseline and the mean FHR (OR [95%CI]: 1.292 [1.174-1.424]), the baseline range (1.027 [1.014-1.040]), fetal bradycardia (1.038 [1.003-1.075]) and the late deceleration area (1.002 [1.000-1.005]). The area under the curve for the multivariate model was 0.79 [0.76; 0.81].

CONCLUSION

In addition to clinical predictors, the automated FHR analysis highlighted other significant predictors, such as the baseline range, the instability of the FHR signal and the late deceleration area. This study further extends the routine application of automated FHR analysis during labor and, ultimately, contributes to the development of predictive scores for fetal acidosis.

Linear Phase Sharp Transition BPF to Detect Noninvasive Maternal and Fetal Heart Rate

Fetal heart rate (FHR) detection can be monitored using either direct fetal scalp electrode recording (invasive) or by indirect noninvasive technique. Weeks before delivery, the invasive method poses a risk factor to the fetus, while the latter provides accurate fetal ECG (FECG) information which can help diagnose fetal's well-being. Our technique employs variable order linear phase sharp transition (LPST) FIR band-pass filter which shows improved stopband attenuation at higher filter orders. The fetal frequency fiduciary edges form the band edges of the filter characterized by varying amounts of overlap of maternal ECG (MECG) spectrum. The one with the minimum maternal spectrum overlap was found to be optimum with no power line interference and maximum fetal heart beats being detected. The improved filtering is reflected in the enhancement of the performance of the fetal QRS detector (FQRS). The improvement has also occurred in fetal heart rate obtained using our algorithm which is in close agreement with the true reference (i.e., invasive fetal scalp ECG). The performance parameters of the FQRS detector such as sensitivity (Se), positive predictive value (PPV), and accuracy (F₁) were found to improve even for lower filter order. The same technique was extended to evaluate maternal QRS detector (MQRS) and found to yield satisfactory maternal heart rate (MHR) results.

Linear and nonlinear features of fetal heart rate on the assessment of fetal development in the course of pregnancy and the impact of fetal gender

OBJECTIVE

This work aims to investigate the impact of gestational age and fetal gender on fetal heart rate (FHR) tracings.

APPROACH

Different linear and nonlinear parameters indicating correlation or complexity were used to study the influence of fetal age and gender on FHR tracings. The signals were recorded from 99 normal pregnant women in a singleton pregnancy at gestational ages from 28 to 40 weeks, before the onset of labor. There were 56 female fetuses and 43 male.

MAIN RESULTS

Analysis of FHR shows that the means as well as measures of irregularity of FHR, such as approximate entropy and algorithmic complexity, decrease as gestation progresses. There were also indications that mutual information and multiscale entropy were lower in male fetuses in early pregnancy.

SIGNIFICANCE

Fetal age and gender seem to influence FHR tracings. Taking this into consideration would improve the interpretation of FHR monitoring.

Is Abdominal Fetal Electrocardiography an Alternative to Doppler Ultrasound for FHR Variability Evaluation?

Great expectations are connected with application of indirect fetal electrocardiography (FECG), especially for home telemonitoring of pregnancy. Evaluation of fetal heart rate (FHR) variability, when determined from FECG, uses the same criteria as for FHR signal acquired classically—through ultrasound Doppler method (US). Therefore, the equivalence of those two methods has to be confirmed, both in terms of recognizing classical FHR patterns: baseline, accelerations/decelerations (A/D), long-term variability (LTV), as well as evaluating the FHR variability with beat-to-beat accuracy—short-term variability (STV). The research material consisted of recordings collected from 60 patients in physiological and complicated pregnancy. The FHR signals of at least 30 min duration were acquired dually, using two systems for fetal and maternal monitoring, based on US and FECG methods. Recordings were retrospectively divided into normal (41) and abnormal (19) fetal outcome. The complex process of data synchronization and validation was performed. Obtained low level of the signal loss (4.5% for US and 1.8% for FECG method) enabled to perform both direct comparison of FHR signals, as well as indirect one—by using clinically relevant parameters. Direct comparison showed that there is no measurement bias between the acquisition methods, whereas the mean absolute difference, important for both visual and computer-aided signal analysis, was equal to 1.2 bpm. Such low differences do not affect the visual assessment of the FHR signal. However, in the indirect comparison the inconsistencies of several percent were noted. This mainly affects the acceleration (7.8%) and particularly deceleration (54%) patterns. In the signals acquired using the electrocardiography the obtained STV and LTV indices have shown significant overestimation by 10 and 50% respectively. It also turned out, that ability of clinical parameters to distinguish between normal and abnormal groups do not depend on the acquisition method. The obtained results prove that the abdominal FECG, considered as an alternative to the ultrasound approach, does not change the interpretation of the FHR signal, which was confirmed during both visual assessment and automated analysis.

A novel technique for fetal heart rate estimation from Doppler ultrasound signal

Background

The currently used fetal monitoring instrumentation that is based on Doppler ultrasound technique provides the fetal heart rate (FHR) signal with limited accuracy. It is particularly noticeable as significant decrease of clinically important feature - the variability of FHR signal. The aim of our work was to develop a novel efficient technique for processing of the ultrasound signal, which could estimate the cardiac cycle duration with accuracy comparable to a direct electrocardiography.

Methods

We have proposed a new technique which provides the true beat-to-beat values of the FHR signal through multiple measurement of a given cardiac cycle in the ultrasound signal. The method consists in three steps: the dynamic adjustment of autocorrelation window, the adaptive autocorrelation peak detection and determination of beat-to-beat intervals. The estimated fetal heart rate values and calculated indices describing variability of FHR, were compared to the reference data obtained from the direct fetal electrocardiogram, as well as to another method for FHR estimation.

Results

The results revealed that our method increases the accuracy in comparison to currently used fetal monitoring instrumentation, and thus enables to calculate reliable parameters describing the variability of FHR. Relating these results to the other method for FHR estimation we showed that in our approach a much lower number of measured cardiac cycles was rejected as being invalid.

Conclusions

The proposed method for fetal heart rate determination on a beat-to-beat basis offers a high accuracy of the heart interval measurement enabling reliable quantitative assessment of the FHR variability, at the same time reducing the number of invalid cardiac cycle measurements.

Comparison of short term variability indexes in cardiotocographic foetal monitoring

Concise indexes related to variability of foetal heart rate (FHR) are usually utilised for foetal monitoring; they enrich information provided by cardiotocography (CTG). Most attention is paid to the short term variability (STV), which relates to activity and reaction of autonomic nervous control of foetal heart. There is not a unique method to compute short term variability of the FHR but different formulas have been proposed and are employed in clinical and scientific environments: this leads to different evaluations and makes difficult comparative studies. Nine short term variability indexes: Arduini, Dalton, Organ, Sonicaid 8000, Van Geijn, Yeh, Zugaib a modified version of Arduini index and Standard Deviation were considered and compared to test their robustness in CTG applications. A large set of synthetic foetal heart rate series with known features were used to compare indexes performances. Different amounts of variability, mean foetal heart rate, storage rates, baseline variations were considered. The different indexes were in particular tested for their capability to recognise short term heart rate variability variation, their dependence on heart rate signal storage rate (as those provided by commercial cardiotocographic devices), on mean value of the foetal heart rate and on modifications of the floatingline, such in case of accelerations or decelerations. Concise statistical parameters relative to indexes scores were presented in comparative tables. Results indicate that although the indexes are able to recognise STV variation, they show substantial differences in magnitude and some in sensibility. Results depend on the frequency used to acquire and store FHR data (depending on devices); in general, the lower is data rate the more degraded are the results. Furthermore, results differently depend on FHR mean, some for their intrinsic definition; differences arise also in correspondences of accelerations and decelerations. Our results demonstrate that only indexes which refer directly to differences in FHR values, such as Organ and SD indexes, not show dependence on FHR mean. The use of the Standard Deviation index may provide efficient information while showing independence from the considered variables. Indexes performance in case of real cardiotocographic signals were also presented as examples.

Comparison of Doppler ultrasound and direct electrocardiography acquisition techniques for quantification of fetal heart rate variability

A method for comparison of two acquisition techniques that are applied in clinical practice to provide information on fetal condition is presented. The aim of this work was to evaluate the commonly used Doppler ultrasound technique for monitoring of mechanical activity of fetal heart. Accuracy of beat-to-beat interval determination together with its influence on indices describing the fetal heart rate (FHR) variability calculated automatically using computer-aided fetal monitoring system were examined. We considered the direct fetal electrocardiography as a reference technique because it ensures the highest possible accuracy of heart interval measurement, and additionally all the definitions of popular time domain parameters quantifying FHR variability formerly have been created using the fetal electrocardiogram. We evaluated the reliability of various so called short-term and long-term variability indices, when they are calculated automatically using the signal obtained via the Doppler US from a fetal monitor. The results proved that evaluation of the acquisition technique influence on fetal well-being assessment can not be accomplished basing on direct measurements of heartbeats only. The more relevant is the estimation of accuracy of the variability indices, since analysis of their changes can significantly increase predictability of fetal distress.

Quality of intrapartum cardiotocography in twin deliveries

OBJECTIVE

Intrapartum fetal heart rate (FHR) recordings in twins were compared for fetal signal loss during both stages of labor to assess the quality of these recordings by the method that had been used: external ultrasound or directly via a scalp electrode.

STUDY DESIGN

Analysis of recordings collected between January 1, 1994, and January 1, 2002, from consecutive twin deliveries at the Vrije Universiteit Medical Center in Amsterdam. One hundred seventy-two twins that delivered via the vaginal route were included in the study. FHR recordings had a duration of at least 1 hour before the birth of the second twin. Subdivision took place on the basis of the recording technique, ie, ultrasound or scalp electrode. FHR data was obtained with HP-M1350 cardiotocographs. The status (pen on, pen off, maternal signal) and the mode of the signals were acquired. The duration of pen lifts and maternal signals was divided by the total duration of the recording. Statistical analyses were performed with the Mann-Whitney U test and the Wilcoxon signed ranks test.

RESULTS

Recordings obtained via ultrasound demonstrated significantly more fetal signal loss than those obtained via the direct mode, particularly in the second stage. Approximately 26% to 33% of first stage and 41% to 63% of second stage ultrasound intrapartum FHR recordings in twins exceeded the International Federation of Gynecology and Obstetrics (FIGO) criteria for fetal signal loss.

CONCLUSION

Intrapartum FHR monitoring via ultrasound provides far poorer quality FHR signals than the direct mode. The direct mode deserves a more prominent position in fetal surveillance than it currently has.

The quality of intrapartum fetal heart rate monitoring

OBJECTIVE

To determine the quality of fetal heart rate (FHR) recordings during the first and second stage of labor by quantifying the amount of fetal signal loss in relation to the method of monitoring: external ultrasound or directly via a scalp electrode.

STUDY DESIGN

Analysis of 239 intrapartum recordings stored between 1 January 2001 and 1 July 2001 from consecutive deliveries at the Vrije Universiteit Medical Center in Amsterdam. Singletons delivered via the vaginal route were included in the study. FHR recordings had duration of at least 1h prior to birth of the infant. Subdivision in three groups took place on the basis of the recording technique which had been used; i.e. ultrasound, scalp electrode or a combination of both methods. FHR data was obtained using HP-M1350 cardiotocographs. The status (pen on, pen off, maternal signal) and the mode of the signals were acquired. The duration of pen lifts and maternal signals was divided by the total duration of the recording. Statistical analyses were performed with the Mann-Whitney U-test and the Wilcoxon signed ranks test.

RESULTS

Recordings obtained via ultrasound demonstrated significantly more fetal signal loss than those obtained via the direct mode, particularly in the second stage. The FIGO criteria for fetal signal loss with external ultrasound were not fulfilled during this stage for about half the cases.

CONCLUSION

Intrapartum FHR monitoring via a scalp electrode provides far better quality FHR signals than external ultrasound and deserves a more prominent position in fetal surveillance than it currently has.

Linear and nonlinear parameters for the analysis of fetal heart rate signal from cardiotocographic recordings

Antepartum fetal monitoring based on the classical cardiotocography (CTG) is a noninvasive and simple tool for checking fetal status. Its introduction in the clinical routine limited the occurrence of fetal problems leading to a reduction of the precocious child mortality. Nevertheless, very poor indications on fetal pathologies can be inferred from the even automatic CTG analysis methods, which are actually employed. The feeling is that fetal heart rate (FHR) signals and uterine contractions carry much more information on fetal state than is usually extracted by classical analysis methods. In particular, FHR signal contains indications about the neural development of the fetus. However, the methods actually adopted for judging a CTG trace as "abnormal" give weak predictive indications about fetal dangers. We propose a new methodological approach for the CTG monitoring, based on a multiparametric FHR analysis, which includes spectral parameters from autoregressive models and nonlinear algorithms (approximate entropy). This preliminary study considers 14 normal fetuses, eight cases of gestational (maternal) diabetes, and 13 intrauterine growth retarded fetuses. A comparison with the traditional time domain analysis is also included. This paper shows that the proposed new parameters are able to separate normal from pathological fetuses. Results constitute the first step for realizing a new clinical classification system for the early diagnosis of most common fetal pathologies.

Acoustic stimulation in hypoxic-ischemic encephalopathy: heart rate analysis

Two groups of 30 term newborns (normal and anoxic) were analyzed to study the influence of anoxia in the heart rate. The experiment consisted in a polygraphic EEG register before, during and after an acoustic stimulus. In basal conditions of active sleep, the heart rate in the asphyctic newborn was higher than in the normal group. The acoustic stimulation caused a transitional tachycardia. The variability of the heart rate was smaller in the asphyctic group. The basal heart rate varied in relation to the grade of Hypoxic-Ischemic Encephalopathy (HIE) and there was also a difference in the capacity of response to acoustic stimulation between the moderate HIE group and the normal group.

Developments in CTG analysis

FHR monitoring has been the subject of many debates. The technique, in itself, can be considered to be accurate and reliable both in the antenatal period, when using the Doppler signal in combination with autocorrelation techniques, and during the intrapartum period, in particular when the FHR signal can be obtained from a fetal ECG electrode placed on the presenting part. The major problems with FHR monitoring relate to the reading and interpretation of the CTG tracings. Since the FHR pattern is primarily an expression of the activity of the control by the central and peripheral nervous system over cardiovascular haemodynamics, it is possibly too indirect a signal. In other specialities such as neonatology, anaesthesiology and cardiology, monitoring and graphic display of heart rate patterns have not gained wide acceptance among clinicians. Digitized archiving, numerical analysis and even more advanced techniques, as described in this chapter, have primarily found a place in obstetrics. This can be easily explained, since the obstetrician is fully dependent on indirectly collected information regarding the fetal condition, such as (a) movements experienced by the mother, observed with ultrasound or recorded with kinetocardiotocography (Schmidt, 1994), (b) perfusion of various vessels, as assessed by Doppler velocimetry, (c) the amount of amniotic fluid or (d) changes reflected in the condition of the mother, such as the development of gestation-induced hypertension and (e) the easily, continuously obtainable FHR signal. It is of particular comfort to the obstetrician that a normal FHR tracing reliably predicts the birth of the infant in a good condition, which makes cardiotocography so attractive for widespread application. However, in the intrapartum period, many traces cannot fulfil the criteria of normality, especially in the second stage. In this respect, cardiotocography remains primarily a screening and not so much a diagnostic method. As long as continuous monitoring of fetal acid-base balance has not been extensively tested in clinical practice, microblood sampling of the fetal presenting part (Saling, 1994) is a useful adjunct. The problem with non-normal tracings is that their significance is very often unclear. They may indicate serious fetal distress, finally resulting in preventable destruction of critical areas in the fetal brain and damage to various organs; or, on the contrary, they may indicate temporary changes in cardiovascular control as a reaction to the intermittent effects on fetal haemodynamics of, for example, uterine contractions, whether or not in combination with partial or complete compression of umbilical cord vessels or the vessels on the chorionic plate (van Geijn, 1994). Many factors influence the FHR and its variability, which further complicates the interpretation of FHR patterns; some have been discussed here in some detail. Undoubtedly, there is a need for quantitative and objective FHR analysis, as long as it does not lead to erroneous results. Close collaboration between engineers and clinicians is a prerequisite for further advances in this field. Decision support systems certainly have a future but only if they are able to take into account a large set of clinical data and can combine it with data obtained from FHR signals and other parameters referring to the fetal condition, such as fetal growth, Doppler velocimetry, amniotic fluid volume and biochemical and biophysical data obtained from the mother. Basic technical concepts inherent in computerized CTG analysis, such as sampling rate (Chang et al, 1995), signal loss, artefact detection (van Geijn et al, 1980), further processing of intervals, archiving in digitized format and monitor display, should receive considerable attention. There is still a long way to go until decision support systems find their way into obstetric practice. Further developments can only be achieved thanks to efforts of many basic and clinical researchers, wo

Fetal heart rate patterns and chronic exposure to antiepileptic drugs

Fetal heart rate (FHR) characteristics of fetuses exposed and not exposed to antiepileptic drugs (AEDs) were studied. FHR is considered to reflect central nervous system (CNS) integrity. Three intervals during pregnancy were investigated: 20, 32, and 38 weeks. At 32 and 38 weeks, FHR was studied in relation to quiet (C1F) and active (C2F) sleep periods. For each tracing, a baseline was determined and accelerations and decelerations were identified. To assess FHR variability, the long-term irregularity, interval difference and absolute beat-to-beat indexes, and the bandwidth were calculated for 30-s intervals between accelerations and decelerations. No marked differences were noted between study and control groups concerning basal FHR and the occurrence of accelerations. For FHR derived from the fetal ECG, all indexes of FHR variability and the bandwidth were lower for the study group as compared with the control group, although the differences did not reach statistical significance. Our study shows that chronic prenatal exposure to AEDs does not seriously interfere with modulation of fetal heart rhythm by the CNS.

Quantitated fetal heart rhythm at 20, 32 and 38 weeks of gestation and dependence on rest-activity patterns

Quantitative parameters of fetal heart rate (FHR) were automatically analysed at 20, 32 and 38 weeks of pregnancy. FHR was obtained both by the fetal ECG method and by wide range Doppler ultrasound with autocorrelation. At 32 and 38 weeks, FHR was studied in relation to fetal rest-activity according to the fetal behavioural state concept (coincidence 1F and 2F). Basal fetal heart rate was significantly higher at 20 weeks of gestation than at 32 and 38 weeks. The number of accelerations increased significantly from 20 weeks to 32 and 38 weeks for C2F periods. Parameters of FHR variability, i.e. ID, ABB, LTI indices and bandwidth, were higher during periods C2F compared to periods C1F. Lowest values of all four parameters were found at 20 weeks gestation. The ID index, which is a measure of short-term variability increased significantly between 32 to 38 (C2F). The absolute values of ID, ABB and LTI were lower for ultrasound recordings than for the fetal ECG.

Influences on heart rate variability in spontaneously breathing preterm infants

To investigate the influence of maturational and physiological factors on heart rate variability in spontaneously breathing very preterm infants (n = 29) a multiparametric study was performed during the first 3 days of life in infants born at a gestational age below 33 weeks. Four times a day, RR-intervals, respiration curve and rate, transcutaneously measured blood gases and observed body movements were recorded while the infants were asleep. All data were stored simultaneously in a micro-computer. Non-invasively measured blood pressure and patency of the ductus arteriosus were documented as well. Four sets of short- (STV) and long term variability (LTV) indices were calculated. Both STV and LTV appeared to be significantly influenced by conceptional and postnatal age in the appropriate for gestational age infants. LTV was influenced by the behavioural state and body movements. During state coincidence 2 ('active sleep') LTV was influenced by respiratory rate and the variations in transcutaneous PO2. An effect of blood pressure or ductus patency could not be demonstrated.

The influence of respiratory distress syndrome on heart rate variability in very preterm infants

In a multi-parametric study the influence of pathological neonatal conditions on heart rate variability was investigated in 60 preterm infants born at a gestational age below 33 weeks. Measurements were performed during the first 3 days of life. Four times a day, RR-intervals, respiration curve and rate, transcutaneously measured blood gases and observed body movements were recorded while the infants were asleep. All data were stored simultaneously in a micro-computer. Severity of respiratory distress syndrome (RDS), patency of ductus arteriosus and periventricular haemorrhage were documented as well. Four sets of short- (STV) and long-term variability (LTV) indices were calculated. Severe RDS was associated with a significant decrease in LTV. The influence of RDS on LTV persisted after correction for conceptional age, postnatal age, behavioural state and variations in respiratory rate and in transcutaneous PO2. Infants with a symptomatic patent ductus arteriosus had lower LTV than controls with the same severity of RDS. STV was predominantly influenced by postnatal and conceptional age, and tended to be lower in infants with periventricular haemorrhage.

Properties of fetal heartbeat intervals during labour

This study was designed to investigate the range of beat-to-beat changes in fetal inter-beat (RR) intervals during routine clinical monitoring in labour. Fetal RR intervals were automatically measured and collected from 10 fetuses. Intervals which were incorrectly measured were excluded, and the remaining 23,510 intervals were used to compile the distribution of beat-to-beat changes. The inter-quartile range of this distribution was 23 ms and the 99th centile fell at approximately 50 ms. No relationship could be established between beat-to-beat changes and the absolute RR interval. These findings differ from the results published by other workers on the basis of data obtained antenatally or during early labour. In addition, the results suggest possibilities for improving algorithms designed to enhance data quality in fetal heart rate monitoring.

Computer analysis of antepartum fetal heart rate: 1. Baseline determination

A consequent and reproducible determination of baseline is an essential prerequisite for objective interpretation of fetal heart rate. A fully automated off-line method of baseline determination has been developed and tested on 50 normal antepartum fetal heart rate recordings of two hours duration. The method is constructed around two functional units, a digital filter and a trim function, which interact in an iterative process. The results were evaluated in comparison with automated baseline determination according to Dawes and coworkers. A panel of 3 experts agreed that in 14 of the 50 recordings (28%), the new developed procedure resulted in a substantially better baseline fit. In the remaining 34 recordings (72%), baseline fit from both methods was judged as equivalent. The described procedure of baseline determination provides a solid base for automated detection of accelerations and decelerations in fetal heart rate recordings. It enables the study of the relation between the fetal heart rate pattern and fetal movements. Finally, it provides an objective tool for analysis of variables within the fetal heart rate with the highest predictive value with respect to fetal outcome.

Computer analysis of antepartum fetal heart rate: 2. Detection of accelerations and decelerations

Based upon a previously described procedure for automated baseline determination in antepartum fetal heart rate recordings, a programme has been developed for recognition of accelerations and decelerations. Detection of these deviations from the baseline depends on criteria for amplitude and duration, with special account for signal loss. The described automated analysis provides an objective tool for description of antepartum fetal heart rate patterns. A first evaluation with respect to its capacity to discriminate between different fetal heart rate patterns in relation to fetal rest-activity patterns shows promising results.

Quantitative analysis of fetal rate: its application in antepartum clinical monitoring and behavioural pattern recognition

A system based on an IBM compatible personal computer is described for the computerized analysis of the cardiotocograms. The system performs a quantification of both fetal heart rate and uterine contractions providing automatically several variables including heart rate baseline; number, duration and area of accelerations and decelerations; latency time between decelerations and uterine contractions; uterine resting tone; number, duration and area of contractions. The computerized description was compared to the visual analysis of the cardiotocogram and a good agreement was evidenced. This system was applied to the analysis of heart rate variability in order to recognize the different patterns characterizing fetal behavioral states. Up to now, however, the results obtained in this latter application are not sufficiently accurate and do not allow a reliable automatic recognition of heart rate patterns.

The influence of physiological parameters on long term heart rate variability in healthy preterm infants

The instantaneous heart rate shows a variation around the mean heart rate caused by cardioregulatory mechanisms which are mediated through the sympathetic and vagal autonomic nervous system. To gain more insight into the influence of physiological parameters on neonatal heart rate variability a study was performed in four healthy preterm newborns during the first five days of life. Instantaneous heart rate, respiration rate, transcutaneous pO2, blood pressure and behaviour were recorded during 40 minutes four times a day. Long term heart rate variability was calculated as the difference between p95 and p5 of instantaneous heart rate values sampled during three minutes. A clear relationship between long term variability and age (maturity of the autonomic nervous system), respiration rate (respiratory sinus arrhythmia or a tidal volume mediated effect) and behaviour (increase of sympathetic tone during REM sleep) was found. No influence of blood pressure, heart rate, and transcutaneous pO2 within physiological ranges could be detected. The relative influence of the different physiological parameters on heart rate variability has to be established before the value of heart rate variability as a monitoring tool in neonatal intensive care can be investigated.

Real time processing and analysis of fetal phonocardiographic signals

The monitoring of fetal heart rate (FHR) is commonly used in assessing the general health of the fetus. Although certain periodic cycles may be indicative of fetal problems, only short term observations are routinely employed in clinical practice. This is due to cost considerations, inconvenience to the patient and concern about long term ultrasonic monitoring. Therefore only a low confidence assessment can be established between detected rhythms and the health of the fetus. The technique advocated in this paper makes use of an inexpensive, non-invasive phonocardiographic (phono) transducer which facilitates safe long-term patient monitoring. A variable comb filter applied to the frequency domain is used in order to take full advantage of the harmonic content of fetal heart signals. Real time estimation of FHR has been achieved on pre-recorded phono signals lasting eight hours. Recordings with a reasonable signal quality were analysed and some of the results are given. Advanced signal processing techniques followed by Artificial Intelligence (AI) algorithms reduce the number of erroneous estimates during periods of low signal to noise ration (SNR). The resulting FHR time series is stored on the host computer for further processing, display and parameter extraction. This paper outlines the processing steps involved.

Computerized assessment of fetal behavioral states

Fetal heart rate and fetal movements provide information on the fetal condition. In the near term human fetus, four behavioural states have been described based upon heart rate patterns and presence or absence of eye and body movements. For our studies concerning fetal physiology as well as the influence of maternal antiepileptic medication and the effects of intrauterine growth retardation on the fetal condition, we developed a computerized system for acquisition and storage of fetal heart rate signals and observed fetal movements. Fetal heart rate is recorded using a commercially available monitor combined with a home-made computer interface. Fetal movements are observed using two real-time ultrasound units. The observers handle keyboards to mark occurrence and duration of various types of fetal movements, and pedals to mark the visibility of the observed part of the fetal body. Keyboards and pedals are scanned by the computer. Special techniques are used to store heart rate and movement signals in an efficient way. Three experts determine fetal heart rate patterns by application of a Delphi group opinion procedure. Fetal behavioral states are identified by the computer using the results of the Delphi procedure, and performing the so-called extended automatic window procedure. This procedure identifies periods of presence and periods of absence of fetal eye and body movements, and incorporates the loss of visibility of the ultrasound images during the recording session. Fetal heart rate variability indices and distributions of fetal movements can be computed in the context of the fetal behavioral state concept.

Fetal behavioural states in epileptic pregnancies

Possible negative effects of maternal antiepileptic medication on fetal motility and heart rate patterns were examined at 32 and 38 wk of gestation. Fetal eye and body movements were recorded using 2 real-time ultrasound units. Comparison between pregnancies with antiepileptic medication and control pregnancies did not show marked differences in patterns of motility and heart rate. Duration of sleep states, occurrence and duration of body movements in state 2F and statistical parameters of heart rate level and heart rate variability were very similar for both groups. This preliminary study, limited to mostly combined treatment with antiepileptic drugs, could not demonstrate any obvious effect on fetal neuromuscular development from maternal antiepileptic medication.

Validity of mathematical methods of quantitating fetal heart rate variability

Twenty-two mathematical formulas purporting to quantitate fetal heart rate variability, 13 for short-term and nine for long-term variability, were examined by using artificially generated numbers representing either short-term variability or long-term variability. Numbers representing short-term variability consisted of alternating beats, and those representing long-term variability were sine waves of differing amplitude or frequency spanning the generally accepted clinical range of fetal heart rate variability. The validity of the indices was determined by applying certain selected criteria. The results showed that 11 of the 13 short-term variability indices validly measured short-term variability, whereas only one of the nine long-term variability indices did so. The two major defects in the long-term variability indices were that there was an increase in the long-term variability index as the short-term variability increased, and the long-term variability indices did not increase linearly with increasing frequency of long-term variability complexes. This study suggests that short-term variability is relatively easy to quantify, but there is little relationship between the long-term variability indices and what is clinically regarded as long-term variability.

Midtrimester fetal heart rate variability and maternal hemodynamics in association with smoking

Maternal heart rate (MHR), blood pressure (BP), the differential index (DI) describing the short-term component of fetal heart rate (FHR) variability, and the interval index (II) describing the long-term component were measured in eight subjects in the midtrimester before, during, and after the mothers smoked a standard cigarette. The analyses of FHR variability were performed by an "on-line" method with an abdominal fetal electrocardiogram used as a triggering signal and with a sample time of 1 minute. An increase of MHR, FHR, and BP with a concomitant decrease of II was observed. Unlike our findings regarding the third trimester, no significant change of DI was observed. A correlation analysis revealed fetomaternal hemodynamic relationships different from those in the third trimester: There was a negative correlation between DI and FHR (p less than 0.01) and between DI and MHR (p less than 0.01). There was a positive correlation between FHR and MHR (p less than 0.001). Unlike our findings regarding the third trimester, no correlation was found between DI and BP or between II and MHR. We suggest that the midtrimester fetus shows the narcotic effect (decrease of II) of one cigarette, as does the term fetus, but fails to show the hypoxic effect (decrease of DI), which has been observed in the term fetus.

Significant correlation between maternal hemodynamics and fetal heart rate variability

The differential index, describing the short-term component, and the interval index, describing the long-term component, of fetal heart rate variability were measured by an on-line method from the abdominal electrocardiograms of eight fetuses during the third trimester of normal pregnancies while mothers were smoking a cigarette. Maternal heart rate and maternal blood pressure were measured intermittently before, during, and after smoking. Analyses of fetal heart rate variability were performed continuously with the sample time of 1 minute during the experiments. An increase in maternal heart rate and blood pressure, with concomitant decrease in the interval index and the differential index, was observed. The correlation analysis revealed a negative correlation between maternal heart rate and the interval index (p less than 0.001), between diastolic pressure and the differential index (p less than 0.01), and between systolic pressure and the differential index (p less than 0.01).