Respiratory sinus arrhythmia in the human fetus

Fetal respiratory movements improve reliability of heart rate variability and suggest a coupling between fetal respiratory arrhythmia and vagal activity

Fetal heart rate variability (FHRV) reflects autonomic cardiac regulation. The autonomic nervous system constantly adjusts the heart rate to maintain homeostasis. By providing insight into the fetal autonomic state, FHRV has the potential to become an investigational and clinical instrument. However, the method needs standardization and the influence of fetal movements, including fetal respiratory movements, is not well explored. Therefore, in a highly standardized setting, the aim was to evaluate the association between fetal movements and fetal heart rate variability (FHRV) including their impact on reliability. Fetal heart rate was obtained by noninvasive fetal electrocardiography (NI-FECG) and fetal movements by simultaneous ultrasound scanning in 30 healthy singleton pregnant women on two occasions with a maximum interval of 7 days. The standard deviation of normal-to-normal RR-intervals (SDNN), root mean square of successive RR-interval differences (RMDDS), high-frequency power (HF-power), low-frequency power (LF-power), and LF/HF were measured. A multivariate mixed model was used and reliability was defined as acceptable by a coefficient of variance (CV) ≤15% and an intraclass correlation coefficient (ICC) ≥0.80. During time periods with fetal respiratory movements, the highest reliability was achieved. Intra- and inter-observer reliability measurements were very high (CV: 0-9%; ICC ≧ 0.86). Within the same recording, SDNN and RMSSD achieved acceptable reliability (CV: 14-15%; ICC ≧ 0.80). However, day-to-day reliability displayed high CV's. In time periods with fetal respiratory movements, as compared to periods with quiescence RMSSD and HF-power were higher (Ratio: 1.33-2.03) and LF/HF power lower (Ratio: 0.54). In periods with fetal body movements SDNN, RMSSD and HF-power were higher (Ratio: 1.27-1.65). In conclusion, time periods with fetal respiratory movements were associated with high reliability of FHRV analyses and the highest values of parameters supposed to represent vagal activity.

Physiological control of fetal heart rate variability during labour: Implications and controversies

The interpretation of fetal heart rate (FHR) patterns is the only available method to continuously monitor fetal wellbeing during labour. One of the most important yet contentious aspects of the FHR pattern is changes in FHR variability (FHRV). Some clinical studies suggest that loss of FHRV during labour is a sign of fetal compromise so this is reflected in practice guidelines. Surprisingly, there is little systematic evidence to support this observation. In this review we methodically dissect the potential pathways controlling FHRV during labour-like hypoxaemia. Before labour, FHRV is controlled by the combined activity of the parasympathetic and sympathetic nervous systems, in part regulated by a complex interplay between fetal sleep state and behaviour. By contrast, preclinical studies using multiple autonomic blockades have now shown that sympathetic neural control of FHRV was potently suppressed between periods of labour-like hypoxaemia, and thus, that the parasympathetic system is the sole neural regulator of FHRV once FHR decelerations are present during labour. We further discuss the pattern of changes in FHRV during progressive fetal compromise and highlight potential biochemical, behavioural and clinical factors that may regulate parasympathetic-mediated FHRV during labour. Further studies are needed to investigate the regulators of parasympathetic activity to better understand the dynamic changes in FHRV and their true utility during labour. This article is protected by copyright. All rights reserved.

Fetal Heart Rate Variability Is Affected by Fetal Movements: A Systematic Review

Introduction: Fetal heart rate variability (FHRV) evaluates the fetal neurological state, which is poorly assessed by conventional prenatal surveillance including cardiotocography (CTG). Accurate FHRV on a beat-to-beat basis, assessed by time domain and spectral domain analyses, has shown promising results in the scope of fetal surveillance. However, accepted standards for these techniques are lacking, and the influence of fetal breathing movements and gross movements may be especially challenging. Thus, current standards for equivalent assessments in adults prescribe rest and controlled respiration. The aim of this review is to clarify the importance of fetal movements on FHRV. Methods: A systematic review in accordance with the PRISMA guidelines based on publications in the EMBASE, the MEDLINE, and the Cochrane Library databases was performed. Studies describing the impact of fetal movements on time domain, spectral domain and entropy analyses in healthy human fetuses were reviewed. Only studies based on fetal electrocardiography or fetal magnetocardiography were included. PROSPERO registration number: CRD42018068806. Results: In total, 14 observational studies were included. Fetal movement detection, signal processing, length, and selection of appropriate time series varied across studies. Despite these divergences, all studies showed an increase in overall FHRV in the moving fetus compared to the resting fetus. Especially short-term, vagal mediated indexes showed an increase during fetal breathing movements including an increase in Root Mean Square of the Successive Differences (RMSSD) and High Frequency power (HF) and a decrease in Low Frequency power/High Frequency power (LF/HF). These findings were present even in analyses restricted to one specific fetal behavioral state defined by Nijhuis. On the other hand, fetal body movements seemed to increase parameters supposed to represent the sympathetic response [LF and Standard Deviation of RR-intervals from normal sinus beats (SDNN)] proportionally more than parameters representing the parasympathetic response (RMSSD, HF). Results regarding entropy analyses were inconclusive. Conclusion: Time domain analyses as well as spectral domain analyses are affected by fetal movements. Fetal movements and especially breathing movements should be considered in these analyses of FHRV.

Frequency and Time Domain Analysis of Foetal Heart Rate Variability with Traditional Indexes: A Critical Survey

Monitoring of foetal heart rate and its variability (FHRV) covers an important role in assessing health of foetus. Many analysis methods have been used to get quantitative measures of FHRV. FHRV has been studied in time and in frequency domain and interesting clinical results have been obtained. Nevertheless, a standardized definition of FHRV and a precise methodology to be used for its evaluation are lacking. We carried out a literature overview about both frequency domain analysis (FDA) and time domain analysis (TDA). Then, by using simulated FHR signals, we defined the methodology for FDA. Further, employing more than 400 real FHR signals, we analysed some of the most common indexes, Short Term Variability for TDA and power content of the spectrum bands and sympathovagal balance for FDA, and evaluated their ranges of values, which in many cases are a novelty. Finally, we verified the relationship between these indexes and two important parameters: week of gestation, indicator of foetal growth, and foetal state, classified as active or at rest. Our results indicate that, according to literature, it is necessary to standardize the procedure for FHRV evaluation and to consider week of gestation and foetal state before FHR analysis.

Respiratory sinus arrhythmia in growth restricted fetuses with normal Doppler hemodynamic indices

BACKGROUND

The autonomic behavior of growth-restricted fetuses at different evolving hemodynamic stages has not been fully elicited.

AIM

To analyze the respiratory sinus arrhythmia (RSA) of growth-restricted fetuses that despite this severe condition show normal Doppler hemodynamics.

SUBJECTS

10 growth-restricted fetuses (FGR group) with normal arterial pulsatility indices (umbilical, uterine, middle cerebral, ductus venosus and aortic isthmus), and 10 healthy fetuses (Control group), 32-37weeks of gestation.

METHOD

B-mode ultrasound images for visualizing fetal breathing movements (FBM) or breathing akinesis (FBA), and the simultaneous RR-interval time series from maternal abdominal ECG recordings were obtained. The root-mean-square of successive differences of RR-intervals (RMSSD) was considered as a RSA-related parameter among the instantaneous amplitude of the high-frequency component (AMPHF) and its corresponding instantaneous frequency (IFHF), both computed by using empirical mode decomposition. Mean fetal heart-periods and RSA-related parameters were assessed during episodes of FBM and FBA in 30s length windows.

RESULTS

FGR and Control groups presented RSA-related fluctuations during FBM and FBA. Also, both groups showed significant higher (p<0.001) values for the mean heart-period, RMSSD and AMPHF during FBM. No-significant differences (p>0.05) were found for the IFHF regardless of breathing activity (FBM vs. FBA).

CONCLUSION

Growth-restricted fetuses without evident hemodynamic compromise exhibit a preserved autonomic cardiovascular regulation, characterized by higher values of RSA and mean heart-period in the presence of FBM. This physiological response reflects a compensatory strategy that may contribute to preserve blood flow redistribution to vital organs.

Fetal heart rate variability during pregnancy, obtained from non-invasive electrocardiogram recordings

OBJECTIVE

Non-invasive spectral analysis of fetal heart rate variability is a promising new field of fetal monitoring. To validate this method properly, we studied the relationship between gestational age and the influence of fetal rest-activity state on spectral estimates of fetal heart rate variability.

DESIGN

Prospective longitudinal study.

SETTING

Tertiary care teaching hospital.

POPULATION

Forty healthy women with an uneventful singleton pregnancy.

METHODS

Non-invasive fetal electrocardiogram measurements via the maternal abdomen were performed at regular intervals between 14 and 40 weeks of gestation and processed to detect beat-to-beat fetal heart rate. Simultaneous ultrasound recordings were performed to assess fetal rest-activity state.

MAIN OUTCOME MEASURES

Absolute and normalized power of fetal heart rate variability in the low (0.04-0.15 Hz) and high (0.4-1.5 Hz) frequency band were obtained, using Fourier Transform.

RESULTS

14% of all measurements and 3% of the total amount of abdominal data (330 segments) was usable for spectral analysis. During 21-30 weeks of gestation, a significant increase in absolute low and high frequency power was observed. During the active state near term, absolute and normalized low frequency power were significantly higher and normalized high frequency power was significantly lower compared with the quiet state.

CONCLUSIONS

The observed increase in absolute spectral estimates in preterm fetuses was probably due to increased sympathetic and parasympathetic modulation and might be a sign of autonomic development. Further improvements in signal processing are needed before this new method of fetal monitoring can be introduced in clinical practice.

Effects of fetal respiratory movements on the short-term fractal properties of heart rate variability

We evaluated the effect of fetal respiratory movements (RM) on the heart rate (HR) fractal dynamics.Abdominal ECG recordings were collected from low-middle-risk pregnant woman at rest. Mean gestational age was 34.8 ± 3.7 weeks. Ultrasound images were simultaneously acquired determining if RM were exhibited by fetuses. 13 pairs of HR series were compared. Each pair included 5 min of data from the same fetus either during the manifestation of RM or when there was no persistent indication of them. Detrended fluctuation analysis was applied to these series for obtaining the scaling exponent α1. HR series were also assessed using the conventional parameters RMSSD and HF power.The main findings of this contribution were the lack of significant changes in the scaling exponent α1 of fetal HR fluctuations as a result of RM. By contrast, HF power and RMSSD did show significant changes associated with the manifestation of fetal RM (p < 0.001 and p < 0.05, respectively). Yet the scaling exponent was the only parameter showing a significant relationship with the particular frequency of fetal RM (r s  = 0.6, p < 0.03). Given the invariability of α1 regarding the manifestation of fetal RM, we consider that the HR short-term fractal properties are convenient for assessing the cardiovascular prenatal regulation.

Characterization of the fetal diaphragmatic magnetomyogram and the effect of breathing movements on cardiac metrics of rate and variability

Breathing movements are one of the earliest fetal motor behaviors to emerge and are a hallmark of fetal well-being. Fetal respiratory sinus arrhythmia (RSA) has been documented but efforts to quantify the influence of breathing on heart rate (HR) and heart rate variability (HRV) are difficult due to the episodic nature of fetal breathing activity. We used a dedicated fetal biomagnetometer to acquire the magnetocardiogram (MCG) between 36 and 38 weeks gestational age (GA). We identified and characterized a waveform observed in the raw data and independent component decomposition that we attribute to fetal diaphragmatic movements during breathing episodes. RSA and increased high frequency power in a time-frequency analysis of the IBI time-series was observed during fetal breathing periods. Using the diaphragmatic magnetomyogram (dMMG) as a marker, we compared time and frequency domain metrics of heart rate and heart rate variability between breathing and non-breathing epochs. Fetal breathing activity resulted in significantly lower HR, increased high frequency power, greater sympathovagal balance, increased short-term HRV and greater parasympathetic input relative to non-breathing episodes confirming the specificity of fetal breathing movements on parasympathetic cardiac influence. No significant differences between breathing and non-breathing epochs were found in two metrics reflecting total HRV or very low, low and intermediate frequency bands. Using the fetal dMMG as a marker, biomagnetometry can help to elucidate the electrophysiologic mechanisms associated with diaphragmatic motor function and may be used to study the longitudinal development of human fetal cardiac autonomic control and breathing activity.

Magnetographic assessment of fetal hiccups and their effect on fetal heart rhythm

Fetal hiccups emerge as early as nine weeks post-conception, being the predominant diaphragmatic movement before 26 weeks of gestation. They are considered as a programmed isometric inspiratory muscle exercise of the fetus in preparation for the post-natal respiratory function, or a manifestation of a reflex circuitry underlying the development of suckling and gasping patterns. The present paper provides the first evidence of non-invasive biomagnetic measurements of the diaphragm spasmodic contractions associated with fetal hiccups. The magnetic field patterns generated by fetal hiccups exhibit well-defined morphological features, consisting of an initial high frequency transient waveform followed by a more prolonged low frequency component. This pattern is consistent across recordings obtained from two fetal subjects, and it is confirmed by signals recorded in a neonatal subject. These results demonstrate that fetal biomagnetometry can provide insights into the electrophysiological mechanisms of diaphragm motor function in the fetus. Additionally, we study the correlation between hiccup events and fetal cardiac rhythm and provide evidence that hiccups may modulate the fetal heart rate during the last trimester of pregnancy.

An estimate of fetal autonomic state by time-frequency analysis of fetal heart rate variability

In this study we present a noninvasive method that enables the investigation of the fetal heart rate (FHR) fluctuations. The objective was to design a quantitative measurement to assess the fetal autonomic nervous system and to investigate its development as a function of the gestational age. Our Medical Physics group has developed a complex algorithm for online beat-to-beat detection of the fetal ECG (FECG), extracted from the maternal abdominal ECG signal. We used our previously acquired FECG data, which includes noninvasive recordings of 200 maternal abdominal ECG signals. From these, we chose 35 cases of healthy pregnancies that we divided into three groups according to gestational age: Group 1, 23 +/- 2 wk; Group 2, 32 +/- 1 wk; and Group 3, 39 +/- 1 wk. The FHR variability was analyzed by a time-frequency decomposition based on a continuous wavelet transform. We showed that, independent of the gestational age, most of the FHR power is concentrated in the very-low-frequency range (0.02-0.08 Hz) and in the low-frequency range (0.08-0.2 Hz). In addition, there is power in the high-frequency range that correlates with the frequency range of fetal respiratory motion (0.4-1.7 Hz). In the intermediate-frequency range (0.2-0.4 Hz), the power is significantly smaller. The changes in the average power spectrum in relation to gestation time were carefully and quantitatively examined. The results imply that there is a neural organization during the last trimester of the pregnancy, and the sympathovagal balance is reduced with the gestational age.

Changes in the frequency power spectrum of fetal heart rate in the course of pregnancy

OBJECTIVE

The aim of this study was to examine changes in the heart rate variability based on the frequency power spectrum of healthy fetuses during the second and third trimester of pregnancy.

METHODS

We analyzed 222 fetal magnetocardiograms recorded in 49 healthy singleton pregnancies between the 16th and 42nd week. Discrete Fourier transformation was performed on the time-based step function of the RR-intervals. Changes of spectral density in the frequency spectrum in various bands between 0.003 to 1 Hz, including low-frequency (LF: 0.04-0.15 Hz) and high-frequency (HF: 0.15-0.40 Hz) bands, were examined as a function of gestational age.

RESULTS

Spectral density between 0.003 to 1.0 Hz increased with gestational age with large changes, in particular, at lower frequencies. At approximately the 32nd week, the rate of increase in power slowed substantially. Prior to this time, the rates of change in power were different for the bands 0.003 to 0.40 Hz, 0.40 to 0.60 Hz and 0.60 to 1.0 Hz. LF and HF showed similar development, with HF increasing slightly more rapidly.

CONCLUSION

We conclude that characteristic spectral bands that increase in spectral density at different rates during the second and third trimester may be identified. They most likely reflect developmental changes and behavioral states during pregnancy.

Computerised intrapartum diagnosis of fetal hypoxia based on fetal heart rate monitoring and fetal pulse oximetry recordings utilising wavelet analysis and neural networks

OBJECTIVE

To develop a computerised system that will assist the early diagnosis of fetal hypoxia and to investigate the relationship between the fetal heart rate variability and the fetal pulse oximetry recordings.

DESIGN

Retrospective off-line analysis of cardiotocogram and FSpO2 recordings.

SETTING

The Maternity Unit of the 2nd Department of Obstetrics and Gynaecology, Aretaieion Hospital, University of Athens.

POPULATION

Sixty-one women of more than 37 weeks of gestation were monitored throughout labour.

METHODS

Multiresolution wavelet analysis was applied in each 10-minute period of second stage of labour focussing on long term variability changes in different frequency ranges and statistical analysis was performed in the associated 10-minute FSpO2 recordings. Self-organising map neural network was used to categorise the different 10-minute fetal heart rate patterns and the associated 10-minute FSpO2 recordings.

MAIN OUTCOME MEASURES

Umbilical artery pH of < or = 7.20 and Apgar score at 5 minutes of < or = 7 formed the inclusion criteria of the risk group.

RESULTS

After using k-means clustering algorithm, the two-dimensional output layer of the self-organising map neural network was divided into three distinct clusters. All the cases that mapped in cluster 3 belonged in the risk group except one. The sensitivity of the system was 83.3% and the specificity 97.9% for the detection of risk group cases.

CONCLUSIONS

A relationship between the fetal heart rate variability in different frequency ranges and the time in which FSpO2 is less than 30% was noticed. Fetal pulse oximetry seems to be an important additional source of information. Computerised analysis of the fetal heart rate monitoring and pulse oximetry recordings is a promising technique in objective intrapartum diagnosis of fetal hypoxia. Further evaluation of this technique is mandatory to evaluate its efficacy and reliability in interpreting fetal heart rate recordings.

Fetal arrhythmias

Fetal arrhythmias may be benign or life-threatening. Benign disturbances in fetal cardiac rhythm are relatively common, and their clinical manifestations are reviewed. Life-threatening fetal arrhythmias include supraventricular tachycardias, atrial flutter, ventricular or junctional tachycardia, chaotic atrial tachycardia, and bradyarrhythmias such as second or third degree AV block. The incidence, diagnostic characteristics and treatment of each of these are reviewed. In addition, the pathophysiology of hydrops fetalis, embryology of the conduction system and transplacental drug transfer characteristics are reviewed.

Human fetuses have nonlinear cardiac dynamics

Approximate entropy (ApEn) is a statistic that quantifies regularity in time series data, and this parameter has several features that make it attractive for analyzing physiological systems. In this study, ApEn was used to detect nonlinearities in the heart rate (HR) patterns of 12 low-risk human fetuses between 38 and 40 wk of gestation. The fetal cardiac electrical signal was sampled at a rate of 1,024 Hz by using Ag-AgCl electrodes positioned across the mother's abdomen, and fetal R waves were extracted by using adaptive signal processing techniques. To test for nonlinearity, ApEn for the original HR time series was compared with ApEn for three dynamic models: temporally uncorrelated noise, linearly correlated noise, and linearly correlated noise with nonlinear distortion. Each model had the same mean and SD in HR as the original time series, and one model also preserved the Fourier power spectrum. We estimated that noise accounted for 17.2-44.5% of the total between-fetus variance in ApEn. Nevertheless, ApEn for the original time series data still differed significantly from ApEn for the three dynamic models for both group comparisons and individual fetuses. We concluded that the HR time series, in low-risk human fetuses, could not be modeled as temporally uncorrelated noise, linearly correlated noise, or static filtering of linearly correlated noise.

High vagal tone is associated with more efficient regulation of homeostasis in low-risk human fetuses

Homeostasis is maintained primarily by the parasympathetic nervous system and is thought to provide a physiological substrate for the development of complex behaviors. This investigation was undertaken to test the hypothesis that infants with high parasympathetic tone are more efficient regulators of homeostasis than infants with low parasympathetic tone. Respiratory sinus arrhythmia (RSA) was used as a measure of parasympathetic tone, and the efficiency of homeostatic control was quantified, for each infant, by the slope (SRSA) and correlation coefficient (RRSA) of the regression line relating fluctuations in heart period and fluctuations in RSA. To test our hypothesis, we examined the relationship between RSA and both SRSA and RRSA in 34 low-risk human fetuses between 36 and 40 weeks gestation. We found that fetuses who were parasympathetic-dominated had larger SRSA and RRSA values, and hence were more efficient regulators of homeostasis, than fetuses who were sympathetic-dominated. The results of our analyses are important because they establish, very early in development, a physiological basis for the relationship between vagal tone and the development of complex behaviors.

Gestational age related changes in cardiac dynamics of the fetal baboon

To provide insight into the maturation of neural mechanisms governing fetal heart rate and rate variability, seven chronically instrumented fetal baboons were monitored under steady state conditions between 120 and 165 days gestation (term 175 d). Forty records of 24 h duration (5-7 records/fetus) were evaluated. For each fetus, heart rate decreased with gestational age (mean+/-SD, r = -0.530+/-0.324, P <0.05). In contrast, there were increases with age in markers of various components of autonomic control of fetal R-wave to R-wave interval (RRi) variability as reflected in a positive correlation with age for all fetuses of SD RRi (r = 0.656+/-0.347, P < 0.01), root mean squared differences in RRi (r = 0.686+/-0.223, P <0.05), and power at low frequency in the RRi spectrum (r = 0.800+/-0.161 P < 0.01). In each of the seven fetuses, scatter plots of RRi as a function of the prior RRi (Poincare plots) had increased dispersion around the median with gestational age (0.605+/-0.371, P<0.05). Additional measures of variability evaluated changes in RRi from one interval to the next (deltaRRi). The incidence of sustained deltaRRi changes, either decelerations or accelerations, rose with gestation (r = 0.920+/-0.057, P < 0.001) while the incidence of no detected deltaRRi changes (<+/-1 ms) diminished (r = - 0.649+/-0.364, P <0.05). Sequential decreases in fetal heart rate, increases in RRi variability and increases in changes in RRi and deltaRRi with age imply an overall maturation in autonomic cardio-regulatory control processes. Increases with gestation in measures of high frequency components of variability are compatible with enhanced parasympathetic modulation of fetal heart rate.

Time-frequency analysis of fetal heartbeat fluctuation using wavelet transform

We examined whether the nonlinear control mechanism of the fetal autonomic nervous system would change in various fetal states. Eight thousand or more fetal heartbeats were detected from normal, hypoxemic, and acidemic fetuses. Fetal heart Doppler-signal intervals were determined in a high-precision autocorrelation method, and a time series of fetal heart rate fluctuation was obtained. The distribution of the amplitude of temporal fluctuation in the low-frequency component of fetal heart rate frequency was studied using a method of time-frequency analysis called wavelet transform. Spline 4 was used as the mother wavelet function. A gamma distribution was observed from 17 wk of gestation onward. The value of the parameter nu of this gamma distribution was approximately 1.6 and remained constant regardless of the gestational age or the time of day. The value of nu decreased significantly to 0.77 when the fetus developed acidemia and was 1.51 in hypoxemia and 1.54 in a normal condition. This study elucidates a nonlinear structure of the time series of heart rate fluctuation of the gamma distribution in the human fetus. This technique may provide a new quantitative index of fetal monitoring to diagnose fetal acidemia.

Heart rate dynamics in low risk human fetuses

Evaluation of nonlinear heart rate (HR) dynamics has received considerable attention in the pediatric literature because such analyses not only provide insight into underlying control mechanisms, but may also help to differentiate between normal and abnormal infants. The purpose of this study was to determine, in eight low risk human fetuses, if nonlinear HR dynamics could be identified by analyzing the dispersion of interbeat intervals at slow (Ds) and fast (Df) HRs. The fetal cardiac electrical signal was captured transabdominally at a resolution of +/- 1 ms. To test the null hypothesis, that the time series is the result of a linear stochastic process, Ds and Df for the original time series were compared with the values calculated for three linear models. The linear models were constructed to preserve the major statistical properties of the original time series, including the mean, SD, and the Fourier power spectrum. For each fetus, there was no evidence of nonlinear cardiac dynamics based on analyses of Ds and Df. In contrast, the distribution of adjacent R-R intervals and the pattern of change across three successive interbeat intervals both revealed significant nonlinearities in HR control in each fetus. If the difference between normal and abnormal infants is the result of aberrant control of nonlinear processes, then our findings indicate that parameters which describe the nonlinearity may be more useful then Ds and Df in assigning a risk status.

How useful is intrapartum electronic fetal heart rate monitoring?

It is essential for an obstetric anesthesiologist to be aware of the fetal status before undertaking care of the laboring mother. In the last 20 years electronic fetal monitoring has been the most widely used technique of evaluating the fetus in labor. Recently however, the ability to predict or improve fetal outcome using traditional interpretation has been questioned. This review presents a summary of the current technology and interpretation of intrapartum electronic fetal monitoring, as well as a discussion of its limitations and some of the developments in this field which may help improve the accuracy of fetal assessment. The new developments in fetal monitoring discussed in this article are computerized assessment of fetal heart tracings, heart rate variability analysis, fetal electrocardiogram waveform analysis, abdominal detection of fetal ECG, fetal scalp oxygen saturation, fetal pH sampling and transcutaneous oxygen and carbon dioxide measurement.

Spectral analysis of fetal heart rate in flat recordings

Flat heart rate recordings may be observed in different fetal states such as chronic distress and sleep. Their visual analysis do not allow the distinction between these two states. We used spectral analysis to study the heart rate patterns in 25 fetuses. Two significant (P < 5 x 10(-5)) groups were apparent from the determination of the position of the maximum energy peak (PMEP) in the high-frequency band (0.20-0.50 Hz): a PMEP at about 0.20 Hz (group 1), and another around 0.30 Hz (group 2). The two groups did not differ in spectral density (SD). The outcome of neonates showed that group 1 fetuses made good progress and produced healthy neonates; whereas group 2 comprised cases of chronic fetal distress, or even death in utero, and neonatal distress. The significance of this difference in PMEP between fetal heart rate patterns in chronic distress and sleep is unclear. Studies combining the assessment of fetal movements and the determination of PMEP are planned.

Foetal magnetocardiogram amplitude oscillations associated with respiratory sinus arrhythmia

We show that oscillations at foetal breathing frequencies observed in foetal heart rate (FHR) tracings obtained from foetal magnetocardiogram (FMCG) recordings are often accompanied by synchronous modulation of FMCG signal amplitude. This implies an association between the FHR oscillations and foetal chest wall movements, corroborating the hypothesis that the oscillations are due to respiratory sinus arrhythmia.

Vagal tone during quiet sleep in normal human term fetuses

The purpose of this paper was to calculate vagal tone (V) for 17 normal human fetuses in quiet sleep (QS) between 36 and 40 weeks gestation. The fetal cardiac electrical signal was captured transabdominally in 3-min blocks at a rate of 833 times per second and fetal R-waves were extracted using adaptive signal processing techniques. Fetal R-wave interbeat intervals were converted to equally spaced, time-based data, and the low-frequency component was removed using a 21-point third-order moving polynomial. The parameter V was calculated by taking the natural logarithm of the sum of the power densities between 0.3 Hz and 1.3 Hz. We found that fetal breathing was associated with an approximately 25% increase in V as compared to nonbreathing, 3.33 +/- 0.48 versus 2.57 +/- 0.47, p < 0.0001. Furthermore, there was a significant linear relationship between the mean single-fetus V during spontaneous respiration and the mean single-fetus V during normally occurring apneic periods, r = 0.772, p < 0.002. We conclude that respiratory activity is associated with a significant increase in vagal tone for normal human fetuses in QS.

Power spectral analysis of fetal heart rate

This chapter examines the role of power spectral analysis (PSA) in elucidation of the physiological control mechanisms of fetal heart rate and as a potential indicator of fetal well-being. The importance of fetal heart rate variability (FHRV) as an indicator of fetal oxygenation is discussed, and the limitations in the current methods of measurement of FHRV are highlighted. Evidence is presented for the paramount influence of the autonomic nervous system in the control of heart rate variability. The basic proposition underlying spectral analysis is that the two autonomic branches influence heart rate in a frequency-dependent way, and their differential effects can be determined by PSA which breaks down the heart rate trace into its component frequencies. The application of PSA to heart rate variability data is an established tool in cardiology, and the published literature related to its use in the adult, neonate and fetus is reviewed. The power spectrum is sensitive to the activity state of the fetus, particularly fetal breathing movements, which have a variable effect on short- and long-term FHRV. There are a variety of mathematical approaches to the construction of power spectra, and a particular method of data acquisition and analysis is presented together with some theoretical background. Recent experimental evidence indicates a role for PSA as an indicator of fetal activity state, and the effect of hypoxia on the spectrum of the fetus in labour is discussed. There are some problems with the technique of PSA, particularly in regard to accepted definitions and methods of analysis. It is a powerful non-invasive tool in the elucidation of fetal cardiac control, but its value in the detection of the compromised fetus has yet to be tested in a clinical trial.

Quantification of the fetal heart rate variability by spectral analysis of fetal well-being and fetal distress

Our objectives were to increase the discrimination between fetal distress and fetal well-being, using fetal heart rate spectral analysis. Monitoring of the heart rate from 259 fetuses was done between 26 and 42 weeks, interpreted with classical criteria, and analysed with the spectral analysis method we developed. The fetal heart rate spectrum analysis performed on these recordings allow discrimination of fetal distress from the normal state using the energy value and frequency of the maximal energy in the high frequency band. We can conclude that the spectral analysis produces two significant parameters which could contribute to a multivariate approach to assessments of the physiological mechanisms of heart rate variability.

Spectral analysis of antepartum fetal heart rate variability from fetal magnetocardiogram recordings

Fetal heart rate variability was derived from fetal magnetocardiogram recordings in ten subjects at gestation ages 32-38 weeks. Maternal interference was negligible and R-wave detection was highly reliable. Oscillations suggestive of respiratory sinus arrhythmia (RSA) were prominent in many of the heart rate tracings. Spectral analysis was used to quantify heart rate variability and to examine the influence of the RSA-like oscillations on heart rate variability. The oscillations were associated with increased power in the frequency range 0.4-1.0 Hz (P < or = 0.05). Magnetic recording appears to offer significant advantages for investigation of beat-to-beat fetal heart rate throughout the latter stages of pregnancy.

Computer analysis of heart rate variation and breathing movements in fetal lambs

A quantitative method for studying the frequency-specific relationships between heart rate (HR) and fetal breathing movements (FBM) was developed. The reactivity of periodic HR variation in relation to FBM was investigated by means of power spectral analysis. Seven fetal lambs were studied during the third trimester of gestation using a chronic animal model. HR variability increased at the rate of FBM, as shown by an increase of spectral density at > 0.35 Hz in the HR autospectrum and in the cross-spectrum of HR and respirogram, as well as by an increase in the short-term variability index CVS. FBM were associated with the increased HR variation in all but the lowest frequency bands (0.07-1.0 Hz). Although respiratory sinus arrhythmia was found, only 10 per cent of the total HR variability and 25 per cent of the joint-density of HR and respirogram appeared at > 0.35 Hz during FBM. The greatest variation in both the HR and respirogram spectra appeared at < 0.07 Hz. Although the low-frequency variability of HR and respirogram was simultaneous, it was on the whole not synchronised. The existence of multiple control systems that simultaneously link the cardiac and respiratory control mechanisms to each other in the fetal lamb is postulated.

Association of breathing movements to the variability of heart rate and blood pressure in foetal lambs

Heart rate (HR) variability and arterial blood pressure (BP) variability were analysed as functions of foetal breathing movements (FBMs) by means of power spectral analysis in seven foetal lambs during the third trimester of gestation. No evidence of FBM-related changes, either in mean HR, mean systolic or diastolic arterial pressures, were found. Mean arterial pulse pressure, HR variability, and BP variability increased during FBMs. The increase in BP variability occurred at frequencies higher than 0.35 Hz, i.e. those of FBMs. The increase in HR variability occurred at 0.07-1.0 Hz, i.e. at every frequency band except the lowest one. Thus, the increase in HR variability was not frequency-specifically related to FBMs. During FBMs the periodic variability of HR at frequencies > 0.35 Hz was only 10% of total HR variability. We suggest that the FBM-related changes of BP variability may be mediated by direct peripheral, hydraulic mechanisms. HR changes involve autonomic control systems: the vagal component of baroreflex seems to be relatively insensitive, whereas the very slow vasomotor component of HR variability is dominant.

Fetal heart rate variability and behavioral state: analysis by power spectrum

OBJECTIVE

We attempted to determine the relationship between the fetal heart rate power spectrum and fetal state.

STUDY DESIGN

Interbeat intervals, electrocortical activity, and fetal breathing movements were recorded from five near-term fetal lambs. Interbeat intervals were taken from epochs of low-voltage electrocortical activity with breathing, low-voltage electrocortical activity without breathing, and high-voltage electrocortical activity without breathing. Power spectral techniques were applied to determine the underlying frequencies contributing to fetal heart rate variability. Spectral analysis was also performed on fetal breathing data from three animals.

RESULTS

Significant differences were found between low-voltage electrocortical activity with breathing and high-voltage electrocortical activity without breathing at 0.62 Hz and from 1.09 to 1.56 Hz. There was no clear relationship between the breathing and heart rate spectra.

CONCLUSIONS

Fetal heart rate is mediated by both state and respiratory variables. The respiratory component is not strictly related to respiratory rate.

High-frequency and low-frequency heart-rate fluctuation analysis in newborns--a review of possibilities and limitations

Respiratory sinus arrhythmia (RSA) has been traditionally defined as the high-frequency component of heart-rate fluctuations (HRF) synchronous with the respiratory movements (RM), i.e., the frequency of RSA corresponds to the respiration rate. It can be shown that at defined relations of mean heart and respiration rate some essential effects must be taken into consideration in studies using RSA parameters. The relevance of these effects is shown and a new strategy of RSA quantification demanded, at least in neonates. The diagnostic importance, physiological background and future application of the low-frequency components of the neonatal HRF are reviewed on the basis of our own results. Nonrespiratory cardiovascular HRF cannot always be detected in the neonate by power spectral analysis. Movement-related HRF have a potential diagnostic importance per se, but may also artefactually disturb the quantification of other HRF components. Long-term trends can be used to describe sleep-state related trends and their disturbances.

What is fetal distress?

Fetal distress is a widely used but poorly defined term. This confusion of definition compounds the difficulty of making an accurate diagnosis and initiating appropriate treatment. The fetus reacts at the onset of asphyxia with a remarkable series of responses, primarily a complexly regulated redistribution of blood flow that serves to limit the deleterious effects of oxygen limitation in vital organs. This enables the fetus to survive asphyxia intact unless the insult is profound or prolonged. The most common asphyxial stresses imposed on the fetus during labor are insufficiency of uterine blood flow, or insufficiency of umbilical blood flow, and occasionally decrease in uterine arterial oxygenation. Each of these stresses produces characteristic fetal heart rate patterns: late decelerations, variable decelerations, or prolonged bradycardia. There is strong evidence that the presence of normal fetal heart rate variability represents normal central nervous system integrity, including adequate oxygenation. A decrease or loss of variability in the presence of these patterns is a sign that the physiologic compensations are overwhelmed as a result of the severity of asphyxia. Knowledge of the fetal responses to asphyxia, together with the known evolution of fetal heart rate patterns during asphyxia, should allow a more accurate definition of the onset of unacceptable asphyxia, and more rational management and timing of intervention.

Cardiac aliasing--a possible cause for the misinterpretation of cardiorespirographic data in neonates

Cardiac aliasing is a physiological phenomenon generating respiratory sinus arrhythmia in a frequency range lower than that of respiration if the mean respiration rate exceeds half of the mean heart rate. This was found in 8 out of 39 healthy fullterm newborns, 5 out of 33 fullterm and 3 out of 63 preterm newborns during intensive care. Quantification of cardiorespirographic data is considerably impaired in those cases when the mean respiration rate markedly exceeds half of the mean heart rate (two out of the 39 healthy full term newborns).

Diminished respiratory sinus arrhythmia in asphyxiated term infants

Spectral analysis techniques were used to quantitate the association between respiration and heart rate variability in eight healthy and eight asphyxiated infants born at term gestation. Respiratory sinus arrhythmia was demonstrated in all healthy infants. This arrhythmia was significantly diminished in asphyxiated newborn infants. We conclude that newborn infants with low Apgar scores have a reduced respiratory sinus arrhythmia and that this reduction could account for the loss of short-term heart rate variability commonly associated with asphyxia.

Biophysical profile in the fetus from a phonographic sensor

We have recently developed a phonographic transducer which is compliance-matched to the maternal abdomen. Using it, it is possible to monitor for long time periods, non-invasively and without discomfort, the sound and infra-sound produced by the fetus. Fetal heart sounds, fetal breathing, and fetal body movements can be recorded overnight in hospital or at home, and thereby provide a biophysical profile of fetal activity. Overnight recordings of fetal sounds and infra-sound together with maternal ECG and maternal breathing movements are currently being used to study mothers with normal and abnormal pregnancies.

The fetal phonogram: a measure of fetal activity

In 12 pregnant mothers fetal sounds and infrasounds were recorded by means of a new compliance matched transducer and compared with a simultaneous ultrasound record of fetal activity. A defined pattern on the fetal phonograph correlated with 86% of the total fetal breathing detected with ultrasound, and a further distinctive pattern was associated with 90% of fetal movements. Examination of the fetal phonocardiogram when the fetus was breathing showed a significant increase in the short-term variability of both the systolic and diastolic times when compared with non-breathing episodes. The median amplitude variabilities for both the first and the second heart sounds were also significantly increased during fetal breathing. Measurement of fetal sounds and infrasounds with a compliance matched transducer offers a non-invasive method for assessment of fetal activity for long periods of time.