Power spectral analysis of fetal heart rate

Detection of Suspicious Cardiotocographic Recordings by Means of a Machine Learning Classifier

Cardiotocography (CTG) is one of the fundamental prenatal diagnostic methods for both antepartum and intrapartum fetal surveillance. Although it has allowed a significant reduction in intrapartum and neonatal mortality and morbidity, its diagnostic accuracy is, however, still far from being fully satisfactory. In particular, the identification of uncertain and suspicious CTG traces remains a challenging task for gynecologists. The introduction of computerized analysis systems has enabled more objective evaluations, possibly leading to more accurate diagnoses. In this work, the problem of classifying suspicious CTG recordings was addressed through a machine learning approach. A machine-based labeling was proposed, and a binary classification was carried out using a support vector machine (SVM) classifier to distinguish between suspicious and normal CTG traces. The best classification metrics showed accuracy, sensitivity, and specificity values of 92%, 92%, and 90%, respectively. The main results were compared both with results obtained by considering a more unbalanced dataset and with relevant literature studies in the field. The use of the SVM proved to be promising in the field of CTG classification. However, appropriate feature selection and dataset balancing are crucial to achieve satisfactory performance of the classifier.

A Comprehensive Review of Techniques for Processing and Analyzing Fetal Heart Rate Signals

The availability of standardized guidelines regarding the use of electronic fetal monitoring (EFM) in clinical practice has not effectively helped to solve the main drawbacks of fetal heart rate (FHR) surveillance methodology, which still presents inter- and intra-observer variability as well as uncertainty in the classification of unreassuring or risky FHR recordings. Given the clinical relevance of the interpretation of FHR traces as well as the role of FHR as a marker of fetal wellbeing autonomous nervous system development, many different approaches for computerized processing and analysis of FHR patterns have been proposed in the literature. The objective of this review is to describe the techniques, methodologies, and algorithms proposed in this field so far, reporting their main achievements and discussing the value they brought to the scientific and clinical community. The review explores the following two main approaches to the processing and analysis of FHR signals: traditional (or linear) methodologies, namely, time and frequency domain analysis, and less conventional (or nonlinear) techniques. In this scenario, the emerging role and the opportunities offered by Artificial Intelligence tools, representing the future direction of EFM, are also discussed with a specific focus on the use of Artificial Neural Networks, whose application to the analysis of accelerations in FHR signals is also examined in a case study conducted by the authors.

Systematic Review of Intrapartum Fetal Heart Rate Spectral Analysis and an Application in the Detection of Fetal Acidemia

It is fundamental to diagnose fetal acidemia as early as possible, allowing adequate obstetrical interventions to prevent brain damage or perinatal death. The visual analysis of cardiotocography traces has been complemented by computerized methods in order to overcome some of its limitations in the screening of fetal hypoxia/acidemia. Spectral analysis has been proposed by several studies exploring fetal heart rate recordings while referring to a great variety of frequency bands for integrating the power spectrum. In this paper, the main goal was to systematically review the spectral bands reported in intrapartum fetal heart rate studies and to evaluate their performance in detecting fetal acidemia/hypoxia. A total of 176 articles were reviewed, from MEDLINE, and 26 were included for the extraction of frequency bands and other relevant methodological information. An open-access fetal heart rate database was used, with recordings of the last half an hour of labor of 246 fetuses. Four different umbilical artery pH cutoffs were considered for fetuses' classification into acidemic or non-acidemic: 7.05, 7.10, 7.15, and 7.20. The area under the receiver operating characteristic curve (AUROC) was used to quantify the frequency bands' ability to distinguish acidemic fetuses. Bands referring to low frequencies, mainly associated with neural sympathetic activity, were the best at detecting acidemic fetuses, with the more severe definition (pH ≤ 7.05) attaining the highest values for the AUROC. This study shows that the power spectrum analysis of the fetal heart rate is a simple and powerful tool that may become an adjunctive method to CTG, helping healthcare professionals to accurately identify fetuses at risk of intrapartum hypoxia and to implement timely obstetrical interventions to reduce the incidence of related adverse perinatal outcomes.

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.

Gender-specific evolution of fetal heart rate variability throughout gestation: A study of 8823 cases

BACKGROUND

Fetal heart rate (FHR) variability throughout gestation reflects aspects of fetal development, and its analysis has been used for the assessment of fetal well-being.

AIM

The objective of this study was to provide a gender-specific analysis of the evolution of FHR variability indices throughout gestation, using linear time-domain, spectral and nonlinear FHR indices.

STUDY DESIGN

A large retrospective study was carried out using antepartum FHR recordings obtained from 4713 male and 4110 female fetuses, with normal pregnancy outcome, between 2004 and 2013, with gestational ages ranging between 25 and 40weeks.

OUTCOME MEASURES

FHR variability was analysed through linear time-domain methods, as well as using spectral analysis and entropy indices. Evolution of FHR indices throughout gestation was analysed through Spearman correlation coefficient. Comparison between male and female fetuses was performed using nonparametric bootstrap 95% confidence intervals for the median.

RESULTS

Mean FHR decreased significantly throughout gestation, whereas most variability indices increased. Sympatho-vagal balance measured by spectral analysis exhibited two local maxima at 29-30 and 34-35weeks and decreased afterwards. Entropy indices increased until around the 34th week, slightly decreasing after the 37th week. Female fetuses presented higher mean FHR and entropy from the 34th week afterwards, and lower short-term variability and sympatho-vagal balance in the same period.

CONCLUSIONS

Spectral and entropy analysis should be considered as a complement to conventional FHR variability analysis, aiming at a better characterization and follow-up of fetal development/maturation throughout gestation. Additionally, gestational age needs to be considered when defining reference ranges for FHR indices in systems of computerized analysis.

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.

Fetal heart rate monitoring: from Doppler to computerized analysis

The monitoring of fetal heart rate (FHR) status is an important method to check well-being of the baby during labor. Since the electronic FHR monitoring was introduced 40 years ago, it has been expected to be an innovative screening test to detect fetuses who are becoming hypoxic and who may benefit from cesarean delivery or operative vaginal delivery. However, several randomized controlled trials have failed to prove that electronic FHR monitoring had any benefit of reducing the perinatal mortality and morbidity. Also it is now clear that the FHR monitoring had high intra- and interobserver disagreements and increased the rate of cesarean delivery. Despite such limitations, the FHR monitoring is still one of the most important obstetric procedures in clinical practice, and the cardiotocogram is the most-used equipment. To supplement cardiotocogram, new methods of computerized FHR analysis and electrocardiogram have been developed, and several clinical researches have been currently performed. Computerized equipment makes us to analyze beat-to-beat variability and short term heart rate patterns. Furthermore, researches about multiparameters of FHR variability will be ongoing.

Software for computerised analysis of cardiotocographic traces

Despite the widespread use of cardiotocography in foetal monitoring, the evaluation of foetal status suffers from a considerable inter and intra-observer variability. In order to overcome the main limitations of visual cardiotocographic assessment, computerised methods to analyse cardiotocographic recordings have been recently developed. In this study, a new software for automated analysis of foetal heart rate is presented. It allows an automatic procedure for measuring the most relevant parameters derivable from cardiotocographic traces. Simulated and real cardiotocographic traces were analysed to test software reliability. In artificial traces, we simulated a set number of events (accelerations, decelerations and contractions) to be recognised. In the case of real signals, instead, results of the computerised analysis were compared with the visual assessment performed by 18 expert clinicians and three performance indexes were computed to gain information about performances of the proposed software. The software showed preliminary performance we judged satisfactory in that the results matched completely the requirements, as proved by tests on artificial signals in which all simulated events were detected from the software. Performance indexes computed in comparison with obstetricians' evaluations are, on the contrary, not so satisfactory; in fact they led to obtain the following values of the statistical parameters: sensitivity equal to 93%, positive predictive value equal to 82% and accuracy equal to 77%. Very probably this arises from the high variability of trace annotation carried out by clinicians.

Fractal Analysis and Hurst Parameter for Intrapartum Fetal Heart Rate Variability Analysis: A Versatile Alternative to Frequency Bands and LF/HF Ratio

Background

The fetal heart rate (FHR) is commonly monitored during labor to detect early fetal acidosis. FHR variability is traditionally investigated using Fourier transform, often with adult predefined frequency band powers and the corresponding LF/HF ratio. However, fetal conditions differ from adults and modify spectrum repartition along frequencies.

Aims

This study questions the arbitrariness definition and relevance of the frequency band splitting procedure, and thus of the calculation of the underlying LF/HF ratio, as efficient tools for characterizing intrapartum FHR variability.

Study Design

The last 30 minutes before delivery of the intrapartum FHR were analyzed.

Subjects

Case-control study. A total of 45 singletons divided into two groups based on umbilical cord arterial pH: the Index group with pH ≤ 7.05 (n = 15) and Control group with pH > 7.05 (n = 30).

Outcome Measures

Frequency band-based LF/HF ratio and Hurst parameter.

Results

This study shows that the intrapartum FHR is characterized by fractal temporal dynamics and promotes the Hurst parameter as a potential marker of fetal acidosis. This parameter preserves the intuition of a power frequency balance, while avoiding the frequency band splitting procedure and thus the arbitrary choice of a frequency separating bands. The study also shows that extending the frequency range covered by the adult-based bands to higher and lower frequencies permits the Hurst parameter to achieve better performance for identifying fetal acidosis.

Conclusions

The Hurst parameter provides a robust and versatile tool for quantifying FHR variability, yields better acidosis detection performance compared to the LF/HF ratio, and avoids arbitrariness in spectral band splitting and definitions.

Changes in spectral power of fetal heart rate variability in small-for-gestational-age fetuses are associated with fetal sex

BACKGROUND

Little is known about the influences of fetal weight and sex on spectral analysis of fetal heart rate (FHR) variability.

AIM

The study aims to assess whether there are differences in spectral power of FHR variability according to fetal weight and sex during labor.

STUDY DESIGN

Case-control study. A total of 414 singleton term deliveries without fetal acidemia were divided into small-for-gestational-age (SGA) (n=29) and non-SGA (n=385) groups. Analyses were performed separately according to fetal sex.

SUBJECTS

FHR recordings obtained with cardiotocography during the last 2h of labor preceding delivery.

OUTCOME MEASURES

Our outcome measures include spectral power of FHR variability.

RESULTS

For the male group, SGA fetuses had significantly lower values for low, movement, high, and total frequencies of spectral power compared with non-SGA fetuses (all P<0.005). Normalized low frequency (LFn) was significantly higher, and normalized high frequency (HFn) was significantly lower in SGA fetuses compared with non-SGA fetuses (all P<0.005). In contrast, for the female group, there were no significant differences in any of the indices of spectral power between the SGA and non-SGA fetuses. In addition, SGA males had significantly higher LFn spectral power and lower HFn spectral power compared to SGA females (P=0.016, and 0.041, respectively).

CONCLUSIONS

SGA males have decreased spectral power of FHR variability compared with non-SGA males during labor. However, there are no differences between SGA and non-SGA female fetuses. It is important in the clinical setting to take fetal weight and sex into account during FHR monitoring using spectral analysis.

Simulation of foetal phonocardiographic recordings for testing of FHR extraction algorithms

A valuable alternative to traditional diagnostic tools, such as ultrasonographic cardiotocography, to monitor general foetal well-being by means of foetal heart rate analysis is foetal phonocardiography, a passive and low cost recording of foetal heart sounds. In this paper, it is presented a simulator software of foetal phonocardiographic signals relative to different foetal states and recording conditions (for example different kinds and levels of noise). Before developing the software, a data collection pilot study was conducted with the purpose of specifically identifying the characteristics of the waveforms of the foetal and maternal heart sounds, since the available literature is not rigorous in this area. The developed software, due to the possibility to simulate different physiological and pathological foetal conditions and recording situations simply modifying some system parameters, can be useful as a teaching tool for demonstration to medical students and others and also for testing and assessment of foetal heart rate extraction algorithms from foetal phonocardiographic (fPCG) recordings. On this purpose, the simulator software was used to test an algorithm developed by the authors for foetal heart rate extraction considering different foetal heart rate parameters and signal to noise ratio values. Our tests demonstrated that simulated fPCG signals are very close to real fPCG recordings.

Specific change in spectral power of fetal heart rate variability related to fetal acidemia during labor: comparison between preterm and term fetuses

BACKGROUND

Spectral analysis of fetal heart rate (FHR) variability is a useful method to assess fetal condition. There have been several studies involving the change in spectral power related to fetal acidemia, but the results have been inconsistent.

AIMS

To determine the change in spectral power related to fetal umbilical arterial pH at birth, dividing cases into preterm (31-36 weeks) and term (≥37 weeks) gestations.

STUDY DESIGN

Case-control study. The 514 cases of deliveries were divided into a low-pH group (an umbilical arterial pH <7.2) and a control group (pH≥7.2).

SUBJECTS

FHR recorded on cardiotocography during the last 2h of labor.

OUTCOME MEASURES

The spectral powers in various bands of FHR variability.

RESULTS

In preterm fetuses, the total, low (LF), and movement (MF) frequency spectral powers and LF/HF ratio were significantly lower in the low-pH group than the control group (all P<0.05). In contrast, in term fetuses, the total frequency, LF, and MF powers were significantly higher in the low-pH group than the control group (all P<0.05). The area under the receiver operating characteristic of LF power to detect a low pH at birth was 0.794 in preterm fetuses and 0.595 in term fetuses. The specificity was 86.8% and 93.3% in preterm and term fetuses, respectively.

CONCLUSIONS

The changes in spectral power responding to a low pH are different between term and preterm fetuses. Spectral analysis of FHR variability may be useful fetal monitoring for early detection of fetal acidemia.

Analysis of heart rate variability in a rat model of induced pulmonary hypertension

Monocrotaline (MCT) is commonly used to experimentally induce pulmonary hypertension (PH), which might lead to chronic heart failure. In this study, linear and non-linear heart rate (HR) dynamics were weekly assessed in MCT-treated and non-treated Wistar rats. The HR of 10 adult Wistar rats injected with MCT (MCT group) and of 10 similar rats injected with vehicle (non-MCT group), anesthetized with Ketamine, was weekly recorded during 4 weeks. The first four segments of 1-min length of each HR recording were analysed using linear, time and frequency domains, and approximate (ApEn) and sample (SampEn) entropy indices, considering recently proposed values for the threshold parameter of ApEn and SampEn. Statistical analysis was performed using 95% confidence intervals and statistical tests. Along the study period, an overall weekly maintenance of HR indices, or a decrease, namely in weeks 1-2, was manifest, in the MCT group, except for LF and LF/HF, in week 1, denoting a short-term increase in sympathetic activity without any other changes. On the other hand, a maintenance of HR indices, or an increase, namely on week 4, was observed in the non-MCT group, except for LF/HF, denoting a long-term increase of the overall activity of HR control systems, with a parasympathetic like dominance. Studies on long-term HR dynamics should be performed in very carefully controlled experimental settings, as significant weekly changes may occur, both among anesthetized MCT-treated and non-treated rats.

A longitudinal description of heart rate variability in 28--34-week-old preterm infants

The purpose of this study was to longitudinally describe changes in heart rate variability (HRV) from 28 to 34 weeks postmenstrual age (PMA). A convenience sample of 31 low-risk preterm infants participated. HRV was quantified using a spectral analysis of heart periods and recorded during seven weekly test sessions from an electrocardiogram (ECG) signal. The total range of frequency components (0.04-2.0 Hz), high-frequency (HF) components (0.30-1.3 Hz), and ratio of low-to-high frequency (LF/HF) components (0.04-0.20/ 0.30-1.3 Hz) were measured. A mixed general linear model analysis revealed no significant change over weekly test sessions for the total, the high, and the ratio of LF/HF components. A significant interaction effect was, however, noted in the HF components for test session x gender (df = 1; F = 4.85; p = .030). With increasing age, the HF components for females increased or displayed a pattern of HRV indicative of a more mature autonomic nervous system (ANS). Study findings warrant further investigation of the impact of gender on normative descriptions of HRV.

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.

Heart rate variability analysis allows early asphyxia detection in ovine fetus

Fetal heart rate (FHR) monitoring is commonly used to predict asphyxia but clinical and experimental studies have questioned its diagnostic value. We examined the usefulness of fetal heart rate variability (fHRV) measures in detecting early asphyxia using chronically instrumented fetal sheep under normoxic (n = 6) and asphyxic conditions (3 umbilical cord occlusions, n = 6). The occlusions consistently led to pH decreases from 7.35 +/- 0.01 to 7.09 +/- 0.03 ( P < .05). FHR showed biphasic deceleration during each occlusion, associated with increasing arterial blood pressure ( P < .05). RMSSD, an index of vagal modulation of fHRV, increased consistently during repeated occlusion induced FHR decelerations ( P < .05). Under normoxic conditions, RMSSD did not change during FHR decelerations and decreased during FHR accelerations ( P < .05). Our results suggest that an increase of RMSSD in association with FHR decelerations reflects initial vagal activation during fetal asphyxia. RMSSD may accurately identify asphyxic fetuses early. Clinical validation is needed.

Linear and nonlinear analysis of heart rate patterns associated with fetal behavioral states in the antepartum period

BACKGROUND

Fetal behavioral states are important indicators of fetal physiology and pathology associated to typical fetal heart rate (FHR) patterns.

AIM

To provide linear and nonlinear analysis of FHR patterns associated with fetal behavioral states regarding a better understanding of these states and patterns.

STUDY DESIGN AND SUBJECTS

Fifty FHR tracings from normal term pregnancies with a median duration of 40.3 min were acquired with the SisPorto 2.01 system for computerized analysis of cardiotocograms. Each tracing was divided into consecutive 10-minute segments and each segment was classified by two experts as pattern A, B, C or D.

OUTCOME MEASURES

Linear and nonlinear indices were computed in each segment, namely mean FHR, long-term irregularity index (LTI), very low (VLF), low (LF) and high (HF) frequency spectral indices, approximate entropy (ApEn) and sample entropy (SampEn). Kappa statistic (kappa) and proportions of agreement (Pa) were used for assessment of inter-observer agreement. Bootstrap percentile confidence intervals and nonparametric statistical tests were calculated for statistical inference.

RESULTS

Overall agreement between experts in pattern classification was good to excellent with values for kappa and Pa of 0.74 (95% CI: 0.64-0.94) and 0.94 (95% CI: 0.92-0.96), respectively. Most linear domain indices increased significantly with rising fetal activity whereas the opposite occurred with nonlinear indices, except for SampEn(2, 0.1). LF/(MF+HF) ratio also significantly increased with fetal activity, denoting an increased sympatho-vagal balance.

CONCLUSIONS

Results support the hypothesis that entropy and linear variability indices measure different FHR features. FHR patterns associated with active sleep (B) and active wakefulness (D) evidenced more signs of autonomous nervous system activity, with sympatho-vagal imbalance, and less signs related to complexity or irregularity control systems than patterns associated with calm sleep (A) and calm wakefulness (C).

Change in heart rate variability in relation to a significant ST-event associates with newborn metabolic acidosis

OBJECTIVE

To find whether low-to-high frequency (LF/HF) ratio of fetal heart rate (FHR) variability changes in relation to a significant ST-event during delivery, and if the change is predictive of metabolic acidosis of the newborn.

DESIGN

A case-control study.

SETTING

Data from a multicentre project.

SUBJECTS

Acidotic and control fetuses with abnormal cardiotocography together with a ST-event in fetal electrocardiogram (ECG).

METHODS

We studied intrapartum FHR variability with spectral analysis from 34 fetuses with a significant ST-event in the fetal ECG. LF/HF ratio of FHR variability was measured within a period of 1 hour before and 1 hour after a significant ST-event. Sensitivity and specificity of the change in LF/HF ratio of FHR variability in prediction of metabolic acidosis (pH < or = 7.05 and base deficit value > 12.0 mmol/l) of the newborn were described by means of the receiver operating characteristic curve.

MAIN OUTCOME MEASURES

Change in LF/HF ratio of FHR in relation to a significant ST-event.

RESULTS

We found that a relative change in LF/HF ratio greater than 30% in relation to a significant ST-event predicted cord arterial metabolic acidosis with a sensitivity of 89% (95% CI 68-100%) and specificity of 80% (95% CI 64-96%).

CONCLUSIONS

Relative changes in LF/HF ratio of FHR variability in relation to a significant ST-event are more pronounced in fetuses born with metabolic acidosis.

Linear and nonlinear fetal heart rate analysis of normal and acidemic fetuses in the minutes preceding delivery

Linear and nonlinear fetal heart rate (FHR) indices, namely mean FHR, interval index (II), very low, low and high frequencies, approximate (ApEn) and sample entropy (SampEn), were computed, immediately before delivery, in the initial and final FHR tracing segments, from 48 normal, 10 mildly acidemic and 10 moderate-to-severely acidemic fetuses. Progression of labor was associated with a significant increase in linear frequency domain indices whereas nonlinear indices were significantly decreased. Moderate-to-severe fetal acidemia was associated with a significant decrease in nonlinear indices. The best discrimination between moderate-to-severe acidemic fetuses and the remaining cases was obtained combining II and ApEn(2,0.15), with a specificity of 71% and a sensitivity of 80%. These findings support the hypothesis of increased autonomic nervous system activity in the final minutes of labor and of decreased central nervous system activity, both in the final minutes of labor and in moderate-to-severe acidemic fetuses.

An algorithm for the recovery of fetal heart rate series from CTG data

Cardiotocography (simultaneous recording of fetal heart rate (FHR) and uterine contractions) is one of the most used diagnostic techniques to evaluate fetal well-being and to investigate the functional state of the fetal autonomic nervous system. Recently, great interest has been paid to the variability of the FHR, and its frequency analysis, as a base for a more objective analysis of the cardiotocographic (CTG) tracings. FHR signals are unevenly sampled series. To obtain evenly sampled series, cardiotocographs often use zero-order interpolation. Such process is simple and fast but results unsuitable for frequency analyses because it introduces alterations in the FHR power spectrum. An algorithm for the recovery of the true FHR series out of the zero-order interpolated CTG data was developed and evaluated.

Foetal heart rate power spectrum response to uterine contraction

Cardiotocography is the most diffused prenatal diagnostic technique in clinical routine. The simultaneous recording of foetal heart rate (FHR) and uterine contractions (UC) provides useful information about foetal well-being during pregnancy and labour. However, foetal electronic monitoring interpretation still lacks reproducibility and objectivity. New methods of interpretation and new parameters can further support physicians' decisions. Besides common time-domain analysis, study of the variability of FHR can potentially reveal autonomic nervous system activity of the foetus. In particular, it is clinically relevant to investigate foetal reactions to UC to diagnose foetal distress early. Uterine contraction being a strong stimulus for the foetus and its autonomic nervous system, it is worth exploring the FHR variability response. This study aims to analyse modifications of the power spectrum of FHR variability corresponding to UC. Cardiotocographic signal tracts corresponding to 127 UC relative to 30 healthy foetuses were analysed. Results mainly show a general, statistically significant (t test, p<0.01) power increase of the FHR variability in the LF 0.03-0.2 Hz and HF 0.2-1 in correspondence of the contraction with respect to a reference tract set before contraction onset. Time evolution of the power within these bands was computed by means of time-varying spectral estimation to concisely show the FHR response along a uterine contraction. A synchronised grand average of these responses was also computed to verify repeatability, using the contraction apex as time reference. Such modifications of the foetal HRV that follow a contraction can be a sign of ANS reaction and, therefore, additional, objective information about foetal reactivity during labour.

Internal versus external intrapartum foetal heart rate monitoring: the effect on linear and nonlinear parameters

The effect of foetal heart rate (FHR) acquisition mode on linear and nonlinear parameters is still largely unknown. In 33 normal labouring women, FHR signals were acquired simultaneously by an external ultrasound sensor applied to the maternal abdomen and an internal scalp electrode, in the minutes preceding delivery. For each case, the initial and final 5, 10 and 20 min segments were analysed, considering FHR signals at a frequency of 4 Hz (the frequency at which they are transmitted by the majority of commercialized foetal monitors). Several time and frequency domain linear and nonlinear FHR indices were computed in these segments, namely mean FHR, very low frequency (VLF), low frequency (LF), high frequency (HF), approximate entropy (ApEn) and sample entropy (SampEn). Parametric confidence intervals, statistical tests and correlation coefficients were calculated in order to evaluate the effect of internal versus external FHR monitoring modes on the considered indices. The whole evaluation was repeated using FHR signals at a frequency of 2 Hz. Most time domain linear indices were similar with external and internal monitoring in the initial and final segments of the tracings. However, linear frequency domain indices were poorly correlated in the final segments and had significantly different mean values in the initial segments. Nonlinear indices were significantly different in both initial and final segments. The correlation between 4 and 2 Hz sampled parameters was high for both linear and nonlinear indices (most correlation coefficient values ranging between 0.95 and 1) but nonlinear index values were significantly higher at 2 Hz. In conclusion, the mode used to acquire FHR signals and the sampling rate employed can significantly affect most FHR indices.

Antepartum cardiotocography: a study of fetal reactivity in frequency domain

Cardiotocography (CTG) is the most widely used diagnostic technique in clinical practice to monitor fetal health. Cardiotocographic recording also permits to assess maturation of the fetal autonomous nervous system (ANS): fetal heart rate (FHR) modifications may reveal ANS' reactions to stimuli. To assess fetal reactivity, physicians evaluate specific clinical CTG parameters, generally, by means of visual inspection, thus depending on observer's expertise, with lack of reproducibility. Still nowadays, there is a very high intra- and inter-observer variation in the assessment of FHR patterns. More objective methods for CTG interpretation are of crucial importance. For adults, frequency analysis of heart rate variability (HRV) is a non-invasive and powerful method to investigate ANS activity. This frequency analysis can also be a valid support for a better knowledge of fetal ANS functional state and reactions. Indeed, fetal HRV is a good indicator of fetal well-being in non-stress conditions. Fetal reactivity is a very important CTG characteristic used to diagnose fetal distress, but its interpretation is still uncertain. The aim of this study is to characterise fetal reactivity proposing new fetal HRV frequency parameters to support a more exhaustive CTG analysis.

Marked fetal acidosis and specific changes in power spectrum analysis of fetal heart rate variability recorded during the last hour of labour

OBJECTIVE

To assess whether intrapartum acidosis affects specific components of fetal heart rate variability.

DESIGN

Prospective clinical study.

SETTING

Twelve Nordic delivery units.

SUBJECTS

Fetal heart rate variability was studied in 334 fetuses divided into two groups according to cord pH value: the acidotic group (cord arterial pH < 7.05 at birth, n= 15) and the control group (cord arterial pH > or =7.05 at birth, n= 319).

METHODS

In spectral analysis of fetal heart rate variability, frequencies were integrated over the total frequency band (0.04-1.0 Hz), low-frequency band (0.04-0.15 Hz) and high-frequency band (0.15-1.0 Hz). We also calculated the low-to-high frequency ratio.

MAIN OUTCOME MEASURES

The spectral bands of fetal heart rate variability were compared between the acidotic and control fetuses.

RESULTS

We found that during the last hour of monitoring, baseline fetal heart rate gradually decreased, whereas total, low-frequency and high-frequency fetal heart rate variability initially increased but then, near the delivery, decreased in the acidotic fetuses when compared with the controls. Low-to-high frequency ratio was greater in the acidotic group during the whole study period (P= 0.002). Cord artery pH was inversely associated with total fetal heart rate variability (P < 0.001), low-frequency fetal heart rate variability (P < 0.001) and low-to-high frequency ratio (P= 0.004).

CONCLUSIONS

Marked fetal acidosis was associated with frequency-specific changes in fetal heart rate variability as reflecting the compensation ability of autonomic nervous activation during the last hour of labour.

Diurnal changes in the power spectral characteristics of eye movements and heart rate variability in the human fetus at term

It is well known that 1/f characteristics in power spectral patterns exist in various biological factors including heart rate variability. In the present study, we tried to elucidate the diurnal variation in spectral properties of eye movement and heart rate variability in the human fetus at term, via continuous 24-h observation of both these parameters. Studied were five uncomplicated fetuses at term. We observed eye movement and fetal heart rate (FHR) with real-time ultrasound and Doppler cardiotocograph, respectively, and analyzed the diurnal change in spectral properties, using the maximum entropy method. In four of five cases, the slope values of power spectra for both eye movement frequency and FHR, ranging approximately between 0.5 and 1.8, indicated diurnal variation, where the slopes tended to have high values during the day and low values at night. These findings suggest that, in the human fetus at term, eye movement and FHR are under the control of a common central mechanism, and this center changes its complexity as seen through diurnal rhythm.

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