T. Kupka

Determination of fetal heart rate from abdominal signals: evaluation of beat-to-beat accuracy in relation to the direct fetal electrocardiogram

Abstract The main aim of our work was to assess the reliability of indirect abdominal electrocardiography as an alternative to the commonly used Doppler ultrasound monitoring technique. As a reference method, we used direct fetal electrocardiography. Direct and abdominal signals were acquired simultaneously, using dedicated instrumentation. The developed method of maternal signal suppression as well as fetal QRS complexes detection was presented. Recordings were collected during established labors, each consisted of four signals from the maternal abdomen and the reference signal acquired directly from the fetal head. After assessing the performance of the QRS detector, the accuracy of fetal heart rate measurement was evaluated. Additionally, to reduce the influence of inaccurately detected R-waves, some validation rules were proposed. The obtained results revealed that the indirect method is able to provide an accuracy sufficient for a reliable assessment of fetal heart rate variability. However, the method is very sensitive to recording conditions, influencing the quality of signals. Our investigations confirmed that abdominal electrocardiography, even in its current stage of development, offers an accuracy equal to or higher than an ultrasound method, at the same time providing some additional features.

Towards noise immune detection of fetal QRS complexes

The noninvasive fetal electrocardiography is a source of more precise information on the fetal heart activity than the measurements based on Doppler ultrasound signals. However, the clinical diagnostic applications of this technique are limited by difficulty with successful detection of small amplitude fetal QRS complexes. In this study, we investigate the influence of different stages of fetal signals processing on the detection performance. The main propositions of the paper are: application of normalized matched filtering to fetal QRS complexes enhancement and a new approach to the final detection of the complexes. Compared to the classical detectors, the proposed new one allows a significant increase of the detection performance for signals of very different quality.

The influence of coincidence of fetal and maternal QRS complexes on fetal heart rate reliability

Bioelectrical fetal heart activity being recorded from maternal abdominal surface contains more information than mechanical heart activity measurement based on the Doppler ultrasound signals. However, it requires extraction of fetal electrocardiogram from abdominal signals where the maternal electrocardiogram is dominant. The simplest technique for maternal component suppression is a blanking procedure, which relies upon the replacement of maternal QRS complexes by isoline values. Although, in case of coincidence of fetal and maternal QRS complexes, it causes a loss of information on fetal heart activity. Its influence on determination of fetal heart rate and the variability analysis depends on the sensitivity of the heart-beat detector used. The sensitivity is defined as an ability to detect the incomplete fetal QRS complex. The aim of this work was to evaluate the influence of the maternal electrocardiogram suppression method used on the reliability of FHR signal being calculated.

Timing events in Doppler ultrasound signal of fetal heart activity

Among various methods of monitoring fetal heart activity a Doppler ultrasound technique is the most often used. Complexity and variability of Doppler signal make difficult the precise measurement of timing dependences defining individual phases of cardiac cycle. Aim of the work was to carry out detailed comparative analysis of Doppler echo coming from movement of two different objects within fetal heart: valve and wall. Joint time frequency analysis were applied. Fetal monitor performed a role of input device in our measurement station based on LabView environment. Doppler signal was acquired from analog outputs with a help of dedicated data acquisition card. Average recording time in a group of 15 patients was 20 minutes. Analysis comprised determination and comparison of spectrograms and power density spectrums corresponding to individual phases of cardiac cycle.

Extraction of Fetal Heart-Rate Signal as the Time Event Series From Evenly Sampled Data Acquired Using Doppler Ultrasound Technique

Analysis of variability of fetal heart rate (FHR) is very important in prediction of the fetal wellbeing. The beat-to-beat variability is described quantitatively by the indices originated from invasive fetal electrocardiography which provides the FHR signal in a form of time event series. Today, monitoring instrumentation is based on Doppler ultrasound technology. We used two bedside fetal monitors with different processing methods for heartbeat detection and FHR signal determination: the autocorrelation and cross-correlation techniques. Both monitors provide the output signal in a form of evenly spaced samples. The goal of this paper is to present a new method for the FHR signal processing, which enables extraction of series of consecutive heartbeat intervals from the sampled signal. The proposed correction algorithms allow recognition and removal of the FHR signal distortions typical for fetal monitors-invalid and duplicated samples. The correction efficiency has been verified based on the FHR variability indices calculated for the sampled signal and the corresponding event series. For both monitors, considerable influence of the signal representation on indices values was noted. Concluding, we recommended implementing these algorithms in fetal surveillance system as a preprocessing stage for the determination of FHR variability indices.

Detection of low amplitude fetal QRS complexes

The significance of the most important operations performed by the noninvasive systems for fetal heart rate determination is investigated. The method of template subtraction for maternal ECG suppression is compared to the method based on independent component analysis. The QRS detector based on the classical slope-responsive preprocessing competes with the one that employs normalized matched filtering for QRS enhancement. A small database containing the four-channel abdominal ECG signals with the marked positions of the fetal QRS complexes is prepared to enable the investigations. The performed experiments show the factors that have the greatest impact on the results of the fetal QRS detection, and an effective approach to cope the problem is proposed.

Instrumentation for Fetal Cardiac Performance Analysis During the Antepartum Period

Cardiotocography as a simultaneous recording of fetal heart rate (FHR) and uterine contraction activity is a basic method for evaluation of fetal condition. Continuous variability of the fetal heart rate is an indirect sign of adequate oxygenation of a fetus. Unfortunately, the reverse case is not always true, signs suggesting pathological changes can also appear in recording when the fetal is not at risk. The cardiotocography shall then be recognized as a more screening than diagnostic method. It will be interesting to develop a noninvasive method being complementary to routine cardiotocography. This method should allow the adequate prediction of a bad clinical outcome when the test is abnormal. The paper presents the system that makes possible cardiotocograms analysis in parallel with the assessment of additional parameters determined from comparison of mechanical and electrical fetal heart activity signals. The studies are aimed at development of set of parameters that are high correlated with fetal outcome

Evaluation of Fetal Heart Rate Baseline Estimation Method Using Testing Signals Based on a Statistical Model

Computer-aided fetal monitoring is based on automated analysis of the fetal heart rate (FHR) variability. The first and the main step in the automated signal interpretation is the estimation of the so called FHR baseline. There are various algorithms for baseline estimation, of different efficiency. For its evaluation, the method of modeling of FHR signal based on the preset baseline component has been developed. The best algorithm is expected to provide the same baseline as the component baseline used to model the FHR signal. Generated signals were used to compare the baselines that have been estimated by two algorithms: the first one relying on artificial neural networks and the classical one using nonlinear filtering of FHR signal

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.

Classification of Uterine Electrical Activity Patterns for Early Detection of Preterm Birth

Preterm birth is the leading cause of a neonatal death, so it is extremely important to distinguish the pregnancy at risk of preterm threatening labour. Monitoring an electrical activity of the uterine muscle seems very promising as a method which enables noninvasive recording of good quality electrohysterographic signals. The developed instrumentation enabled recording of signals by means of electrodes attached to abdominal wall and determination of quantitative parameters describing the contractions detected. Our research material comprised 3 groups of patients: with physiological pregnancy, with the symptoms of premature threatening labour and patients during at term labour. The classification of uterine activity signals in each pair of groups was made using nonlinear Lagrangian Support Vector Machines which allows for improving the computational efficiency and learning quality of the SVM algorithm. The obtained results show that the proposed approach is able to differentiate between the contractile activity in physiological pregnancy and that connected with a risk of premature labour. Identification of these high-risk pregnancies leads to an enhanced perinatal surveillance.