Critical analysis of electrohysterographic methods for continuous monitoring of intrauterine pressure.

Monitoring the progression of uterine activity provides important prognostic information during pregnancy and delivery. Currently, uterine activity monitoring relies on direct or indirect mechanical measurements of intrauterine pressure (IUP), which are unsuitable for continuous long-term observation. The electrohysterogram (EHG) provides a non-invasive alternative to the existing methods and is suitable for long-term ambulatory use. Several published state-of-the-art methods for EHG-based IUP estimation are here discussed, analyzed, optimized, and compared. By means of parameter space exploration, key parameters of the methods are evaluated for their relevance and optimal values. We have optimized all methods towards higher IUP estimation accuracy and lower computational complexity. Their accuracy was compared with the gold standard accuracy of internally measured IUP. Their computational complexity was compared based on the required number of multiplications per second (MPS). Significant reductions in computational complexity have been obtained for all published algorithms, while improving IUP estimation accuracy. A correlation coefficient of 0.72 can be obtained using fewer than 120 MPS. We conclude that long-term ambulatory monitoring of uterine activity is possible using EHG-based methods. Furthermore, the choice of a base method for IUP estimation is less important than the correct selection of electrode positions, filter parameters, and postprocessing methods. The presented review of state-of-the-art methods and applied optimizations show that long-term ambulatory IUP monitoring is feasible using EHG measurements.

Blind Source Separation for Clutter and Noise Suppression in Ultrasound Imaging: Review for Different Applications.

Blind source separation (BSS) refers to a number of signal processing techniques that decompose a signal into several "source" signals. In recent years, BSS is increasingly employed for the suppression of clutter and noise in ultrasonic imaging. In particular, its ability to separate sources based on measures of independence rather than their temporal or spatial frequency content makes BSS a powerful filtering tool for data in which the desired and undesired signals overlap in the spectral domain. The purpose of this work was to review the existing BSS methods and their potential in ultrasound imaging. Furthermore, we tested and compared the effectiveness of these techniques in the field of contrast-ultrasound super-resolution, contrast quantification, and speckle tracking. For all applications, this was done in silico, in vitro, and in vivo. We found that the critical step in BSS filtering is the identification of components containing the desired signal and highlighted the value of a priori domain knowledge to define effective criteria for signal component selection.

Visual inspection of transvaginal ultrasound videos to characterize uterine peristalsis: an inter-observer agreement study.

PURPOSE: Contractions in non-pregnant uterine can be assessed by visual inspection of transvaginal ultrasound (TVUS). Many authors have used this method to extract features like contraction frequency and direction. However, visual inspection is a subjective method and the outcome is dependent on the sonographers and video analysts. In this study, we wanted to see which uterine feature is reproducible enough, in terms of inter-observer agreement, to serve as a reliable control for future research. METHODS: Six observers assessed 80 TVUS videos, and rated video quality, contraction frequency, direction and timing. One observer assessed operating time. A Fleiss' kappa (κ) or an intra-class correlation (ICC) was calculated to determine the inter-observer agreement of all features. RESULTS: The inter-observer agreement in frequency was substantial (ICC = 0.68). Conversely, there was just slight to fair agreement in contraction timing and direction and in video quality: ICC = 0.26, κ = 0.17 and κ = 0.16, respectively. Overall, agreement among technical engineers was better than between medical professionals. The level of agreement was correlated with video quality, phase of the menstrual cycle and individual patient (all χ2 with p < 0.00). The time to analyze one video ranged between 6 and 20 min. CONCLUSIONS: This study shows that visual inspection of TVUS videos is a fairly reproducible method to assess contraction frequency. However, the operating time is too extensive to implement this method in daily practice. Automated methods could offer a solution for this problem in the future.

Investigation of The Neural Drive During Vibration Exercise by High-density Surface-electromyography.

Mechanical vibration applied directly to the muscle belly or tendon has been reported to elicit a specific reflex loop named tonic vibration reflex (TVR), which involves motor unit (MU) activation synchronized and un-synchronized within the vibration cycle. Indirect application of vibration to the muscle by vibration exercise (VE) has also been suggested to evoke TVR, as evidenced by the spectral peaks observed at the vibration frequency in the surface electromyography (sEMG). However, other studies interpreted these spectral peaks as the result of motion artifacts (MAs). The aim of the present study is, therefore, to investigate MU activation patterns during VE in order to clarify the nature of those spectral peaks. To this end, low-intensity isometric contractions were executed with and without VE, and high-density sEMG measurements were performed during the contraction tasks. MU action potential (MUAP) trains were extracted by decomposing the recorded high-density sEMG signals. The spectra of the MUAP trains were then calculated and compared between vibration and no-vibration conditions. Clear MU synchronization was observed during VE, confirming the spectral peaks at the vibration frequency to be mainly due to the reflex loop rather than MAs.

Effects of vibration-induced fatigue on the H-reflex.

Vibration exercise (VE) has been suggested as an effective training for improving muscle strength and coordination. However, the underlying physiological adaptation processes are not yet fully understood, limiting the development of safe and effective exercise protocols. To better understand the neuromuscular responses elicited by VE, we aimed at investigating the acute effects of superimposed vibration on the Hoffmann reflex (H-reflex), measured after fatiguing exercise. Twenty-five volunteers performed four isometric contractions of the right Flexor Carpi Radialis (FCR) with baseline load at 80% of their maximal voluntary contraction (MVC), both with no vibration and with superimposed vibration at 15, 30, and 45 Hz. Fatigue was estimated by MVC test and estimation of electromyographic spectral compression. H-reflex suppression was estimated as the relative decrease after exercise. Our results show that fatiguing exercise determined a decrease in H-reflex amplitude compared to rest condition while vibration determined a lower H-reflex suppression as compared to no vibration. The superimposition of 30-Hz vibration determined the largest acute reduction in force generating capacity (36 N, p < 0.05) and the lowest H-reflex suppression (20%, p < 0.05). These results suggest VE to be particularly suitable in rehabilitation programs for rapid restoration of muscle form and function after immobilization periods.

Towards Real-Time Estimation of Muscle-Fiber Conduction Velocity Using Delay-Locked Loop.

Decrease in muscle-fiber conduction velocity (MFCV) during sustained contraction has been widely accepted as myoelectric manifestation of muscle fatigue. Several methods have been proposed in the literature for MFCV estimation by analysing surface electromyography (EMG), e.g., cross-correlation (CC) function and maximum likelihood (ML). However, for all the availablemethods, windowing of the EMG signal and computationally demanding calculations are required, limiting the possibility to continuously monitor muscle fatigue in real time. In the present study, an adaptive scheme is proposed that permits real-time estimation of MFCV. The proposed scheme is based on a delay-lockedloop (DLL). Asecond-orderloop is adopted to track the delay variationover time. An error filter is employed to approximate a ML estimation in case of colored noise. Furthermore, the DLL system is extended for multichannel CV estimation. The performance of the proposed method is evaluated by both dedicated simulations and real EMG signals. Our results show the accuracy of the proposed method to be comparable to that of theML method formuch lower (1/40) computational complexity, especially suited for real-time MFCV measurements. Use of this method can enable new studies onmyoelectric fatigue, possibly leading to new insight on the underlying physiological processes.

Feasibility Study of a New Method for Low-Complexity Fetal Movement Detection From Abdominal ECG Recordings.

Fetal movement counting can provide valuable information on the fetal health, as a strong decrease in the number of movements can be seen as a precursor to fetal death. Typically, assessment of fetal health by fetal movement counting relies on the maternal perception of fetal activity. The percentage of detected movements is strongly subject dependent and with undivided attention of the mother varies between 37% and 88%. Various methods to assist in fetal movement detection exist based on a wide spectrum of measurement techniques. However, these are unsuitable for ambulatory or long-term observation. In this paper, a novel low-complexity method for fetal movement detection is presented based on amplitude and shape changes in the abdominally recorded fetal ECG. This method was compared to a state-of-the-art method from the literature. Using ultrasound-based movement annotations as ground truth, the presented method outperforms the state-of-the-art abdominal-ECG based method, with a sensitivity, specificity, and accuracy of 56%, 68%, and 63%, respectively. Additionally, a significant reduction in algorithm complexity is achieved, possibly enabling continuous ambulatory fetal movement detection and early detection of reduced fetal motility.

Electrohysterographic detection of uterine contractions in term pregnancy.

Uterine-contraction detection is a fundamental component of pregnancy monitoring. Electrohysterography (EHG) provides a non-invasive and accurate alternative to intrauterine pressure (IUP) measurements, and several techniques provide an estimated IUP (eIUP) based on the EHG alone. Commonly, EHG contraction detection is based on amplitude thresholding of the eIUP. We aim at improving the reliability of contraction detection, such that automatic contraction detection can be realized. An algorithm for template-matching of the eIUP signal is proposed. This method is based on Bayesian evidence using a Gaussian likelihood function to classify uterine activity. Gaussian templates are matched to the input signal, with weights obtained empirically from manually-annotated contraction events in a training data-set. The results show an improvement in contraction detection accuracy compared to threshold-based methods. The template-matching method is adaptable to relevant features in the input training data, and is thus less sensitive to differences in eIUP derivation or measurement variability. The method allows for improved automatic uterine contraction detection in labor EHG data, while being extensible to e.g. preterm contraction detection.

Propagation of electrical activity in uterine muscle during pregnancy: a review.

The uterine muscle (the myometrium) plays its most evident role during pregnancy, when quiescence is required for adequate nourishment and development of the foetus, and during labour, when forceful contractions are needed to expel the foetus and the other products of conception. The myometrium is composed of smooth muscle cells. Contraction is initiated by the spontaneous generation of electrical activity at the cell level in the form of action potentials. The mechanisms underlying uterine quiescence during pregnancy and electrical activation during labour remain largely unknown; as a consequence, the clinical management of preterm contractions during pregnancy and inefficient uterine contractility during labour remains suboptimal. In an effort to improve clinical management of uterine contractions, research has focused on understanding the propagation properties of the electrical activity of the uterus. Different perspectives have been undertaken, from animal and in vitro experiments up to clinical studies and dedicated methods for non-invasive parameter estimation. A comparison of the results is not straightforward due to the wide range of different approaches reported in the literature. However, previous studies unanimously reveal a unique complexity as compared to other organs in the pattern of uterine electrical activity propagation, which necessarily needs to be taken into consideration for future studies to be conclusive. The aim of this review is to structure current variegated knowledge on the properties of the uterus in terms of pacemaker position, pattern, direction and speed of the electrical activity during pregnancy and labour.

Influence of electrode placement on signal quality for ambulatory pregnancy monitoring.

Noninvasive fetal health monitoring during pregnancy has become increasingly important in order to prevent complications, such as fetal hypoxia and preterm labor. With recent advances in signal processing technology using abdominal electrocardiogram (ECG) recordings, ambulatory fetal monitoring throughout pregnancy is now an important step closer to becoming feasible. The large number of electrodes required in current noise-robust solutions, however, leads to high power consumption and reduced patient comfort. In this paper, requirements for reliable fetal monitoring using a minimal number of electrodes are determined based on simulations and measurement results. To this end, a dipole-based model is proposed to simulate different electrode positions based on standard recordings. Results show a significant influence of bipolar lead orientation on maternal and fetal ECG measurement quality, as well as a significant influence of interelectrode distance for all signals of interest.

Fetal movement detection based on QRS amplitude variations in abdominal ECG recordings.

Evaluation of fetal motility can give insight in fetal health, as a strong decrease can be seen as a precursor to fetal death. Typically, the assessment of fetal health by fetal movement detection relies on the maternal perception of fetal activity. The percentage of detected movements is strongly subject dependent and with undivided attention of the mother varies between 37% to 88%. Various methods to assist in fetal movement detection exist based on a wide spectrum of measurement techniques. However, these are typically unsuitable for ambulatory or long-term observation. In this paper, a novel method for fetal motion detection is presented based on amplitude and shape changes in the abdominally recorded fetal ECG. The proposed method has a sensitivity and specificity of 0.67 and 0.90, respectively, outperforming alternative fetal ECG-based methods from the literature.

Low-complexity intrauterine pressure monitoring by Teager energy estimation.

Monitoring the progression of maternal uterine activity provides important prognostic information during pregnancy and parturition. Currently used methods, however, are unsuitable for long-term observation of uterine activity. The abdominally measured electrohysterogram (EHG) provides a non-invasive alternative to the existing methods for long-term ambulatory uterine contraction monitoring. A new EHG signal analysis method for intrauterine pressure (IUP) estimation based on the Teager energy estimate is proposed. The new method is compared to existing methods from the literature in terms of estimation accuracy and computational complexity. An accurate IUP estimate, with a complexity up to 40 times lower than that of algorithms from the literature is obtained. Therefore, the proposed method offers a valuable new option for long-term uterine monitoring.

Characterization of a novel instrument for vibration exercise.

Vibration exercise (VE) has been suggested as an effective option to improve muscle strength and power performance. Several studies link the effects of vibration training to enhanced neuromuscular stimulation and typically to involuntary reflex mechanisms. However, the underlying mechanisms are still unclear and information for the most appropriate vibration training protocols is limited. This study proposes to realize a new vibration exercise system for the biceps brachii. Amplitude, frequency, and baseline of the vibrating load, which is generated by an electromechanical actuator, can be adjusted dynamically by a feedback control loop. A second-order model is employed to identify the relation between the mechanical load and the input voltage driving the actuator. An adaptive normalized least mean square algorithm is proposed to remove the motion artifacts from the measured electromyography (EMG) data. Our results show a high correlation (0.99) between the second-order model fit and the measured data, permitting accurate control on the supplied load for vibrations up to 80 Hz. Furthermore, preliminary validation with 4 volunteers showed an excellent performance in the motion artifact removal, enabling reliable evaluation of the neuromuscular activation.

Analysis of muscle fatigue induced by isometric vibration exercise at varying frequencies.

An increase in neuromuscular activity, measured by electromyography (EMG), is usually observed during vibration exercise. The underlying mechanisms are however unclear, limiting the possibilities to introduce and exploit vibration training in rehabilitation programs. In this study, a new training device is used to perform vibration exercise at varying frequency and force, therefore enabling the analysis of the relationship between vibration frequency and muscle fatigue. Fatigue is estimated by maximum voluntary contraction measurement, as well as by EMG mean-frequency and conduction-velocity analysis. Seven volunteers performed five isometric contractions of the biceps brachii with a load consisting of a baseline of 80% of their maximum voluntary contraction (MVC), with no vibration and with a superimposed 20, 30, 40, and 50 Hz vibrational force of 40 N. Myoelectric and mechanical fatigue were estimated by EMG analysis and by assessment of the MVC decay, respectively. A dedicated motion artifact canceler, making use of accelerometry, is proposed to enable accurate EMG analysis. Use of this canceler leads to better interpolation of myoelectric fatigue trends and to better correlation between mechanical and myoelectric fatigue. In general, our results suggest vibration at 30 Hz to be the most fatiguing exercise. These results contribute to the analysis of vibration exercise and motivate further research aiming at improved training protocols.

Electromyographic assessment of muscle fatigue during isometric vibration training at varying frequencies.

Resistance exercise is essential to improve or maintain muscle performance. Vibration training has been suggested as an alternative option for muscle conditioning, aiming especially at improving muscle strength and power. Several studies link the effects of vibration training to enhanced neuromuscular stimulation, measured by electromyography (EMG) and typically ascribed to involuntary reflex mechanisms. However, the underlying mechanisms are still unclear, limiting the use of vibration training. This paper proposes additional methods to analyze the mechanisms involved in vibration training. A dedicated measurement setup was realized to relate vibration parameters to muscle fatigue in the biceps brachii. Fatigue is estimated by EMG mean frequency and conduction velocity assessments as well as by maximum voluntary contraction (MVC) force measurements. A modified maximum likelihood algorithm is proposed for the conduction velocity estimation based on high-density EMG recording. Five volunteers performed four isometric contractions of 50 s at 80% MVC with no vibration (control) and with superimposed vibration at 20, 30, and 40 Hz. Fatigue was estimated from the decay of force, EMG mean frequency, and EMG conduction velocity. 30-Hz vibrations represented the most fatiguing stimulus. Our preliminary results also show a better correlation between force and conduction velocity decay than between force and mean frequency decay, indicating the former as a better EMG indicator of fatigue. The proposed methods provide important advancements for the analysis of vibration exercise and guidance towards the definition of optimal training protocols.

Nifedipine-induced changes in the electrohysterogram of preterm contractions: feasibility in clinical practice.

Objective. Evaluating changes in the power spectral density (PSD) peak frequency of the electrohysterogram (EHG) caused by nifedipine in women with preterm contractions. Methods. Calculation of the PSD peak frequency in EHG contraction bursts at different times of nifedipine treatment in women in gestational age 24 to 32 weeks with contractions. Results. A significant (P < .05) decrease of PSD peak frequency between EHG signals measured before and 15 minutes after administration of nifedipine. A significant (P < .05) decrease in PSD peak frequency comparing signals recorded within 24 hours after administration of nifedipine to signals 1 day after tocolytic treatment. A higher average PSD peak frequency for patients delivering within 1 week than that for patients delivering after 1 week from nifedipine treatment (P > .05). Conclusions. EHG signal analysis has great potential for quantitative monitoring of uterine contractions. Treatment with nifedipine leads to a shift to lower PSD peak frequency in the EHG signal.

Myometrium electromechanical modeling for internal uterine pressure estimation by electrohysterography.

During delivery, quantitative information on the uterine activity can be provided by internal uterine pressure (IUP) recordings using an invasive intrauterine pressure catheter (IUPC). The electrohysterogram, which measures the electrical signal that drives the mechanical contraction of the uterine muscle and the consequent IUP increase, is recorded by electrodes placed on the abdomen. Recent work demonstrated the possibility of reliably estimating the IUP noninvasively by electrohysterographic (EHG) signal analysis. To further improve the accuracy of IUP estimates, we investigated the use of three nonlinear functions for modeling the relationship between the electrical activation measured by the EHG signal and the mechanical response of the uterine muscle. The feature employed for obtaining a first estimate of the IUP is the unnormalized first statistical moment of the EHG spectrum. The relationship between the extracted feature and the IUP is modeled by a second-order polynomial, a logarithmic, and an exponential function. For validation, the IUPC and the EHG signals were recorded on nine women in labor. A second-order polynomial model already provided estimates that are highly correlated with the IUPC signal (r = 0.73). However, the logarithmic model resulted to be the most accurate, especially in terms of root mean squared error (RMSE = 5.13 mmHg).

Electrohysterographic conduction velocity estimation.

Monitoring and analysis of the fetal-heart and the uterine-muscle activity, referred to as electrohysterogram (EHG), is essential to permit timely treatment during pregnancy. While remarkable progress is reported for monitoring of the fetal cardiac activity, the EHG measurement and interpretation remains challenging, and limited knowledge is available on the underlying physiological processes. In particular, little attention has been paid to the analysis of the EHG propagation, whose characteristics might indicate the presence of coordinated uterine contractions leading to intrauterine pressure increase. Therefore, this study focuses for the first time on the noninvasive estimation of the conduction velocity of EHG action potentials by means of multichannel EHG recording and surface high-density electrodes. A maximum likelihood algorithm, initially proposed for skeletal-muscle electromyog-raphy, is modified for the required EHG analysis. The use of clustering and weighting is introduced to deal with poor signal similarity between different channels. The presented methods were evaluated by specific simulations, proving the combination of weighting and clustering to be the most accurate method. A preliminary EHG measurement during labor confirmed the feasibility of the method. An extensive clinical validation will however be necessary to optimize the method and assess the relevance of the EHG conduction velocity for pregnancy monitoring.

Relationship between electrohysterogram and internal uterine pressure: a preliminary study.

Electrohysterography (EHG) is a promising technique for monitoring the uterine activity, based on electrical recordings on the abdominal surface. However, a quantitative estimation of the internal uterine pressure (IUP) by means of EHG is not available for clinical practice. In this paper we present a preliminary study on the estimation of the mechanical uterine activity from abdominal EHG measurements. For the EHG analysis we use two different Time Frequency Distributions (TFD): the spectrogram and the Wigner-Ville Distribution. We assume the EHG to be the sum of frequency modulated signals. Based on this assumption, the IUP is estimated from the unnormalized first moment of the TFD. Eventually, a third order polynomial model is applied to the estimated IUP in order to improve the estimate accuracy. A recent method for detection of uterine contraction by EHG is employed for comparison of the performance. The algorithms were tested on two patient recordings. The results were compared with a reference IUP which is simultaneously measured by an intrauterine catheter. The IUP estimated by our method showed a correlation coefficient with the reference IUP (R=0.93) higher than that achieved by the comparison method (R=0.85). Therefore the proposed method may be considered as a promising clinical technique for accurate non invasive IUP measurements.