Trends in fetal monitoring through phonocardiography: Challenges and future directions
Introduction
Center for Disease Control and Prevention (CDC) estimates that more than one million fetal deaths occur in the United States per year [1]. Complications such as preterm delivery, hypoxia, intrauterine growth retardation or others not only lead to fetal distress and neonatal death but also can cause risks to maternal health. There is a lesser knowledge about the incidence, etiology and prevention strategies for these complications; therefore it is critical to monitor the status of both fetal and maternal health throughout pregnancy. Consequently, Electronic Fetal Monitoring (EFM) was introduced in 1960s as a valuable tool for diagnosing Fetal Heart Rate (FHR) during antepartum and intrapartum periods of pregnancy [2]. Today, EFM is used in 90% of the labor diagnosis procedures in the United States [3] and includes Electrocardiography (ECG), Phonocardiography (PCG), Pulse Oximetry, Magnetocardiogram (MCG) and Tocodynamometer. Organizations such as the International Federation of Gynecology and Obstetrics (FIGO), the American College of Obstetricians and Gynecologists (ACOG), the National Institute of Child Health and Human Development (NICHHD), the Royal College of Obstetricians and Gynecologists (RCOG), and the National Institute of Clinical Excellence (NICE) have standardized the use of EFM in conjunction with Maternal Uterine Contractions (MUC) known as Cardiotocography (CTG) to optimize the outcomes for the mother and the new born infant [4], [5].
Fetal Phonocardiography (FPCG) was discovered by the interventions of Marsac, Kergardec and Kennedy during the 17th century [6], [7]. Although FPCG was discovered many years ago, interest in this research has only occurred over the last few years. Fig. 1 displays the number of peer reviewed articles published in the Institute of Electrical and Electronics Engineers (IEEE), the Science Direct, and the National Institute of Health (PubMed) databases. Currently, the application of FPCG is limited to FHR analysis and is seen as a noninvasive means for data acquisition; it is only used as a secondary diagnosis tool in the antepartum, and has never been utilized for complete clinical diagnosis. There are few reasons as to why FPCG is not clinically accepted for a complete diagnosis: First, the FPCG is very noisy, owing to the fact that the acquired signal is a mixture of acoustic and pressure components from the fetus, the mother and other noise sources; Second, the characteristics of the aforementioned components are highly dependent on the location of data acquisition, gestational age, fetal and maternal positions which result in the non-stationarity; finally, the non-linear transmission medium dynamically morphs all the components to result in a narrow band signal.
Today’s standard of care in fetal monitoring suspects that the fetal heart rate is predictive of pregnancy complications [8]. As a consequence, EFM relies predominantly on FHR and does not incorporate the characteristics of the FPCG waveform in the assessment of fetal and maternal outcomes. The primary reason for the exclusion of this information from clinical practice is that the technology to measure the Fetal Heart Sounds (FHS) reliably is not yet available. Secondly, the existing signal processing techniques are unable to deliver a FHS signal from the acquired FPCG signal without considerable distortion.
The rest of this paper is organized as follows: Section 2, a description of existing standards of fetal monitoring; Section 3 an overview about morphology of FPCG; Section 4, a comprehensive description about all the fundamental acoustic and pressure components of a FPCG signal; Section 5, information about trends in data collection and databases; Section 6, a survey of the FPCG based signal processing techniques; Section 7, mathematical models of the FPCG signal; Section 8 summarizes the existing challenges and provides potential directions for future research.
Section snippets
Standards of fetal monitoring
Optimizing and improving the fetal and maternal outcomes during pregnancy, labor and delivery is the main objective of fetal monitoring. Existing standards of fetal monitoring assess the wellbeing of the fetus and the mother by performing various tests at different stages of pregnancy and labor. Table 1 provides a summary of all the essential parameters acquired using electronic instrumentation, the current gold standard and the history of FPCG use within the literature throughout pregnancy.
Morphology of fetal phonocardiogram
Acquisition, analysis, and processing of FHS from the maternal body is known as fetal phonocardiography. The signal acquired at the transducer is a superimposition of various time-varying acoustic and pressure components. Essentially, fetal components are contributed by fetal heart sounds, fetal respiration, and fetal movements, and maternal components are contributed by maternal heart sounds, maternal movement, maternal organ sounds and ambient vibrations propagating through the maternal body.
Fetal heart sounds (FHS)
Heart sounds are generated by the displacement and reshaping of the heart muscles, vibrations of the myocardium, the opening/closing of valve leaflets and flow of blood, which occur during a cardiac cycle [43]. Four heart sounds are generated in adults, however in the fetus, the third and fourth heart sounds are practically undetectable [60].
The first heart sound, S1, is a result of vibratory components created by the asynchronous closure of mitral and tricuspid valves during isovolumic
Data collection and databases
Collection of high quality FPCG data is possible only when the impact of data acquisition approach and data acquisition system are considered. The data acquisition approach defines the type of data being acquired and the location of the data acquisition depending on the term of pregnancy. On the other hand data acquisition system configures the hardware that is used for the data acquisition. Fig. 5 depicts a generic block diagram representation of the FPCG data acquisition, signal analysis and
State of the art in FPCG analysis
The literature in the fetal data analysis is classified into different groups based on the signal processing and classification techniques.
Modeling the FPCG
Modeling the individual components of a FPCG signal is a rather complex task. The signal is made up of many time varying acoustic and pressure components. The components exhibit temporal overlap and similar spectral distribution. The knowledge about the characteristics of all the individual components that make up the FPCG and the components themselves remain completely unexplored yet. Finally, the envelope, the shape and the quality of a FPCG is highly dependent on the type and location of
Existing challenges and potential directions for future research
From the survey, it was observed that various challenges related to FPCG, particularly, data acquisition approach, data acquisition systems, available databases, signal analysis, processing and modeling have to be addressed based on further evaluation. The objective is to retrieve all the components of the FPCG signal with highest possible fidelity as required for morphological studies. However, the fidelity is highly affected by various limiting factors. Based on these limiting factors,
Conclusion
The current survey provides an overview of the existing standards in fetal monitoring. It was observed that fetal phonocardiography provides superior information for fetal monitoring in comparison to other methodologies. Further, a comprehensive discussion about morphology and diagnostic potential of FPCG is presented. With respect to FPCG data collection, current trends have been reported and challenges have been identified, specifically with regards to data acquisition approaches and data
Conflict of interest
The authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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