Fetal phonocardiography—Past and future possibilities

Abstract

The paper presents an overview of the 15 year long development of fetal phonocardiography including the works on the applied signal processing methods for identification of sound components. Based on the improvements achieved on this field, the paper shows that beyond the traditional CTG test the phonocardiography may be successfully applied for long-term fetal measurements and home monitoring. In addition, by indication of heart murmurs based on a comprehensive analysis of the recorded heart sound congenital heart defects can also be detected together with additional features in the third trimester. This makes an early widespread screening possible combined with the prescribed CTG test even at home using a telemedicine system.

Introduction

Fetal examinations are an important field of healthcare, but due to the hidden position of the fetus, there appear special measuring problems. The ultrasound echocardiography is the most informative non-invasive method providing reliable data about the morphology of the heart with its deviations, furthermore, about the flowing velocity of the blood. The disadvantage of this examination is that the instrument is expensive and a well skilled expertise is needed for obtaining and evaluating the data.

A more simple measurement is the cardiotocography (CTG) which focuses on measuring the fetal heart rate (FHR) and on simultaneous indication of the womb contractions. This is the traditional test in the third trimester by which some basic fetal parameters may be obtained signalizing the actual status of the fetus. These parameters are derived from a 20-min record producing the FHR-diagram (Fig. 1). One of the main features are the visually well observable accelerations of the heart rate. A further characteristic feature of the diagram is the fluctuation from which the variability is calculated, which is related to the instantaneous oxygen delivery through the umbilical cord. The third parameter is the main value (baseline) of the heart rate calculated as the average value but ignoring the outstanding sections (such as the accelerations) of the diagram. The decrease of the FHR (deceleration) indicates an abnormal situation of the pregnancy, especially when it is followed by a womb contraction in a given delay [1].

In the everyday use, and mostly in normal gestations, the evaluation of the FHR diagram happens visually by the obstetrician observing the baseline, the number of accelerations and the variability. At more intensive examinations, especially in case of risky pregnancies further parameters will also be derived such as the indices calculated from the time function of the beat-to-beat time (T_bb) variations, from the frequency spectrum of the FHR, etc. Obviously, this second type of evaluation requires much more accurate measurement of the heart rate than the first one.

There are four methods to carry out CTG measurements, namely ultrasound Doppler method, electrocardiography (ECG), magnetocardiography (MCG) and phonocardiography (PCG) [2].

Nowadays the widely used noninvasive method for CTG is the ultrasound Doppler CTG. The measurement is based on the Doppler principle detecting the movement of the heart wall by the frequency change of the reflected ultrasound beam. Though the placing of the test head on the maternal abdomen is not critical, it takes into account only the movement of the outer surface of the fetal heart. Consequently, it does not provide any information about the inner part of the heart, such as the real operation of the valves, possible turbulences, etc. Furthermore, if there is any reflection from the movement of the valves then it causes an error leading to inaccurate FHR and variability values. Obtained T_bb values were compared with the R–R times measured simultaneously with electrocardiography and more than 4 ms difference in the standard deviation has been found [3].

CTG measurements can also be made with electrocardiography, but here the access to the fetus is problematic. Measured on the maternal abdomen the fetal signal has a much lower intensity than the maternal one, requiring difficult mathematical processes to separate them. Using this intensive processing of the recorded signal, additional, important features of the cardiac activity can be obtained. The placement of the electrodes practically impedes its use as home monitor. However, its passive nature enables very long measurements, which was utilized for studying the FHR variability [4].

Magnetocardiography can also be used for noninvasive CTG measurements. The problems with this method are very similar to the ECG one, though the obtained R–R time intervals and the corresponding FHR values are very accurate and suitable for deriving further parameters. As an example, the changes in the low frequency bands of the spectral density of the R–R interval function with gestational age were examined in order to find relations to the development of the fetus [5].

The three formerly mentioned methods for CTG measurements have some limitations, namely they cannot be used

• for long-term measurements,

• for widespread screening of pregnant women, and

• they do not have the capability to evaluate some further features, e.g. murmur.

Referring to the long-term measurements it should be noted that the 20-min measurement is rather short to get a true information about the fetal status. In addition, the obtained values may be influenced by the actual state of the mother herself, too. Because of the ultrasound irradiation on the intrauterine environment the long-term ultrasound-based CTG measurement, including its home monitoring application, is not preferred. The long-term application of ECG and MCG is safe and suitable, the only obstacle is the more complex measuring scenario.

The ability of a method for widespread screening is a key factor. The prevalence of congenital heart diseases (CHD) are estimated to be 8/1000 live births. An equal number are though to die in-utero or abort spontaneously. This is an important aspect of widening the possibility of techniques for fetal heart examinations in order to minimize the number of cases requiring urgent treatments after delivery. Therefore, screening is of crucial importance for obstetrics and there is an increasing demand for assessment of abnormalities of the fetal heart during routine screening. In fact, screening of high-risk population is worldwide more or less solved by examining the pregnancies with chromosomal defect of parents or second-degree relatives, maternal diseases, positive family history of CHD, documented fetal anomaly, twin gestation or advanced maternal age [6].

In contrast to the high-risk cases, the screening of low-risk population is problematic because moderate symptoms of the anomaly do not appear noticeably. Additionally, more than 85% of fetuses with CHD will not be detected and classified as high-risk population and, consequently, will not be examined with echocardiography. A further problem of screening is that its verification by follow-up observation of the disease is generally difficult.

Taking into account the factors mentioned above one can see that echocardiography itself is not a suitable tool for prenatal screening of low-risk population, but is unconditionally necessary for the detailed examination of the patients found somehow positive by the screening.

The capability for discovering additional features with Doppler CTG is limited to the well known investigations based on the time- and frequency-analysis of the FHR diagram. These are arrhythmias, the intrauterine growth restriction (IUGR) and the neural development. However, there is no additional information about the signal waveforms which limits the analysis. A further problem is that all these analyses use the poorly accurate T_bb values of the Doppler method. The ECG and MCG methods are much more reliable concerning the timings and the signal shapes, which is a highly researched field nowadays.

Some problems listed above can be solved by fetal phonocardiography (fPCG). It appeared during the last ten years in the course of high-volume CTG measurements that the PCG method is capable to detect some additional features of the fetal heart as well, such as the split valve sound and the murmur due to the turbulent blood flow, which may be used for indications of some CHDs. The aim of the present overview is to highlight how fetal phonocardiography as the Renaissance of classical auscultation can form the future.