Nemo Healthcare

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Science

Adequate fetal monitoring is essential in the optimal management of care for both expectant mother and her fetus during pregnancy and labour.

Ideally, the technology used for fetal monitoring should be reliable, safe and operate without any discomfort to mother and child. Unfortunately, few of the existing fetal monitoring technologies meet all of these requirements.

Conventional monitoring technology


In almost all hospitals in developed countries, fetal monitoring is based on the cardiotocogram (CTG), which is the simultaneous registration of fetal heart rate (FHR) and uterine activity. The conventional CTG technologies can be divided into two categories: invasive and non-invasive.

The invasive technologies are a fetal scalp electrode and an intrauterine pressure catheter, whereas the non-invasive ones are Doppler ultrasound and tocodynamometry.

The invasive technologies typically provide reliable information on FHR and/or uterine activity, but their use is hampered by their invasiveness; they impose risks and can only be applied after the rupturing of the membranes and with sufficient dilatation of the cervix. The non-invasive technologies are safe and can also be applied earlier in pregnancy.

However, the changing position of the pregnant woman, movement of the fetus, and muscle activity hamper the obtaining of good quality signals. In addition, in patients with a high body mass index, the performance of these technologies degrades [1, 2].
 

Even when the CTG is obtained in a safe and reliable way, interpretation of a non-reassuring CTG by healthcare professionals is challenging, inter- and intra-observer variability is high [3], and the positive predictive value for poor fetal outcome is low [4].

In other words, in many situations a non-reassuring CTG does not represent fetal compromise. In the event of an inconclusive diagnosis, obstetricians have to rely on additional methods, such as fetal blood sampling or ST-waveform analysis. Both of these additional methods are invasive and carry the risk of complications.

Non-invasive electrophysiology


An alternative approach to obtain information on both the fetus and the uterus is non-invasive electrophysiology. The fetal heart and uterus both consist mainly of muscles. Muscles contract under the influence of an electrical stimulus (i.e. action potential) that propagates over the muscle. This propagating electrical stimulus induces an electrical field that can be measured with surface electrodes on the skin.

By placing electrodes on a pregnant abdomen, the electrical fields on the underlying fetal heart and uterine muscle can be measured. Unfortunately, these same electrodes also measure the electrical fields from other sources, such as the maternal heart, abdominal muscles (especially active while pushing in the second stage of labour), and even the ubiquitous electricity grid.

The figure below illustrates a typical non-invasive electrophysiological recording on a pregnant woman. The main challenge for using non-invasive electrophysiology as a means of fetal monitoring is to separate the fetal heart and uterine muscle from all other interfering electrical sources. In this process, electrophysiological information from the uterus is referred to as electrohysterography and fetal electrocardiography is used to describe the electrophysiology of the muscle of the fetal heart.

 

Electrohysterography


Electrohysterography (EHG) is a promising non-invasive technique which measures the uterine electrical currents through contact electrodes on the maternal abdomen [5]. The use of EHG for monitoring uterine activity has been demonstrated in ample studies [6-8], showing a good correlation with intrauterine pressure. However, to convert the EHG signal so that it resembles the intrauterine pressure graphs, signal processing methods are required.

In most studies, these signal processing methods can only be applied offline (i.e. data is first collected from a patient and processed afterwards to provide a CTG) or with significant delay between input and output. In CTG interpretation, the FHR is evaluated in relation to uterine activity, which means that the EHG must be processed in real time, without significant delay.

Nemo Healthcare has developed novel signal processing methods to process EHG signals in real time and in such a way that they can be used for CTG interpretation. These methods were implemented in the discontinued PUREtrace product and are currently, with further improvements regarding functionality, implemented in the Nemo Fetal Monitoring System.

Furthermore, these methods have been extensively evaluated to assess performance in comparison to external tocodynamometry especially as a function of maternal BMI. These evaluations show that, when compared to intrauterine pressure as the gold standard, the Nemo Healthcare method has a sensitivity of 89.5% versus 65.3% in a group of 48 women in labour [1]. Here, sensitivity is defined as the number of true positive contractions detected by the EHG-based/tocodynamometry method divided by the number of contractions detected by a simultaneously applied intra-uterine pressure catheter. Contractions were labelled as true positive when their peak was within 30s of the peak in the intrauterine pressure signal.

For the EHG-based method, the sensitivity was not affected by BMI: for the 33 non-obese women (i.e. BMI <30 kg/m2 before pregnancy), the sensitivity was 90.0% and for the 15 obese women (BMI ≥30 kg/m2), the sensitivity was 88.4%. For external tocodynamometry, these sensitivities were 73.0% and 45.8%, respectively, showing a significant drop in performance for high BMI [1]. Other studies have also reported that the EHG is superior to the tocodynamometer [9-12].

In the figure below, an example recording from the evaluation study is shown. From top to bottom, the graphs in this figure represent: FHR, intrauterine pressure, EHG, tocodynamometry. In this figure, the uterine contractions for EHG are much more visible than those for tocodynamometry. It should be noted here that these graphs are displayed at 2 cm per minute.

 

Fetal electrocardiography


Generally, the FHR is obtained non-invasively using Doppler ultrasound. In the event of poor signal quality during labour, obstetricians often switch to the invasive fetal scalp electrode, which provides a good quality signal based on the fetal electrocardiogram (FECG).

A non-invasive alternative is to measure the FECG non-invasively using electrodes placed on the maternal abdomen [13]. This FECG is the electrophysiological signal generated by the fetal heart during each cardiac contraction. This method has the potential to provide reliable FHR measurements and can also be applied antepartum. 

The low invasiveness of the abdominal FECG comes at the cost of a reduced signal-to-noise ratio (SNR) [14]. As mentioned above, the abdominal FECG is contaminated by electrical interferences such as the maternal ECG (MECG), muscle activity, power line interference, and measurement noise. Moreover, in the period between 28 to 32 weeks of gestation, an insulating layer (the vernix caseosa) coats the skin of the fetus and reduces the amplitude and affects the shape of the abdominal FECG [15].

In recent years, abdominal FECG recordings have been extensively studied, most studies focussing on suppression of the MECG, which is the dominant source of interference [13, 16-21]. A variety of algorithms has been presented for MECG suppression, such as template subtraction [13, 16, 17] adaptive filtering [18, 19], blind source separation (BSS) [20-22], or a combination of different algorithms [23-25]. For an extensive review, the reader is referred to [14] or [26].

To determine the FHR, one needs to detect the fetal QRS complexes. These QRS complexes represent the electrical activity of the cardiac muscles involved with contraction of the ventricles. However, even after MECG suppression, the SNR of the abdominal FECG is generally still too low for reliable fetal QRS detection. In addition to the low SNR, the position and orientation of the fetus within the abdomen are a priori unknown and can change during a recording. Therefore, the abdominal FECG is typically recorded using multiple electrodes spread across the abdomen [27]. The SNR and waveform of the FECG in each channel depend on the fetal position and orientation. Hence, fetal movement with respect to the abdominal electrodes can cause variations in the SNR and FECG waveform of a certain channel [28]. In short, the low SNR and the non-stationary nature of the abdominal FECG make fetal QRS detection challenging.

At Nemo Healthcare, we have developed proprietary methods for suppressing MECG and other interferences and detecting fetal QRS complexes to yield reliable FHR, even in challenging low-SNR and non-stationary conditions. These methods have been evaluated in a multi-centre study where the Nemo Fetal Monitoring System was used in a group of 110 patients, simultaneously with a fetal scalp electrode. This study (publication expected in 2019) showed that the Nemo Healthcare methods obtained a reliability of 86.8%, with the FHR from fetal scalp electrode as reference and an accuracy of -1.46 beats per minute (BPM). Here, reliability is the ratio of fetal heartbeats identified that fall within a 10% margin around the FHR obtained from the fetal scalp electrode. From the literature, it is known that the performance for Doppler ultrasound is much lower, with a reliability of 62-73%, where reliability is again defined as the relative number of heart beats that fall within a 10% margin around the FHR obtained from the fetal scalp electrode (which typically is a margin of about 14 BPM) [29, 30].

The figure below depicts a simultaneous registration of FHR from the Nemo Fetal Monitoring System and a fetal scalp electrode. Here, the blue line represents the FHR from the fetal scalp electrode and the red line the FHR from the Nemo Fetal Monitoring System.

 

 

Finally, because the signal processing methods that are used to suppress the MECG also determine maternal heart rate (MHR), not only can the MHR be displayed by the Nemo Fetal Monitoring System, the system can also prevent mix-up issues between the MHR and FHR, which can occur with Doppler Ultrasound-based FHR monitoring technology [31].

Further potential of electrophysiology

As previously mentioned, even in the event of a reliable CTG, correct interpretation of the fetal condition is not always possible without resorting to complementary diagnostic measures such as fetal blood sampling.

The electrophysiological measurements performed by the Nemo Fetal Monitoring System provide ample opportunities for developing other complementary diagnostic measures:

  • The electrode patch of the Nemo Fetal Monitoring System incorporates multiple electrodes. These electrodes theoretically allow measurement of the EHG conduction velocity, which has been reported to be related to the time-till-delivery. This might create opportunities for predicting preterm delivery or distinguishing Braxton-Hicks from labour contractions.
     
  • The Nemo Fetal Monitoring System provides more reliable FHR information than Doppler Ultrasound. A lot of ongoing research focuses on the quantitative analysis of FHR variability for predicting fetal hypoxia [32-34] or intrauterine growth restriction [35]. More reliable inputs could enable more reliable analysis.
     
  • Because the FHR by the Nemo Fetal Monitoring System is based on FECG, the FECG waveform could also be obtained and analysed. It is known that the fetal ST-segment changes under the influence of oxygen deficiency [36], so non-invasive analysis of the fetal ST-segment or any other FECG interval or segment might be possible.
     
  • Furthermore, the multi-channel FECG that could be provided by the Nemo Fetal Monitoring System could be used to screen for congenital heart diseases. An example of FECG waveforms obtained with the Nemo Fetal Monitoring System in a development setting are displayed in the figure below. Here, each of the FECGs is recorded with a different electrode, explaining for the different morphologies. In fact, these FECGs are recorded at two locations on the maternal abdomen for the same fetus at the same point in time.