Electrohysterogram Signals from Patients with Threatened Preterm

Labor: Concentric Ring Electrode Vs Disk Electrode Recordings

Javier Mas-Cabo

, Yiyao Ye-Lin

, Carlos Benalcazar-Parra

, José Alberola-Rubio

, Alfredo Perales

Javier Garcia-Casado

and Gema Prats-Boluda

Centro de Investigación e Innovación en Bioingeniería (Ci2B), Universitat Politècnica de València, Valencia, Spain

Servicio de Obstetricia, Hospital Universitari I Politècnic la Fe, Valencia, Spain

Keywords: Electrohysterogram, Preterm Labor, Concentric Ring Electrodes.

Abstract: Recording of electrohysterogram (EHG) has emerged as a non-invasive method for monitoring uterine

dynamics during pregnancy. Usually EHG is picked up using conventional disk electrodes placed on the

abdominal surface resulting in a limited spatial resolution due to the blurring effect of the volume conductor.

In this respect, concentric ring electrodes have been proposed to pick up uterine myoelectrical activity in term

patients so as to improve spatial resolution and to reduce physiological interferences embedded in these

records. The aim of the present work is to check the feasibility of recording EHG signals using concentric

ring electrodes (BC-EHG) in patients with threatened preterm labor and to compare their capability to

discriminate true preterm labor from false alarms with that of conventional EHG bipolar recording. For this

purpose, 50 sessions with simultaneous EHG recordings with conventional disk electrodes and concentric

ring electrodes were conducted in 26 patients. Compared to conventional bipolar EHG recording, the BC-

EHG presents smaller amplitude and similar spectral characteristics. Statistically significant differences

between women who delivered preterm and those that delivered at term were found for both the average peak-

peak amplitude and the dominant frequency in the frequency range 0.2-1 Hz from BC-EHG recordings.

Nonetheless no EHG parameter from simultaneous conventional bipolar recording showed statistically

significant differences. These results suggest superior performance of BC-EHG recordings in patients with

threatened preterm labor for discriminating true preterm labor from term labor.

1 INTRODUCTION

Preterm birth (<37 weeks of gestation, WG) is one of

the major cause of early neonatal mortality. The

incidence of preterm birth has risen over the last

decades, representing around 12% of all labors

nowadays (Beck, 2010). Preterm labor also

implicates around 85% of newborn deaths and 50%

of newborns’ neurological disorders (Beck, 2010).

Currently due to the lack of specific tools, the

diagnosis of preterm labor is one of the most complex

problems faced by clinicians.

The gold standard for monitoring uterine

dynamics is the use of an intra-uterine catheter, which

provides a quantitative measure of intra-uterine

pressure associated to the uterine contractions. It is an

invasive technique which requires ruptured

membranes and entails risks such as infection, and

thus it is unsuitable for predicting preterm labor

during pregnancy. In common practice, obstetricians

use tocodynamometry (TOCO) for assessing uterine

contractility and evaluating the risk of preterm labor.

This technique measures non-invasively the uterine

pressure transmitted to the abdominal surface.

However, it suffers from recurrent signal failure,

needing re-positioning by a midwife (Miles, 2001,

Schlembach, 2009), and may also fail in obese

patients (Euliano, 2013). Furthermore, TOCO is not

able to distinguish effective contractions which lead

to preterm delivery, from non-effective ones (

Schlembach, 2009, Garfield, 2007). Recording of

electrohysterom (EHG) has emerged as an alternative

technique for non-invasively monitoring uterine

contractility (Devedeux, 1993, Garfield, 2005,

Maner, 2007). The electrohysterogram corresponds

to the uterine myoelectric recordings and it is made

up of action potential bursts (EHG-burst), associated

to uterine contraction, and basal activity. EHG signals

Mas-Cabo J., Ye-Lin Y., Benalcazar-Parra C., Alberola-Rubio J., Perales A., Garcia-Casado J. and Prats-Boluda G.

Electrohysterogram Signals from Patients with Threatened Preterm Labor: Concentric Ring Electrode Vs Disk Electrode Recordings.

DOI: 10.5220/0006155000780083

In Proceedings of the 10th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2017), pages 78-83

ISBN: 978-989-758-212-7

present different characteristics along the gestation

and depending on the proximity of labor (Garfield,

2007, Marque, 2007), which could provide

information for estimating the time horizon of labor.

Some parameters extracted from EHG signals, such

as those related with conduction velocity or

propagation patterns, are very promising for the

prediction of term and preterm delivery (Lucovnik,

2011, Euliano, 2009, Rabotti, 2009, Rabotti, 2010).

Nevertheless, the estimation of conduction velocity

depends on the direction of propagation of

contraction in relation to the electrodes position

(Rabotti, 2011). Moreover, it can also be influenced

by the poor spatial resolution associated to

conventional disk electrodes due to the blurring effect

of the volume conductor (Ye-Lin, 2015).

In this sense, concentric ring electrodes (CRE)

have been proposed to improve the spatial resolution

achieved with conventional disk electrodes and to

reduce physiological interferences embedded in EHG

recordings (Alberola-Rubio, 2013). Specifically,

these electrodes have proven to pick up uterine

myoelectrical activity in patients at term (≥ 37 WG),

(Ye-Lin, 2015). Nonetheless, since the EHG-burst

amplitude of patients at earlier WG is expected to be

lower than those obtained from patients at term

(Devedeux, 1993, Garfield, 2005), the utility of CRE

for predicting preterm labor in patients with

threatened preterm labor still needs to be proven.

Therefore, the first aim of the present work was to

determine if concentric ring electrodes are able to

pick up the EHG in patients at early gestational age

(< 37 WG). Secondly, it was aimed to explore the

possibility to discriminate true preterm labor from

false alarms of patients with threatened preterm labor

based on BC-EHG and bipolar EHG records.

Specifically, in this first approach it was studied the

possible differences in EHG temporal and spectral

parameters of patients who end up delivering preterm

and those who did it at term.

2 MATERIALS AND METHODS

2.1 Data Acquisition

50 recording sessions were conducted in 26 patients

with singleton pregnancies at the Hospital

Universitario y Politécnico La Fe de Valencia. This

study adheres to the Declaration of Helsinki and was

approved hospitably the Institutional Review Board.

The patients were informed about the nature of the

study and the recording protocol and they signed an

informed consent form. The criteria for inclusion in

this study were: women with a gestational age

between 25 and 36 weeks with symptoms of preterm

labor, such as regular uterine contractions and/or

cervical effacement. Patients’ obstetric information

was collected, as shown in Table 1, and patients were

followed up to obtain the details of their final

delivery. It should be highlight that cervical length

shows a shortening as labor approaches, and higher

values of this parameter can be observed for the term

group in comparison to preterm labor group.

Term and preterm labor patients were grouped

into two subgroups depending on the gestational age

at recording moment: term [28, 31[ WG group (6

patients and 8 records), term [31, 34[ WG group (10

patients and 14 records), preterm [28, 31[ WG group

(4 patients and 5 records) and preterm [31, 34[ WG

group (6 patients and 23 records). Patients who

presented threatened preterm labor but did not initiate

labor spontaneously were excluded from the study.

Besides the EHG registers were performed under

common clinical practice conditions, this implies that

most patients were under tocolytic drugs effect. For

each recording session, the abdominal skin was

carefully prepared using an abrasive paste in order to

reduce skin-electrode impedance. Two disposable

Ag/AgCl electrodes (EL501, Biopac SystemsInc,

Santa Barbara, CA, USA) were placed

supraumbilically, symmetrically arranged with

respect to the uterine median axis being the inter-

electrode distance 8 cm (see Figure 1). Furthermore,

a custom-made concentric ring electrode was placed

on the uterine median axis in the subumbilical zone

(see Figure 1).

Table 1. Patients’ obstetrical information at the recording

moment.

Term labor Preterm labor

Gestations 1.59 ±1.10 1.50 ±1.10

Parity 0.23 ±0.43 0.23 ±0.43

Voluntary

Interruption of

Pregnancy

0.13 ±0.47 0.07 ±0.36

Abortions 0.22 ±0.68 0.20 ±0.76

Cesarean

Sections

0.09 ±0.29 0.00 ± 0.00

Maternal age

(years)

32.55 ± 5.70 30.47 ±5.16

Gestational Age

(weeks)

31.05 ± 2.54 31.87 ±1.55

Cervical length

(mm)

24.95 ± 10.17 15.37 ± 11.28

Electrohysterogram Signals from Patients with Threatened Preterm Labor: Concentric Ring Electrode Vs Disk Electrode Recordings

Figure 1: Scheme of the electrodes’ placement on a

woman’s abdomen (left) and CRE dimensions (right).

Taking into account that the EHG-burst amplitude

acquired using CRE with an outer ring of 36 mm was

about 42 V in patients at term (Ye-Lin, 2015), in this

work it has been designed a CRE with an outer ring

of 70 mm so as to facilitate picking up the uterine

electrical activity during early pregnancy whose

amplitude was expected to be smaller than that

associated to term patients. Furthermore, two

disposable Ag/AgCl electrodes were placed in each

hip, as reference and ground electrodes. In this work,

the bipolar recording estimated from the two

monopolar raw signals from conventional disk

electrodes and one bipolar concentric-EHG (BC-

EHG) signal obtained by means of CRE were

analysed (see Figure 1).

Bipolar = M1 – M2; BC-EHG = U2-U1; (1)

Where M1 and M2 are the biopotentials picked up

by conventional disk electrodes and U1 and U2 are

the biopotentials corresponding to the central disk and

external ring respectively. Two custom-made

amplifiers which provide a 2059 V/V gain in the

frequency band between 0.1 and 4 Hz were used for

signal conditioning (Alberola-Rubio, 2015).

The implementation details of the bioamplifier

were described in a previous work (Ye-Lin, 2016).

Signals were digitalized using a 24 bits ADC and

sampled at 20 Hz. TOCO signal was simultaneously

acquired by means of a Corometric 170 from GE

Medical Systems and the digitalized data were

transmitted to a PC with a sampling frequency of 4

Hz.

2.2 Data Analysis

Most studies in this field are focused on the analysis

of the EHG-burst (Garfield, 2005, Maner, 2007),

which usually requires manual segmentation of the

EHG recordings. This process depends on the

experts’ subjectivity and it is a time-consuming task

(Fele-Zorz, 2008). In this work, it was preferred to

analyse the whole EHG recording which greatly

simplifies the signal analysis process. Signal

segments corresponding to motion artefacts were

discarded from the analysis. Subsequently a set of

temporal and spectral parameters was calculated for

the EHG signals: average peak to peak amplitude,

mean frequency, dominant frequency in frequency

range 0.2-1 Hz (DF

) and in frequency range 0.34-1

Hz (DF

), normalized subband energies in frequency

ranges 0.2-0.34 Hz (NE1), 0.34-0.6 Hz (NE2) and

0.6-1 Hz (NE3) and H/L ratio, which is the ratio

between the energy in high frequency range (0.34-1

Hz) respect to the energy in low frequency range (0.2-

0.34 Hz) (Ye-Lin, 2015). Specifically, the peak-peak

amplitude was worked out over a moving window of

120s length and 50% overlapping, and then the

average value of all analysed windows was

calculated. Similarly, for the estimation of the

spectral parameters, the Welch periodogram was

worked out in moving analysis windows of 120 s with

50% overlapping.

Figure 2: Simultaneous recording of TOCO, bipolar, BC-EHG performed in two patients at 33 WG. Left: recordings from a

woman who delivered at term. Right: recordings from a woman who delivered preterm.

0 100 200 300 400 500 600 700

100

TOCO (mmHg)

Term Labor

0 100 200 300 400 500 600 700

-200

200

Bipolar (



0 100 200 300 400 500 600 700

-20

BC-EHG (



Time (s)

0 100 200 300 400 500 600 700

100

TOCO (mmHg)

Preterm Labor

0 100 200 300 400 500 600 700

-200

200

Bipolar (



0 100 200 300 400 500 600 700

-50

BC-EHG (



Time (s)

BIOSIGNALS 2017 - 10th International Conference on Bio-inspired Systems and Signal Processing

Table 2: EHG Parameters obtained from conventional bipolar recordings for both term labor and preterm labor groups,

separated into two sets depending on the gestational age at the recording session. ‘*’ indicates statistically significant

differences (p<0.05) between term labor and preterm labor groups for the same set of WG; and ‘’ indicates statistically

significant differences (p<0.05) between conventional bipolar record and BC-EHG (table 3) for each of the 4 subgroups.

Bipolar

Term labor Preterm labor

Parameter [28, 31[ WG [31, 34[ WG [28, 31[ WG [31, 34[ WG

App (µV) 98.85 ± 35.3 127.56 ± 31.33 157.18 ± 41.34 151.46 ± 77.92

MF (Hz) 0.366 ± 0.018 0.338 ± 0.021 0.381 ± 0.034 0.353 ± 0.025

DF1 (Hz) 0.237 ± 0.023 0.234 ± 0.021 0.229 ± 0.037 0.241 ± 0.035

DF2 (Hz) 0.367 ± 0.031 0.357 ± 0.012 0.355 ± 0.012 0.366 ± 0.024

NE1 0.56 ± 0.12 0.65 ± 0.11 0.53 ± 0.1 0.63 ± 0.08

NE2 0.35 ± 0.12 0.27 ± 0.08 0.36 ± 0.07 0.28 ± 0.06

NE3 0.09 ± 0.02 0.09 ± 0.07 0.11 ± 0.04 0.09 ± 0.03

H/L Ratio 0.85 ± 0.38 0.6 ± 0.35 0.93 ± 0.34 0.6 ± 0.19

Table 3: EHG Parameters obtained from BC-EHG recordings for both term labor and preterm labor group, separated into two

sets depending on the gestational week at the recording session. ‘*’ indicates statistically significant differences between term

and preterm labor groups for the same set of WG.

BC-EHG

Term labor Preterm labor

Parameter [28, 31[ WG [31, 34[ WG [28, 31[ WG [31, 34[ WG

App (µV) 52.72 ± 17.18 60.86 ± 37.96 97.68 ±18.49* 90.1 ± 46.16*

MF (Hz) 0.387 ± 0.017 0.358 ± 0.022 0.379 ± 0.025 0.365 ± 0.031

DF1 (Hz) 0.224 ± 0.029 0.212 ± 0.008 0.234 ± 0.028 0.237 ± 0.044*

DF2 (Hz) 0.378 ± 0.031 0.381 ± 0.026 0.391 ± 0.042 0.371 ± 0.026

0.54 ± 0.06 0.60 ± 0.08 0.53 ± 0.05 0.58 ± 0.09

0.33 ± 0.04 0.31 ± 0.08 0.35 ± 0.04 0.32 ± 0.07

0.13 ± 0.03 0.09 ± 0.03 0.11 ± 0.04 0.10 ± 0.04

H/L Ratio 0.87 ± 0.17 0.70 ± 0.24 0.89 ± 0.17 0.76 ± 0.3

Wilcoxon test was performed in order to

determine if there were statistically significant

differences between the EHG parameters derived

from preterm labor group and term labor group for

both [28, 31[ WG and [31, 34[ WG subgroups.

3 RESULTS

Figure 2 shows 750 s of simultaneous recordings of

TOCO, bipolar and BC-EHG signals performed in

two patients at 33 WG, one who delivered at term and

other who delivered preterm. Firstly, each uterine

contraction observed in the tocographic signal was

associated to an increase of signal amplitude in

bipolar recording for both patients. In the case of the

patient who delivered preterm, it can be clearly

identified the presence of uterine contractions in BC-

EHG signals. By contrast, for the patient who

delivered at term, only uterine contractions around

400 s and 700 s, which are of greater intensity, can be

observed in BC-EHG recording. The contractions

around 200 s and 600 s are hardly noticeable, being

imbedded in the basal activity. Secondly, the

amplitude of EHG signals acquired with CRE was

smaller than that from signals obtained using

conventional disk electrodes. Finally, it can also be

clearly observed that the amplitudes of the EHG-

bursts of the patient whose labor was preterm is

higher than those of the patient that delivered at term

(false alarm), even when both records were made at

the same WG.

Tables 2 and 3 show mean and standard deviation

of the EHG parameters calculated from Bipolar and

BC-EHG recordings respectively. Firstly, it can be

noticed that BC-EHG signals presented lower peak-

peak amplitude than conventional bipolar ones. In

fact, the amplitude is the only parameter that shows

statistically significant difference between BC-EHG

signals and bipolar signals (squares) for both [28, 31[

WG and [31, 34[ WG groups. Comparing EHG

parameters in the sets of same ranges of WG, the

average amplitude of preterm labor group was higher

than those of term labor group for both bipolar and

BC-EHG recording. However, such difference was

statistically significant only for BC-EHG. Regarding

spectral parameters, it is noticeable that only DF1 for

the set [31, 34 WG] presented statistically significant

differences between term labor and preterm labor,

and again only for BC-EHG signals.

Electrohysterogram Signals from Patients with Threatened Preterm Labor: Concentric Ring Electrode Vs Disk Electrode Recordings

4 DISCUSSION

In the present work, it was tested and confirmed the

feasibility of picking up the uterine electrical activity

using CRE at early gestational ages. No statistically

significant differences were found between EHG

parameters derived from bipolar and BC-EHG

recording, apart from the peak to peak amplitude.

Conventional bipolar EHG signals presented higher

amplitude than BC-EHG, which may be due to the

fact that CRE presented a relatively smaller

interelectrode distance. This result agrees with other

authors who has acquired both bipolar and BC-EHG

in patients at term (> 37 weeks of gestation) (Ye-Lin,

2015). Moreover, bipolar and BC-EHG signals

presented similar spectral characteristics in the target

signal bandwidth.

Then it was explored the capability of bipolar and

BC-EHG recordings to discriminate between women

with threatened preterm labor who will deliver

preterm of those who deliver at term. In comparison

to term group, bipolar EHG signals from preterm

group presented higher peak to peak amplitude and

greater spectral content in high frequency range. This

result is accordance with other authors that computed

temporal and spectral parameters to characterize the

EHG recording

(Garfield, 2005, Maner, 2003,

Marque, 2007

). Nevertheless, our data did not show

statistically significant differences in spectral

parameters obtained from bipolar recording as

reported by other authors (Maner, 2003, Maner, 2007,

Fele-Zorz, 2008). Maner et al. focused on the DF2

estimated from the EHG-burst associated to uterine

contractions. In that work DF2 of preterm labor group

presented significantly higher values than those of

term labor group. The fact that in the present work the

whole EHG recording is analysed, including not only

the spikes burst but also the basal activity, could

mitigate the difference of EHG parameters between

preterm and term labor group. On the other hand,

other authors who have also analysed the whole EHG

recording have found statistically significant

differences in the median frequency calculated in

frequency range 0.3-3 Hz (Fele-Zorz, 2008). Firstly,

the target bandwidth of data analysis of our work was

slightly different. Besides, EHG recordings in this

work were performed under common clinical practice

conditions, which implies most of them were carried

on patients under tocolytic drugs effect. This can also

affect the obtained results, but it is closer to real

clinical conditions of patients with threatened of

preterm labor after preliminary explorations in

emergency rooms.

As for BC-EHG signals capability to distinguish

between preterm and term group, it was found

statistically significant difference in both peak to peak

amplitude and DF1 parameters. This result may

suggest that concentric ring electrodes not only can

be used for picking up uterine electrical activity in

patients with threatened preterm labor, but also could

have superior performance for discriminating true

preterm labor from false alarms in comparison to

bipolar recordings with conventional disk electrodes.

This may be associated to a higher spatial resolution

of CRE that permits to discriminate better between

normal local activity during pregnancy and

coordinated global electrical activity as labor

approaches, although this hypothesis has to be

confirmed in further studies with a larger database.

The main limitation of this work is the small size

of the actual database, which on one hand limits the

robustness of the results and the derived conclusions,

and on the other hand binds the study only for patients

with gestational weeks at recording moment between

[28, 34]. When the size of this database is increased,

it will allow us to extend the present analysis to

patients with wider ranges of gestational age [24-36]

WG. It can also be considered a limitation of the

study, the above mentioned use of tocolytic drugs in

most patients. These drugs can modify the spectral

content of EHG signals, since they are intended to

alter uterine contractility in order to delay or prevent

preterm labor. Despite these limitations, CRE have

been proven their capability for monitoring uterine

electrical activity in early gestational ages, suggesting

its potential clinical use for predicting preterm labor.

5 CONCLUSIONS

In conclusion, it has been proved CREs’ capability for

picking up uterine electrical activity during early

weeks of gestation. Moreover, BC-EHG signals

present smaller amplitude but similar spectral

characteristics when compared with conventional

bipolar EHG recordings. Finally, average peak-peak

amplitude and DF1 estimated from BC-EHG

recordings showed statistically difference between

preterm term and term labor group and not from

bipolar EHG, suggesting better performance of the

CRE to disk electrodes for predicting preterm labor

based on EHG records. This prediction capability will

be evaluated in future work since a more

comprehensive database is needed for the

implementation and testing of classifiers.

BIOSIGNALS 2017 - 10th International Conference on Bio-inspired Systems and Signal Processing

ACKNOWLEDGEMENTS

This work was supported in part by the Ministry of

Economy and Competitiveness of the Spanish

Government and the European Regional

Development Fund (DPI2015-68397-R,

MINECO/FEDER).

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Electrohysterogram Signals from Patients with Threatened Preterm Labor: Concentric Ring Electrode Vs Disk Electrode Recordings