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ISSN: 1476-7058 (print), 1476-4954 (electronic)
J Matern Fetal Neonatal Med, Early Online: 1–4
! 2014 Informa UK Ltd. DOI: 10.3109/14767058.2014.964677
ORIGINAL ARTICLE
Factors affecting uterine electrical activity during the active phase of
labor prior to rupture of membranes
Liran Hiersch1,2, Liat Salzer1,2, Amir Aviram1,2, Avi Ben-Haroush1,2, Eran Ashwal1,2, and Yariv Yogev1,2
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1
Department of Obstetrics and Gynecology, Helen Schneider Hospital for Women, Rabin Medical Center, Petach Tikva, Israel and 2Sackler Faculty of
Medicine, Tel Aviv University, Tel Aviv, Israel
Abstract
Keywords
Objective: Limited data exist regarding uterine contraction intensity prior to membrane rupture.
Using a novel technique of electrical uterine myography (EUM) we aimed to determine which
factors affect myometrial activity during active phase of labor.
Methods: EUM was prospectively measured in 37 women with singleton pregnancy at term
during the active phase of labor until membranes’ rupture. EUM was measured using noninvasive nine channels recorder with an EMG amplifier and three-dimensional position sensor.
Uterine electrical activity was quantified with the EUM-index, defined as the mean electrical
activity of the uterine muscle over a period of 10 min and measured in units of micro-Joule
(microwatt per second [mW/s]).
Results: The mean EUM-index at the first 10 min of the measurement was 3.3 ± 0.6 mW/s. In a
stepwise linear regression model accounting potential confounders EUM was significantly
affected by cervical dilatation (p ¼ 0.005), maternal age (p ¼ 0.04) and previous cesarean
delivery status (p ¼ 0.02). In a repeated measurement assessment of non-parametric Fridman’s
test for all subjects who had at least 10 continuouss EUM measurements, there was a significant
increase in electrical uterine activity as labor progressed (p ¼ 0.01).
Conclusion: Electrical uterine activity during the active phase of labor prior to rupture of
membranes is affected by maternal age, previous cesarean delivery status and cervical
dilatation. Moreover, electrical uterine activity is enhanced throughout labor.
Active phase, delivery, uterine activity
Introduction
Uterine activity is characterized by the frequency, intensity,
and duration of uterine muscle contractions. Adequate labor is
defined classically as three to five contractions per 10 min
with advance in cervical effacement and dilation and fetal
descent [1]. Conventional methods for uterine contractions
monitoring are limited as external tocodynamometry is
lacking the capability of quantitative uterine contractions
intensity assessment and intrauterine pressure catheter application is only feasible following membranes rupture [2,3].
Measurement of uterine activity using intrauterine pressure
transducers is quantified using Montevideo units (calculated
by multiplying the average peak strength of contractions in
mm Hg by the number of contractions in 10 min). Using this
method, observational studies have found that adequate labor
Address for correspondence: Yariv Yogev, Department of Obstetrics and
Gynecology, Helen Schneider Hospital for Women, Rabin Medical
Center, Petah Tiqwa 49100, Israel. Tel: +972-3-9377680. E-mail:
[email protected]
History
Received 25 August 2014
Accepted 9 September 2014
Published online 29 September 2014
is usually observed with 200–250 Montevideo units [4,5].
There is no evidence that one method is significantly better
than another [6] and still, despite technologic improvements,
the definition of ‘‘adequate’’ uterine activity during labor
remains vague. Moreover, information regarding quantitative
measurement of contractions intensity through the different
stages of labor is lacking.
Recently, the use of electrical uterine myography
Monitoring (EUM) for the assessment of electrical uterine
activity was introduced. EUM is a non-invasive method
allowing the evaluation of duration, frequency and most
importantly the intensity of uterine contractions [7–9].
Moreover, its accuracy in assessing contractions intensity is
comparable to that of intrauterine pressure catheter [10].
Quantitative measurement of contractions intensity during
active phase may shed light on the uterine factor that
contributes to labor dystocia, and can possibly explain the
mechanism behind known risk factors for protraction and
arrest disorders such as maternal obesity [11,12], neuroaxial
anesthesia [13], etc.
Thus, we aimed to explore which factors could affect
uterine myometrial activity during active phase of labor prior
to membrane rupture using a novel technique of EUM.
2
L. Hiersch et al.
Methods
Statistical analysis
Study population
Data analysis was performed with the SPSS v19.0 package
(Chicago, IL). A non-parametric Friedman’s test was carried
out in order to assess if there were differences in perceived
EUM measurements based on cervical dilatation throughout
labor. A stepwise linear regression model was used to adjust
the effect on labor EUM measurements. Variables that were
hypothesized to potentially affect the likelihood of EUM
measurements based on clinical grounds were entered to the
stepwise linear logistic regression model. Differences were
considered significant when p-value was less than 0.05.
We conducted a prospective observational study of women
undergoing labor and delivery at a single tertiary university
affiliated medical center. Eligibility was limited to singleton term (37 + 0 weeks of gestation) pregnancies with
intact membranes, within the active phase of labor defined
as cervical dilatation between 4 and 9 cm. The study was
approved by the local institutional review board, and all
women provided written informed consent prior to participating in the study.
Study design
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Women in active phase of labor with intact membranes were
connected to EUM. In addition, all women were monitored
by external tocodynamometer and electronic fetal heart rate
monitoring, which are routinely used in all deliveries. All
decisions regarding labor management including the frequency of cervical dilatation examination, need for oxytocin
for augmentation or dosage changing, or artificial rupture of
membranes (AROM) timing were made solely by the
attending physician and were based on the routinely used
external tocodynamometer and other clinical signs and
symptoms. The attending physician and research team
were blinded to the EUM results. Women who were
augmented with oxytocin but had dosage changes during
EUM monitoring period were subsequently excluded from
final analysis.
Data collection
The following data were collected for each participant:
demographic and obstetric characteristics, administration of
oxytocin, cervical dynamics during EUM monitoring period
and EUM recordings.
The EUM device
A novel EMG device, EUM-100 (OB-Tools Ltd,
MigdalHaemek, Israel), was used to measure the electrical
uterine activity. The system is comprised of a multichannel
surface electromyogram, a three-dimensional position sensor
and a personal computer providing data analysis and a
graphical user interface. The EUM signals are acquired and
amplified using a proprietary isolated amplifier box based on
modified Teledyne A0401 modules (Teledyne Inc, Thousand
Oaks, CA). The surface EUM activity is acquired by nine
electrodes evenly placed on the patient’s abdomen and a tenth
common ground electrode on the patient’s left thigh. The
measurement of EUM signals together with their physical
location provides an accurate estimation of the uterine
activity in the three-dimensional space.
The electrical signal from the myometrium is recorded and
processed using a uterine contractility algorithm. Uterine
muscle activity is quantified using the EUM scoring index
which is defined as the mean electrical activity of the uterine
muscle over a period of 10 min and is measured in units of
micro-Joule (micro-Watt-Second, mWS) with an automatic
data analyzer blinded to clinical outcome.
Results
Overall, 37 women with singleton pregnancies at term were
enrolled. The demographic and obstetrical characteristics are
presented in Table 1. Mean maternal age was 30.8 ± 4.5 years
and 24% were nulliparous. Eleven women were evaluated for
more than 100 min. Mean EUM-index at the first 10 min of
the exam (EUM1) for all women was 3.3 ± 0.6mWS, and was
referred as baseline. Sixty-five cervical dilatation examinations during active phase of labor prior to membranes’
rupture were analyzed. Thirty-two (68%) women had more
than one cervical dilatation measurement.
In a stepwise linear regression model accounting for
maternal age, gravity, parity, gestational age, BMI, previous
cesarean delivery status, cervical dilatation and the use of
oxytocin in labor, EUM was significantly affected by cervical
dilatation (p ¼ 0.005, Figure 1), maternal age (p ¼ 0.04) and
previous cesarean delivery status (p ¼ 0.02).
In a repeated measurement, a non-parametric Fridman’s
test for all subjects who have had at least 10 continues EUM
measurements was performed (N ¼ 11). There was a significant increase in electrical uterine activity as labor progressed
(p ¼ 0.01, Figure 2).
Discussion
In the current study, we prospectively assessed factors
affecting uterine electrical activity during active phase of
Table 1. Demographic and obstetrical characteristics of the study group.
Variable
Study group
(n ¼ 37)
Maternal age (years)
Gestational age (weeks)
Gravidity
Parity
Nulliparity
Previous CS
BMI at admission (kg/m2)
Oxytocin use during labory
Number of cervical dilatation measurements per patient
1
2
3
Duration of EUM monitoring (min)*
Duration 4100 min*
30.8 ± 4.5
39.2 ± 1.0
2.7 ± 1.3
1.4 ± 1.0
9 (24.3%)
3 (8.1%)
26.8 ± 3.9
16 (43.2%)
1.8 ± 0.6
12 (32.4%)
22 (59.5%)
3 (8.1%)
98.1 ± 60.0
11 (44%)
Data are presented as mean ± SD or n (%).
*Among 25 women with at least two cervical dilatation measurements.
yNo change in oxytocin dosage during EUM monitoring period.
BMI, body mass index; CS, cesarean section; EUM, electrical uterine
myography.
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DOI: 10.3109/14767058.2014.964677
labor prior to membranes’ rupture using novel EUM
technology. Our main findings were: (1) Overall, EUMindex (uterine activity) is enhanced with increase in cervical
dilation during the active phase as labor progresses. (2)
Previous cesarean delivery status and lower maternal age is
associated with higher uterine activity. BMI was not significantly associated with EUM-index.
The traditional methods for assessment of uterine contraction intensity are limited. Intrauterine pressure catheter
necessitates ruptured membranes prior to application.
Moreover, the method is invasive and thus has potential for
complications. The technology of EUM used in the current
study and its non-invasive nature, enabled performing quantification of uterine contractions intensity in the active phase
while membranes are intact. Previous study from our group
[14] has demonstrated that myometrial electrical activity
recorded by the EUM is significantly enhanced following
AROM. Thus, to avoid confounders, we measured uterine
Figure 1. The association between EUM-index and cervical dilatation
during the active phase of labor. Data represent EUM-index results at the
time of 65 cervical dilatation examinations during active phase labor
prior to membranes’ rupture of 32 women.
Figure 2. The change in mean EUM measurement during 10 consecutive measurements
(a period of 100 min). Borders represent 95%
confidence interval.
Uterine electrical activity in the active phase
3
activity in the active phase only prior to rupture of
membranes.
Recent studies focused on quantification of uterine activity
during labor [1,15–17]. Still, though outdated evidence exist
regarding an increase in uterine activity during labor [18],
information on the different intensities of uterine activity
through the progress of active phase itself is lacking. Our data
may suggest that enhanced electrical myometrial activity
through the active phase, may, in part, be responsible for
adequate progress in labor. In clinical practice, this may shed
light on the mechanisms for pathological progress of labor
and be a future therapeutic target.
Other than cervical dilation, maternal age was also
inversely correlated with EUM-index. It was suggested that
older age is associated with a significant decrease in
contractility for spontaneous contractions, but only in the
non-pregnant state [19]. In the pregnant myometrium, a wide
range of contractile ability was found between women and
little evidence for decreased spontaneous activity appeared
between the ages of 25 and 40. The lack of difference in the
pregnant state over this period was attributed to the response
to gestational hormones. On the other hand, other studies
suggested that advanced maternal age is associated with
increase in operative vaginal deliveries, cesarean section, and
an increased need for oxytocin-augmentation [20,21]. These
findings were shown not to be explained by co-morbidities
such as obesity or gestational diabetes. One hypothesis
proposed for these findings is that with increasing age, the
myometrium is less effective or less responsive to uterotonic
agents such as oxytocin or prostaglandins thereby indicating a
direct effect of aging on myometrial function [20]. Uterine
electrical activity recorded with EUM in our study is in
concordance with this hypothesis.
Moreover, our results might suggest that uterine activity in
women with previous cesarean section is even enhanced.
Previous study [22] measuring in vitro myometrial contractility, found that myometrium from women whose previous
cesarean section was for fetal distress contracted with more
force than that from women whose previous cesarean section
was necessary because of other indications. However, our
sample size is too small to separate the various indications.
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L. Hiersch et al.
Previous studies have also found poor spontaneous and
oxytocin-stimulated uterine contractility in vitro in postdate
pregnancies [23] and obese women [24]. Our study did not
confirm these observations. It is possible that the mechanical
contractile ability of the myometrium tested in these studies
does not necessarily equal electrical activity. Moreover,
these studies were made in vitro and there is a possibility
that other factors play a role in the in vivo environment
that influence the contractility and mask its effect, possibly
hormonal effects.
The prospective nature of our design and the blinding of
both treating physician and research team to EUM measurements during labor as well as blinding of the data analyzer to
clinical results strengthen our findings, Still, our study has
several limitations. The study was done on a small number
of patients. Larger studies are needed to confirm our
findings. Since this was an observational study rather than
an interventional one, we lacked a control arm. Moreover, as
labor was managed by the treating physician, some of the
participants had been administered oxytocin prior to recruitment. However, oxytocin dosage was not altered during the
reported hour of assessment, which minimizes its bias effect.
In conclusion, we demonstrated that electrical uterine
activity during the active phase of labor prior to rupture of
membranes is affected by cervical dilatation, maternal age,
and previous cesarean delivery status. Moreover, electrical
uterine activity is enhanced throughout labor. Further
research is warranted in order to describe electrical uterine
activity in pathological labors for better future clinical
purposes.
Declaration of interest
The authors report no conflict of interest.
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