Uterine contraction monitoring is most often ignored when discussing the benefits of fetal monitoring (Bakker 2007a; Freeman et al, 2012). Monitoring the uterine contraction (UC) and the fetal heart rate (FHR) enables the assessment of the relationship between the UC and the FHR (Bakker et al, 2007a). When using oxytocin in augmented or induced labours, intrapartum guidelines recommend that UCs should be monitored continuously (National Institute for Health and Care Excellence (NICE), 2014). Inadequate UC monitoring without clinical oversight could allow excessive contractions to occur unrecognised, delaying appropriate action. This involves either reducing or discontinuing an oxytocin infusion (Reinhard et al, 2011; Ayres-de-Campos et al, 2015).
The maintenance of normal UCs and sufficient recovery time is essential to restore the supply of well-oxygenated maternal blood to the intervillous space and to ensure that the fetal cerebral oxygen saturation remains stable (Bakker and van Geijn, 2008).
In practice, it can be challenging to obtain an adequate recording of the UCs, especially in obese or active parturients. Obese women are at higher risk of complications such as hypertension, pre-eclampsia and diabetes during pregnancy, and it is therefore recommended that continuous monitoring should be carried out in these high-risk women during the intrapartum period.
This article will evaluate five ways of monitoring UCs (maternal perception, manual palpation, external tocodynamometry, intrauterine pressure catheter (IUPC) and abdominal electrohysterography) in order to compare the methods and parameters that each one provides. Despite the IUPC not being used as the standard of care in Europe, it remains the gold standard by which other methods are judged (Euliano et al, 2013).
Parameters of UCs discussed in this article are taken in part from Simpson and Miller (2011), who assume a well-defined contraction with a clear start, end, peak and stable baseline. The parameters are shown in Box 1.
Box. 1Parameters of uterine contractions (UC)
| Peak/acme | The highest point of the uterine contraction |
| Frequency | Can be the time in minutes from the beginning of one UC to the beginning of the next or the peak of one UC to the peak of the next. It is usually evaluated over a 10-minute window and averaged over 30 minutes |
| Duration | The time in seconds from the start of a UC to its end. This is dependent on the method used as shown in Figure 1 |
| Resting phase/relaxation time/recovery time | The time between two UCs. It is measured from the end of one UC to the onset of the next. Standard guidelines normally recommend 1 minute resting phase in induced or augmented labours |
| Strength/intensity/amplitude | When using manual palpation, UC is expressed as ‘mild’, ‘moderate’ or ‘strong’.When using the intrauterine pressure catheter (IUPC), it is expressed as the peak of the UC in millimetres of mercury (mm Hg) which can be used to provide the Montevideo parameter |
| Resting tone | When using manual palpation, it is generally expressed as ‘soft’ or ‘firm’. When an IUPC is used, it is the intrauterine pressure during relaxation time, expressed in mm Hg.Note: By inference, electrohysterography (EHG) and tocodynamometry (TOCO) cannot provide strength or resting tone |
| Hyperstimulation | More than 5 contractions in 10 minutes averaged over a 30 minute window |
The most widely used and internationally accepted method to monitor UCs electronically is external tocography. It is, however, important that technology evolves to address the new challenges that obesity in obstetrics brings. In obese patients, the distance from the skin to the uterus may be such that the tocodynamometry does not detect contractions reliably (Euliano et al, 2007).
Observation and maternal perception of uterine contractions
In the absence of obstetric tools, the midwife uses her eyes, ears and hands to assist and diagnose pregnant women (Spencer, 2004). The initial assessment of UCs is undertaken using basic listening skills and observation of the woman's demeanour. Although maternal observation is subjective, its value lies in the relationship built with the woman, and her behavioural changes over time will give signs.
It is interesting to note that a minimal intrauterine pressure of 15 mm Hg is required for the distension of the lower segment and cervix. It is this distension that causes the pain and subsequently makes the mother aware of a contraction (Bakker et al, 2007b).
In addition, this assessment is affected by diverse factors including epidural analgesia, which affects the maternal perception of pain. There may also be language barriers where the interpreter will be liaising directly with the midwife. This can sometimes cause some confusion as the woman is not able to communicate with the midwife directly and as a result, the midwife may not be fully aware of the mother's needs. An example of this may be where a family member is interpreting on behalf of the woman and miscommunication then occurs. Maternal beliefs and previous pregnancy experiences can also have an impact on the assessment. The Royal College of Midwives (RCM) reported on a study by Burvill (2002), which found that:
‘Midwifery knowledge can work outside the biomedical paradigm that uses cervical dilatation and abdominal palpation of contractions alone, and seeks to recognise the clues that are found in movements, breathing, conversation and emotional states.’
(Royal College of Midwives, 2012: 3)A study by Cottrill et al (2004) focused on the factors associated with perception of UCs on 7808 participants. The women who had singleton pregnancies were divided into four groups by their pre-pregnancy body mass index (BMI): lean, normal, overweight and obese. The study found that there was a significant reduction in the perception of UCs with increasing BMI. The author concluded that ‘obese, nulliparous patients have the greatest difficulty perceiving contractions and such data may help explain unattended birth or late presentation for care in this group’ (Cottrill et al, 2004: 155).
A small study of 17 women (Eri et al, 2010) highlighted that even how clinicians ask questions can affect an assessment. If the midwife only asks the question related to the contraction frequency, vital information could be lost, and the woman could be deprived of the opportunity to verbalise her experiences and her needs as she perceives them.
Any assessment of UCs requires a holistic approach, ensuring that the woman's perception of her contractions is respected.
Manual palpation
The traditional method used to assess UCs is manual palpation. It is a learned skill that allows the clinician to categorise UCs as either ‘mild’, ‘moderate’ or ‘strong’, based on the indentation of the abdomen during UCs at the fundus of the uterus. One training method uses the adult face to describe the indentation variables on manual palpation. A mild contraction is described as a a tense fundus but easy to indent; pressing the finger to the nose would give a similar feeling. A moderate contraction is described as a firm fundus that is difficult to indent with the fingertips; touching the chin with the finger is described as a similar sensation. A strong contraction is a rigid, board-like fundus, almost impossible to indent; this sensation is said to be like touching the finger to the forehead (Tucker et al, 2009). In relation to the resting tone, it is described as ‘soft’ or ‘firm’.
According to Bakker et al (2007b: 52):
‘The contractions of the pregnant uterus are perceptible by abdominal palpation only after their intensity exceeds 10 mm Hg. This perception threshold is influenced by the thickness and tonus of the abdominal wall and the experience of the obstetrician, midwife and nurse.’
There are mixed messages as to the parameters that manual palpation provides. A study on intermittent auscultation (Maude et al, 2016: 288) stated that uterine palpation using touch ‘will determine the frequency, duration and strength of contractions,’ while the fourth edition of Fetal Heart Monitoring (Freeman et al, 2012), writes that manual palpation of UCs can identify frequency and duration. Arrabal and Nagey (1996) undertook a study of 46 labouring women, using a comparison of manual palpation and the IUPC during the first stage of labour. The contractions were palpated and classified as mild (<30mmHg), moderate (30–50mmHg) or strong (>50mmHg). Their overall finding showed that observers were accurate in predicting the contraction strength 49% of the time, and moderate contractions were observed in 28%. They concluded that manual palpation was an inaccurate means of determining contraction strength.
The duration of the contraction estimated by manual palpation is shorter than the actual duration, when measured by IUPC. Therefore, the parameter provided by using manual palpation is the frequency of the contractions (Bakker et al, 2007b).
In clinical practice, the application of technology does not always produce a better outcome. Therefore, manual palpation of UCs should not be regarded as an obsolete form of monitoring (Olah et al, 1993). Manual palpation, when used with technology, allows the clinician to validate information displayed by the electronic contraction monitoring.
External tocodynamometry
Mechanical devices to monitor contractions externally were introduced as early as 1861 (Freeman et al, 2012). There have been many types of ‘tocometers,’ such as those produced by Frey in 1933, Lorand in 1947 and Smyth's guard-ring tocodynamometer (Smyth, 1957).
Tocodynamometry (tocography or toco) provides a continuous record of UCs. The device itself is called the tocodynamometer. It either has a centrally located, pressure sensitive button, or a guard ring and a central flat plunger held against the woman's abdomen with a belt. It detects the change in shape of the uterus by detecting the changes in the abdominal contour as a result of the UC. In obese women, the adipose tissue separates the transducer from the uterus. If the belt holder of the transducer is too slack or too tight, this will impact on the quality of the trace. Many women find the encircling belt uncomfortable (Hayes-Gill et al, 2012).
Toco has the advantage that it is non-invasive, some commercial devices can be used in water, but even in non-obese women, toco suffers frequent failures and this does not correlate well with the IUPC. Some periods of unreliable pattern on the trace can occur because of inadequate calibration (Euliano et al, 2013). Toco provides the contraction frequency and the approximate duration of labour contractions (Euliano et al, 2013), but cannot be relied on to accurately reflect the contraction intensity.
In summary, as Bakker et al (2007b: 53) concluded:
‘[Toco] can accurately measure contraction frequency if circumstances are optimal: i.e. when used on a non-obese, relaxed patient with adequate positioning of the transducer and correct tightness of the belt.’
The International Federation of Gynaecology and Obstetrics (FIGO) guidelines also highlight that only the frequency of contractions can be reliably evaluated (Ayres-de-Campos et al, 2015).
Toco is sensitive to maternal and fetal movement. Maternal respirations can produce an undulating overlay and during maternal position change, sudden baseline shifts may occur. Pushing effort during the second stage of labour usually produces blunted spikes (Association of Women's Health Obstetric and Neonatal Nurses (AWHONN), 2003). Figure 2 depicts the different variations that may be seen on the trace and the possible causes for this pattern.
The intrauterine pressure catheter
The IUPC, also referred to as internal tocography, is an invasive method of monitoring UCs, requiring ruptured membranes before the insertion of the catheter. The flexible, disposable catheter is inserted into the amniotic cavity of the uterus, via the cervix. The measurements taken from the catheter is in mm Hg (AWHONN, 2003).
The first registration of intrauterine pressure to record UCs was in 1872 by Friedrich Schatz. His method to assess UCs used a small bag of fluid introduced between the membranes and the lower segment of the uterus, connected to a mercury manometer. His internal recordings are regarded as extraordinary and have remained unsurpassed for more than a century.
Quantitative tocography with a fluid-filled IUPC was developed by Alvarez and Caldeyro-Barcia in Montevideo, Uruguay in 1950. Many would consider Caldeyro-Barcia the father of modern continuous tocography due to his many studies (Sailing and Arabin, 1988).
As a result, UC intensity is measured in Montevideo units. For example, if there are 4 UCs in 10 minutes with an average intensity of 40 mm Hg for that time frame, the Montevideo units for that period would be 4 x 40, or 160 Montevideo units. Labour was found to begin when the Montevideo units rose between 80 and 120. A more commonly practiced method is to add the contraction intensities in the 10 minute time frame as it equates to similar numbers (Tucker et al, 2009).
The IUPC is unaffected by maternal position, allows for amnioinfusion and gives quantifiable information on baseline tone, duration, amplitude, and contraction relaxation time (Bakker et al, 2007b).
Several studies have found that using the IUPC to quantify contraction intensity does not significantly change the outcome and that knowing the frequency and the length of contractions is sufficient (Euliano et al, 2007). A study undertaken by Bakker et al (2013) compared internal and external monitoring on women who had labours induced or augmented. They found that there was not a significant reduction in adverse neonatal outcome or operative deliveries.
The use of the IUPC is not widespread, however, in the US it is still being used in the hospital settings (Regan and McElroy, 2013).
Abdominal electrohysterography
One of the first attempts to explain the electrohysterogram (EHG) using surface electrodes was by Larks et al (1957), who concluded that a biphasic pattern can be recognised in the abdominal signal recorded. Wolfs and Rottinghuis (1970) simultaneously recorded intrauterine pressure and abdominal electrical signals, both demonstrating that action potential spikes occurred in both intrauterine and abdominal electrical signals. It proved that it is possible to record the electrical activity of the uterus from the maternal abdominal wall, but the abdominal EHG recorded was of poor quality. Today advancements in EHG have improved the detection of UCs; which has resulted in it being used in clinical practice.
The myometrium is composed of roughly 200 billion small-muscle cells and UCs are dependent on action potentials generated and propagated by the muscle cells. The myometrium is relatively inactive before labour, but at term the electrical activity increases (Buhimschi, 1997). Intrauterine pressure strongly depends on synchronisation of myometrial activity. As Benoit et al (2010) stated:
‘EHG monitors the process by which contractions are formed whereas IUPCs measure the end result of that process.’
The electrodes placed on the maternal abdomen allow for the detection of electrical activity produced by these action potentials. In the Monica Novii solution (one of a number of EHG solutions available today). There are five electrodes that form part of a patch, providing a multi channel solution to detect the fetal and maternal heart rates as well as monitoring the EHG. Figure 3 illustrates the five electrodes of the patch.
Although adipose tissue in the obese woman poses a challenge for toco, for EHG, the presence of adipose tissue is not so challenging.
This was highlighted in a study by Euliano (2007), in which 25 subjects with had a median body mass of 39.6 kg/m2 were monitored with toco, IUPC and EHG. The study found that:
‘[The] electrohysterogram-derived contraction pattern correlated better with the intrauterine pressure catheter than tocodynamometry. Electrohysterography may provide another non-invasive means of monitoring labour, particularly for those women in whom tocodynamometry is inadequate’
(Euliano et al, 2007).Another study (Hayes-Gill et al, 2012), comparing toco, IUPC and EHG, was undertaken on 74 term parturients. The results showed that EHG was more reliable than toco when compared to the IUPC. The study also recognised that both and EHG and toco showed false positive contractions when the internal pressure displayed non-positive. However, the EHG had a non-significant trend towards more frequent false positives. The study concluded that:
‘Electrohysterography was more reliable and similar in accuracy to tocodynamometry in detecting intrapartum uterine contractions’
(Hayes-Gill et al, 2012: 65).In comparison to toco, the EHG contraction waveform remains smooth in appearance. This smooth line is also seen in the baseline as well as in the second stage of labour.
Figure 4 shows a trace from the Monica Novii EHG, toco and the IUPC simultaneously.
Low-level or uncoordinated EHG activity not associated with an increase in intrauterine pressure can appear as small irregular deflections from the baseline. These are easily identified during labour among the larger more regular ‘true’ contractions. Maternal activity or vigorous fetal movements can change maternal abdominal surface contours and produce what appears on the trace to be a UC. This is caused by small changes in the electrode positions in relation to each other and to the underlying skin.
Discussion
When working with nurses and midwives, there can be confusion on the parameters electronic that UC devices provide. Manual palpation is sometimes seen as a skill that is no longer required, as there is an increased reliance on technology. However, as this article has highlighted, diverse factors can affect on the quality of the trace. Electronic monitoring of UCs should never be regarded as a substitute for clinical observation and assessment. It is important to assess the trace in the clinical context.
NICE clinical guidance (2014) recommends a continuous cardiotocography for women with high-risk factors. For low-risk women, continuous monitoring should be avoided, as it has been shown to result in increases in emergency caesareans and operative vaginal delivery without any benefit to the baby (Pateman, 2008).
Every woman's labour is unique to her and UCs that result in cervical dilatation and effacement would be deemed to be adequate. When labour is induced or augmented with oxytocin, it is important to ensure that excessive UCs do not occur. UCs should, therefore, be documented adequately, both electronically and clinically.
Set up protocols for external toco are not standardised, leading to false positives and negatives. Unfortunately, the risk of litigation is significant in obstetrics, even in situations where care is appropriate and poor documentation may impact in cases (Miller, 2011).
Electronic monitoring equipment should not be the main focus of care and realistic expectations need to be used when undertaking the monitoring of UC.
In clinical practice, it is also important to provide an environment that supports oxytocin release (Reed, 2010) and to be aware of other factors that could affect uterine activity/contractions.
External monitors are not a replacement for clinical observation and palpation. In order to assess the accuracy of the electronic monitoring, clinical observation and palpation must be applied at the start and at regular intervals during labour. This is to confirm the veracity of the electronic trace and appropriate management. EHG provides another method of monitoring UCs, which may prove a beneficial tool for women who are classified as obese.
Good practice would dictate that clinical observation and palpation should be documented.
It is important to have a hands-on approach with any form of electronic continuous UC monitoring.