INTRODUCTION — The frequency, duration, and strength of uterine contractions are important factors in the assessment of labor progress and in the interpretation of fetal heart rate patterns. Four methods to evaluate contractions are available: manual palpation, external tocodynamometry, internal tocodynamometry, and electrohysterography [1].
External palpation and external tocodynamometry are the most common methods for assessing uterine activity, and provide adequate information in most patients. However, external palpation is subjective and requires the physical presence of a clinician to perform frequent examinations. Although external tocodynamometry provides an objective record of the frequency and duration of contractions without requiring the physical presence of a clinician, the tracing can be interrupted by maternal movement and contraction strength cannot be quantified since the device is measuring the tension of the abdominal wall over the uterus. Internal tocodynamometry utilizing an intrauterine pressure catheter (IUPC) (picture 1) overcomes these limitations, as it provides a reliable, quantitative measure of uterine activity with minimal artifacts [2,3]. Electrohysterography is a noninvasive technology that detects uterine electrical activity using electrodes placed on the mother's abdomen. It may be as reliable and accurate as internal tocodynamometry, but it has not been studied extensively.
This topic will discuss the clinical use of IUPCs for monitoring uterine activity during labor. Diagnosis of normal and abnormal intrapartum uterine activity and management of protraction and arrest disorders are reviewed separately.
●(See "Labor: Overview of normal and abnormal progression".)
●(See "Labor: Diagnosis and management of an abnormal first stage".)
●(See "Labor: Diagnosis and management of a prolonged second stage".)
CLINICAL USES
When is an IUPC preferable to external tocodynamometry? — IUPCs are not routinely used for monitoring uterine activity during spontaneous labor, induction, or augmentation because routine use does not lead to improved maternal or fetal outcomes and increases costs, complexity, and complications compared with external methods (see 'Complications' below). In a systematic review of three randomized trials comparing internal versus external tocodynamometry in nearly 2000 patients undergoing induction or augmentation of labor, internal tocodynamometry did not result in a statistically significant improvement in any maternal or neonatal outcome (eg, risk of Apgar score less than seven at five minutes, umbilical artery pH <7.15, neonatal intensive care unit admission, operative delivery, or hyperstimulation) [4].
There are, however, clinical situations in which use of an IUPC is preferable to external tocodynamometry. For example, we would place an IUPC in the first stage of labor when:
●External tocodynamometry does not provide a clear, continuous tracing (eg, some patients who are frequently changing position, some patients with obesity).
●The precise relationship between the start/end of fetal heart rate decelerations and the start/end of contractions is not clear on the external tracing but necessary for interpreting the fetal heart rate pattern. (See "Intrapartum fetal heart rate monitoring: Overview", section on 'Physiologic significance of selected FHR characteristics'.)
●Objective calculation of contraction strength (Montevideo units) is desirable to help determine the cause of protracted or arrested labor. This information is most useful when contraction frequency is adequate. (See "Labor: Overview of normal and abnormal progression", section on 'Definitions for the stages and phases of labor'.)
Objective calculation of contraction strength also may be helpful for titrating oxytocin dose to provide optimal contractions while preventing hyperstimulation. This information is particularly reassuring when increasing the oxytocin dose in patients at increased risk of uterine rupture (eg, trial of labor after cesarean).
●A conduit is needed for amnioinfusion (see "Amnioinfusion").
It is unclear whether use of an IUPC is helpful for facilitating the diagnosis of uterine rupture in patients at higher risk for this complication [5-8]. In a series of 39 cases of uterine rupture in which an IUPC was in place, loss of intrauterine pressure was not observed in any patient [5]. In two case reports, however, a stepwise gradual decrease in uterine contraction amplitude (staircase sign) followed by the onset of severe variable decelerations was reported after rupture [8].
No studies have assessed the safety and efficacy of IUPCs in twin pregnancies. Anecdotally, they are rarely used in this setting. An IUPC can be used to gauge the frequency of contractions if this information is needed clinically and cannot be obtained with an external device, but information about contraction intensity may not be as accurate as in singleton gestations. There is also the possibility of rupturing the second sac during insertion of the IUPC; ultrasound visualization during insertion may reduce this risk.
Alternatives — Electrohysterography (EHG), a newer quantitative method of uterine monitoring, measures uterine muscle electrical activity via electrodes placed on the maternal abdomen. Studies have evaluated its use for prediction of preterm labor as well as a potential noninvasive alternative to internal monitoring [9-12]. In a meta-analysis of 20 studies, when compared with external tocodynamometry, EHG appeared to be more accurate for detecting contractions and less affected by maternal obesity in term labor [13]. However, when compared with the IUPC, the peak timing of contractions was different and the frequency of contractions was overpredicted. This review also demonstrated two potential EHG parameters warranting further study for prediction of vaginal birth versus cesarean. We do not use EHG in our practice, and large randomized trials are still needed to demonstrate clinical utility.
PROCEDURE
Prerequisites before placement
●Fetal membranes must be ruptured
●Internal cervical os must be dilated sufficiently to admit the catheter tip in its introducer and the examiner's finger(s) to guide it
Contraindications
●Known or suspected placenta previa or vasa previa are obvious contraindications to use of an IUPC, as well as to labor and vaginal birth.
●The author does not place IUPCs in patients with suspected abruption or bleeding of unknown origin. The IUPC package insert lists "bleeding of undetermined origin" as a contraindication. Because almost all intrapartum bleeding is of unknown origin after placenta previa has been excluded, some clinicians believe that use of an IUPC in these cases may be individualized, based on the suspected etiology and potential risks and benefits.
●The author does not place an IUPC in patients with clinical chorioamnionitis because the resulting data would not impact her decision to expedite delivery in patients with infection and prolonged dysfunctional labor. However, clinical chorioamnionitis is not an absolute contraindication, and some clinicians find an IUPC helpful when titrating oxytocin dose in patients with infection-related hypocontractile uterine activity.
Placement in the second stage of labor is not contraindicated, but would be difficult because of the low fetal station and the information acquired is less useful for decision-making proximate to birth.
Types of catheters — There are three types of IUPCs [14-16]:
●Transducer tip
●Sensor tip
●Fluid-filled
All have clear lumens to enable visualization of amniotic fluid flashback and thus confirm that the tip is positioned in amniotic fluid. The transducer tip and sensor tip catheters are flexible; the fluid-filled catheter is more rigid. The sensor tip and fluid-filled catheters require an external transducer.
Choice of catheter — The choice of IUPC is based on clinician preference, availability, and cost. The limited data of the safety and efficacy of transducer and sensor tip IUPCs suggest that they are equivalent [15,17,18]. In a single randomized trial including 249 patients, the transducer tip catheter had a higher extramembranous (between the membranes and decidua) placement rate than the sensor-tipped catheter (12.5 versus 2.4 percent) but no difference in the frequency of abruption or fetal distress [18].
The fluid-filled IUPC was the first commercially available IUPC and is still available in some areas. In contrast to more contemporary catheters, the tip is open and prone to plugging, particularly in the setting of thick meconium; thus, it is less reliable [16]. It is also more likely to perforate the uterus because of its inflexibility. (See 'Complications' below.)
Technique — Depending on the IUPC brand and transducer type, small differences exist in catheter placement and calibration techniques; device-specific techniques are described in the package insert. The general principles for IUPC insertion are described below [19]:
●Perform a vaginal examination to confirm that membranes are ruptured and the cervix is sufficiently dilated to admit the catheter tip in its introducer and the examiner's finger(s) to guide it.
●Insert the tip of the introducer (with the catheter inside) just inside the cervical os, directing it laterally but inside the amniotic cavity (avoid placement between the decidua and membranes). Lateral placement is easiest because the symphysis and sacrum can increase resistance if anterior or posterior insertions are attempted. Lateral placement is also least likely to cause fetal complications.
Avoid advancing the rigid introducer beyond the examiner's fingers, which are placed just inside the cervix and against the presenting part.
Insert away from the placental site, if possible. A bedside ultrasound examination is reasonable if the placental site is not known.
●Gently thread the catheter 10 to 14 cm into the amniotic cavity (indicated by the first imprinted mark on the catheter). In this position, the tip is usually just beyond the fetal head. Ultrasound guidance is not required.
The catheter should go in easily; if any resistance is met, pull the tip back into the introducer and then change the angle or position of the introducer. The catheter should never be forced as it is possible to pass it completely through the placenta, myometrium, or a velamentous umbilical cord [19].
●Flow of amniotic fluid through the catheter at this point indicates appropriate intra-amniotic placement. Lack of clear fluid suggests extramembranous placement. If the catheter is thought to be in a dry pocket, it can be withdrawn to the fingertips and then redirected to a different area and reinserted. If the absence of fluid is thought to be related to oligohydramnios, 10 to 20 mL of sterile saline or water can be flushed through the port and some of this fluid should return through the catheter. If these approaches are ineffective or gross blood is observed, removing the catheter is prudent. (See 'Extramembranous placement' below.)
●When proper placement in the amniotic cavity is established, the catheter is advanced further (approximately 45 cm, indicated by the second imprinted mark). The introducer is removed and the IUPC is secured to the patient's upper medial thigh. The tip of the catheter should be in the uterine fundus, where the maximum contraction amplitude is recorded.
●For sensor-tipped IUPCs, the monitor should be at zero before the electronic sensor tip is connected to the monitor cable. Transducer-tipped catheters may be zeroed prior to or after insertion of the catheter.
Baseline tone at term is between 5 and 25 mmHg, but may be higher preterm.
Note that extramembranous catheters will still provide readings, but they are not accurate reflections of intrauterine pressure and cannot be used for amnioinfusion. (See 'Extramembranous placement' below.)
●Proper placement and function are confirmed by having the patient cough, which should produce a sharp spike in pressure.
Removal — The IUPC is easily removed by gently pulling on the catheter [20]. We often remove it towards the end of the second stage if the patient can feel the contractions or the contractions are sufficiently palpable that a provider can coach the patient about when to push.
INTERPRETATION OF FINDINGS — The IUPC tracing will accurately show the frequency, duration, and pressure of uterine contractions. Montevideo units (MVUs) are calculated by subtracting the baseline uterine pressure (resting tone) from the peak contraction pressure of each contraction in a 10-minute window and adding the pressures generated by each contraction; this sum is the number of MVUs (figure 1).
In a retrospective report, 91 percent of patients who had a spontaneous vaginal birth after oxytocin induction achieved contractile activity greater than 200 MVUs, and 40 percent reached 300 MVUs; the corresponding figures in patients who had a spontaneous vaginal birth after augmentation of spontaneous labor were 77 and 8 percent, respectively [21]. In a study of spontaneously initiated normal labor, uterine activity averaged approximately 100 MVUs in the early first stage of labor, 175 MVUs in the advanced first stage, and 250 MVUs in the second stage [22].
Based on these and other data [23-25], "adequate" MVUs (ie, expected to lead to a normal rate of cervical change and fetal descent) have traditionally been defined as 200 to 250 MVUs, although this level of force generated does not always represent effective uterine contractility for an individual patient. The dose of oxytocin can be titrated to produce MVUs in this range, or the dose can be increased until there is normal progression of labor or strong contractions occurring at two- to three-minute intervals. No method has been proven to be superior to another. (See "Induction of labor with oxytocin", section on 'Oxytocin administration'.)
COMPLICATIONS — Complications following IUPC placement are rare. However, significant changes in the fetal heart rate pattern, bleeding from the IUPC, or significant maternal pain should prompt immediate evaluation.
Extramembranous placement — Most major complications of IUPC placement are related to incorrect placement in the extramembranous (extraovular) space between the fetal membranes and decidua, which is not uncommon [19,26-28]. Signs of extramembranous placement include lack of fluid return and bloody return, if the catheter has an external transducer (these signs are not reliable signs of extramembranous placement for transducer-tipped catheters). Immediate fetal and maternal assessment is warranted as extramembranous placement can result in placental laceration or abruption. Removal versus redirection of the IUPC depends on the clinical situation (eg, maternal status, fetal status, blood in catheter).
There is no typical pressure reading that is diagnostic of extramembranous placement of an IUPC, but a high baseline with peak contraction pressures damped or a negative baseline reading is suggestive.
Placental trauma may manifest as a nonreassuring fetal heart rate pattern and/or high contraction pressures.
Uterine perforation — If the tip of the catheter perforates the myometrium, clinical findings may include dry or bloody insertion, abdominal pain, and/or signs of intra-abdominal bleeding (eg, tachycardia, hypotension). Tocography will be abnormal. A key finding is increased pressure upon maternal Valsalva but no increase during uterine contractions.
Uterine perforation rates for first-generation, inflexible fluid-filled catheters (1970s) ranged from 1 in 300 to 1 in 1400 insertions [29,30]. This risk has fallen dramatically with the introduction of flexible contemporary catheters: Only one case of uterine perforation has been published, to our knowledge [31]. In this case, fetal distress immediately after the placement of the IUPC led to an emergency cesarean birth that revealed a 5 cm vertical laceration in the posterior lower uterine segment, which was repaired.
Umbilical cord prolapse — Umbilical cord prolapse can occur during placement, especially if the presenting part is dislodged during manipulation of the introducer and catheter. (See "Umbilical cord prolapse", section on 'Management of intrapartum cord prolapse'.)
Other complications — The following have been described in single or multiple case reports:
●Amniotic fluid embolism syndrome (also referred to as anaphylactoid syndrome of pregnancy) occurring immediately after IUPC placement [32,33].
●Umbilical cord entanglement with the IUPC [27].
●Perforation of the umbilical cord, fetal blood vessels on the placental surface, or perforation of the placental parenchyma by the IUPC [19,26,27,34].
●Massive fetomaternal hemorrhage [35].
A literature review documented 26 cases of severe complications after IUPC insertion and seven fetal or neonatal deaths, one neonate with seizures who developed severe cerebral palsy, one neonate with anemic shock, at least three neonates with blood transfusions, and at least 22 emergency cesarean births. One mother developed an intraperitoneal abscess at the site of the perforated uterus [34].
Risk of infection — IUPC use may increase risk of infection, but more data are needed with attention to potential confounding due to an association between infection and abnormal uterine contractility.
●A 2013 systematic review of randomized trials of internal versus external tocodynamometry during induced or augmented labor found no significant increase in maternal or neonatal infection with IUPC use; however, only two trials provided data on infection risk [4]. In one of the trials, the relative risk of intrapartum infection with internal versus external contraction monitoring was RR 0.69 (95% CI 0.44-1.08; infection rate: 30/734 [4.1 percent] versus 43/722 [6 percent]); in the other trial, the relative risk of signs/symptoms of maternal or newborn infection in the first three weeks after birth was RR 0.84 (95% CI 0.61-1.16; infection rate 64/726 [8.8 percent] versus 74/709 [10.4 percent]). Patients with any indication for prophylactic antibiotics during labor were excluded from the analysis.
●In contrast, a subsequent secondary analysis of patients undergoing a trial of labor in the Maternal-Fetal Medicine Units Network Vaginal Birth after Cesarean Registry found that IUPC use was associated with a significant increase in surgical site infections compared with no IUPC use, after adjusting for confounders (adjusted odds ratio 1.28, 95% CI 1.1-1.5) [36].
●Another secondary analysis of a randomized trial of azithromycin prophylaxis for cesarean in labor found an increase in IUPC use in patients with readmission or unexpected visits after an unscheduled cesarean birth [37]. The most common reasons for the admissions and visits were infection and hypertensive disorders.
SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Labor".)
SUMMARY AND RECOMMENDATIONS
●Routine use not advised – An intrauterine pressure catheter (IUPC) is not routinely used for monitoring uterine activity during spontaneous labor, induction, or augmentation because randomized trials have demonstrated no clear improvement in maternal or fetal outcome. Furthermore, routine use may be associated with increased costs, complications, and possibly medical interventions. (See 'Clinical uses' above.)
●Use in selected clinical settings – In selected clinical settings, use of an IUPC provides additional clinically useful information. The most common clinical scenarios for an indicated IUPC include:
•To quantify uterine contraction intensity when contraction frequency is adequate but cervical dilation is not progressing
•To distinguish between late and early fetal heart rate decelerations when the start/end of uterine contractions is not clear on external monitoring
•To obtain a clear uterine activity tracing when external tocodynamometry is not informative
•To quantify uterine contraction intensity for titrating oxytocin dose
•To allow for indicated amnioinfusion
●Contraindications – Known or suspected placenta previa or vasa previa are contraindications to IUPC placement. Placement in the second stage of labor is not contraindicated, but would be difficult because of the low fetal station and the information acquired is much less useful for decision-making proximate to birth. (See 'Contraindications' above.)
●Types of IUPCs – The two major types of catheter tips are transducer tips and sensor tips with an external transducer. We suggest choosing an IUPC based on personal preference, availability, and cost. The limited data of the safety and efficacy of the two main types of IUPCs suggest that they are equivalent. (See 'Types of catheters' above and 'Choice of catheter' above.)
●Complications – Although complications are rare, serious maternal and/or fetal compromise may occur. Significant changes in the fetal heart rate tracing, bleeding from the IUPC, or significant maternal pain should prompt immediate evaluation. (See 'Complications' above.)
ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges John T Repke, MD, who contributed to an earlier version of this topic review.
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