Continuing Education Activity

Hypotonic labor is a dysfunction in the propulsive power of the uterus that presents as an abnormal labor pattern resulting in prolonged or protracted delivery, which is a common indication for primary cesarean section. Management options include supportive measures, medical treatment, and surgical interventions. In the absence of cephalo-pelvic disproportion and other absolute indications for cesarean delivery, the prognosis is good when the dysfunction is diagnosed early. This activity describes and highlights the role of the inter-professional team in managing patients with this condition.

Objectives:

• Identify the etiology of hypotonic labor.

• Outline the typical presentation in hypotonic labor.

• Describe the management considerations in hypotonic labor.

Introduction

Hypotonic labor is an abnormal labor pattern, notable especially during the active phase of labor, characterized by poor and inadequate uterine contractions that are ineffective in causing cervical dilation, effacement, and fetal descent, leading to a prolonged or protracted delivery. Normal labor is divided into three stages, with the first stage typically described in two phases - the latent and active phases. In the active phase, uterine contractions are expected to occur at frequent intervals, increasing the intensity and duration of each contraction. These are deranged in hypotonic labor. Four factors influence the normal progression of labor, denoted as the 4Ps: power (uterine contractions), passage (maternal bony pelvis), passenger (fetus), and fetal presentation.[1] Hypotonic labor is primarily a dysfunction of power; there is inadequate propulsive power to cause fetal descent, cervical dilatation, and eventual expulsion of the fetus(es) and placenta.

Etiology

The cause of hypotonic labor is uterine inertia, also known as hypotonic or hypocontractile uterine dysfunction. Though the etiology of the inertia is unknown, these conditions are commonly associated with hypocontractile uterine dysfunction:

• Uterine overdistension and overuse as seen in multifetal gestation, fetal macrosomia, polyhydramnios, and grand-multiparity
• Mechanical disruption of myometrial function from myoma or distension of the bladder or bowel
• Malpositioning and malpresentation of the fetus, where there is an absent reflex in uterine contraction, due to inadequate contact of the presenting part onto the lower uterine segment
• Abnormal uterine axis as seen in a pendulous abdomen. (There is an exaggerated anteversion of the uterus.)
• Uterine deformities or myometrial disorganization as seen with developmental uterine hypoplasia and extensive myomectomy
• Prematurity below 30 weeks gestation where oxytocin receptors are not fairly established
• Other general/systemic causes may include maternal anemia, maternal exhaustion, and improper use of analgesia in labor

Epidemiology

Most labor abnormalities frequently occur in nulliparous women (25%) than in the multiparous group (10%), including those caused by hypocontractile uterine action. The incidence of labor abnormalities is also noted to be higher in elderly nulliparous women. Uterine inertia is a common indication for primary Cesarean section.[2] Many cesarean deliveries are found to be a result of poor labor progress.[3] Results from studies have found the most common cause of labor progress abnormality to be hypotonic uterine contractions.[4] In 2017, the United States and the United Kingdom had a cesarean section rate of 37% and 27.3% respectively. This increasing trend in cesarean deliveries, especially in developed countries, has drawn attention to assessing the indications for primary Cesarean sections.

Pathophysiology

Normal uterine contractions are generated in a coordinated fashion by two uterine pacemakers located at the cornua portion of the uterine fundus (fundal dominance) and propagated in a synchronous order towards the lower uterine segment. The downward coordinated path of contraction explains the role of contraction in the downward migration of the fetus and cervical changes. In the normal active phase, the contraction frequency is usually 3 to 4 within 10 minutes, with a pressure of 30 to 50 mm Hg above the resting tone. The minimum characteristics required to describe a contraction as effective are:

• Minimum frequent contractions with intervals less than 5 minutes apart 
• Intensity of contractions greater than 25 mm Hg

The duration of each contraction also increases from 30 seconds in early labor to between 60 to 90 seconds in the latter part of labor.[5] Friedman established the accepted pattern for the normal progression of labor in the 1950s. The World Health Organization adopted the Friedman curve on the partograph, a graphical labor monitoring tool. Based on his data on nulliparous and multiparous women, he described that the transition from the latent to the active phase of labor occurs at 3 to 4 cm cervical dilatation. He also described that for most women, the minimum rate of cervical dilatation in the active phase is 1.2 cm/hour in nulliparous and faster in multiparous women at 1.5 cm cervical dilatation/hour.

In hypocontractile uterine dysfunction, although the uterine basal tone is within the normal range (less than 10 mm Hg), the peak/active pressure does not rise higher than 25 mm Hg (normal pressure is about 60 mm Hg or 8 kPa). The hypotonic contractions are described with 4 properties: intensity, duration, relaxation, and interval. Characteristically, there is diminished intensity lower than 10 mm Hg, the duration is shortened by less than 20 to 30 seconds, and there is good relaxation between contractions, but the interval between contractions is increased. The frequency of contractions is often less than 2 to 3 within 10 minutes.

History and Physical

An assessment would confirm that ‘true’ labor has been established, with appropriate documentation of onset time. The patient feels less pain with every contraction, with less frequency and increasing intervals between the contractions. Maternal exhaustion is unusual but appears later as anxiety increases. The fetus is also usually not affected early on. The common clinical finding is inadequate cervical dilatation or fetal descent. With ineffective labor, there may be observed slow progression called protracted labor, absence of progress called the arrest of labor or ineffective expulsive effort. The woman must be in the active phase with cervical dilatation up to 4 cm to be diagnosed with any of these dysfunctions.[6]

Evaluation

Palpation and monitoring tools like an external tocodynamometer can be used to qualitatively evaluate uterine activity. This assessment is subjective since the diagnosis of hypocontractility is based on the perception that contractions are infrequent and weak, with less than 3 to 4 contractions/10 minutes and duration of less than 45 seconds. Another qualitative evaluation of progression is the cervical assessment by digital examination. This tracks the changes in cervical dilatation, effacement, and fetal station performed at standard timed intervals from admission time through the various stages of labor. These changes may be charted on a partogram, which aids in comparison with the expected lower limit of normal labor progress. A deviation to the right of the normal labor curve on the partograph denotes slow progress.[7] A protraction in the active phase would refer to cervical dilation slower than 1 to 2 cm/hour, whereas active phase arrest is diagnosed where there is no cervical change for >6 hours with inadequate contractions.[8] The internal pressure catheter is a device that measures the pressure generated by each contraction, recorded quantitatively in Montevideo units (MVU). Uterine activity less than 200 to 250 MVU is considered inadequate and unlikely to affect expected cervical dilation and fetal descent. This may be a more objective assessment than external monitoring in very obese women.

Treatment / Management

Assessing the mother and fetus to exclude other causes of abnormal labor progression, such as cephalopelvic disproportion (CPD) and fetal malpositioning. Management can be discussed as supportive measures and active measures.

Supportive Measures

• Continuous reassurance to keep the mother calm. Maternal stress increases endogenous adrenaline, which can inhibit uterine contractions.
• Encourage ambulation and avoid the supine position. Although these are not proven to improve contractions or prolonged labor due to hypocontractility, they may improve the comfort of the parturient.[9][10]
• Empty bladder, consider catheterization.
• Maintain adequate hydration.
• Adequate pain relief.

Active Measures

Medical management

These are interventions that improve the quality of uterine contractions:

• Amniotomy: Membrane rupture (amniotomy) stimulates contractions by releasing prostaglandins and reflex stimulation of the uterus when the presenting part becomes closely applied to the lower uterine segment. Amniotomy should be attempted when vaginal delivery is probable; where cervical dilatation >4 cm, there is adequate fetal descent (station -2 or lower), and the presenting part is well-applied to the lower uterine segment.

• Oxytocin: Provided there are no contraindications. Oxytocin is the medication of choice for augmenting contractions. The dosage regimen should be titrated to effect for achieving adequate uterine contractions. However, dosing generally does not exceed 30 milliunits/minute.[11] The usual protocol is 5 units of oxytocin in 500 mL of 5% dextrose intravenous infusion, starting with 10 drops/min and gradually titrating the rate to achieve a contraction rate of at least 3 per minute. When instituted early in the active phase, a combination of amniotomy and oxytocin augmentation is more effective in managing hypocontractile labor than alone.[12]

Surgical management:

• Assisted vaginal delivery may be performed using forceps, vacuum, or breach extraction, provided the cervix is fully dilated and vaginal delivery is indicated and probable.
• Operative delivery by cesarean section should be considered early when the assessment indicates a CPD or fetal malpositioning/malpresentation. However, in the absence of an early indication, cesarean section is performed if all other measures have failed to stimulate the uterine contractions; when oxytocin is contraindicated (including cephalopelvic disproportion), if there is maternal exhaustion, fetal distress (category III fetal heart tracing), or before full cervical dilation.[13]

Differential Diagnosis

Differential diagnosis of hypotonic labor include:

• Braxton-Hicks contractions
• Amnionitis
• Malpresentation/malpositioning
• Uterine rupture

Prognosis

The prognosis is good. The maternal and fetal outcomes are favorable when hypotonic labor is recognized early and timely interventions instituted while ensuring close monitoring. The risk of a surgical intervention increases when decreased uterine activity occurs in the presence of a cephalopelvic disproportion, fetal malpositioning, or fetal distress. The early decision for cesarean section improves both maternal and fetal outcomes.

Complications

Complications of hypotonic labor may be maternal, fetal, or both, and can include:

• Arrest of labor
• Maternal anxiety and exhaustion
• Postpartum hemorrhage due to uterine atony
• Retained placenta due to ineffective myometrial retraction
• Increased risk of instrumental delivery and possible injuries to mother and baby
• Cesarean section risk with the attending surgical and anesthetic complications
• Fetal distress and birth asphyxia

Consultations

In hypotonic labor, consultations may become necessary. A patient under the care of a trained obstetric nurse or midwife may require consultations with an obstetrician. In a health facility without an obstetrician, or where provision for cesarean section and instrumental vaginal delivery is unavailable; an early referral to a higher level of care is recommended if protracted labor is diagnosed or anticipated. At the accepting health facility, the care team must include an obstetrician who should be consulted ahead of a transfer. All records on the progress of labor must be provided. Supportive consultation with the anesthetists and pharmacists is recommended where analgesic or anesthetic support is required. Their involvement is necessary during situations of oxytocin complications (hypotension, tachysystole, or oxytocin hypersensitivity).

Deterrence and Patient Education

The psychological preparation of patients before labor appears to improve pain tolerance during labor. This should begin during routine antenatal visits and the counseling for labor analgesia. This preparation may serve to reduce the need for neuraxial analgesia in labor, which is a probable predisposing factor for hypocontractile labor.

Pearls and Other Issues

Although the prognostic outcome of hypotonic labor is good, it is still important to explore the other components of labor when making a diagnosis of an abnormality with power (uterine contraction). A diagnosis of cephalo-pelvic disproportion should not be missed, as this is an immediate indication for cesarean section. Typically, maternal exhaustion and fetal distress do not present early in hypotonic labor. They occur later in the labor process and should have the utmost priority irrespective of the adequacy of labor progression. Even where cervical dilation is complete or near-complete, fetal distress should not be ignored in favor of a vaginal delivery.

In the absence of an absolute indication for Cesarean section, hypotonic contractions are very responsive to using oxytocin to augment the labor; this should only be administered by qualified professional personnel who understand the oxytocin indications, dosing, contraindications, and signs of complication within a facility where surgical interventions are immediately accessible. Oxytocin is contraindicated or discontinued in the following conditions:

• Fetal prematurity and fetal distress when vaginal delivery is not imminent
• Contraindications for vaginal delivery (vasa previa, cord prolapse/presentation, active genital herpes, or cervical cancer)
• Cephalopelvic disproportion 
• Abnormal fetal presentation/position
• Uterine hypertonicity or hyperactivity
• High risk for uterine rupture (grand multiparity, uterine overdistension, or extensive uterine surgeries)
• Obstetric emergencies that necessitate a surgical intervention

Enhancing Healthcare Team Outcomes

Anticipating hypotonic labor and subsequent diagnosis and management requires interprofessional team management. The trained obstetric nurse or midwife may identify the risk factors during the antenatal visits and ensure appropriate monitoring during labor. The midwife is also key in charting the labor progression using various monitoring tools. They must be able to recognize an abnormal progression pattern and involve the necessary specialist in the care plan. The obstetrician usually takes the lead on the interprofessional team, ensuring early detection of slow progress due to hypocontractility and implementing the appropriate step-wise interventions and monitoring while coordinating the input of other team members. The anesthetist and pharmacists ensure the patient has adequate pain management, especially where assisted or operative delivery is required. For cases that require intrauterine fetal resuscitation, instrumental delivery, or cesarean delivery, early involvement of the neonatologist ensures continuity of postpartum care. The team management approach improves maternal and fetal outcomes and may lower the average cesarean section rates.