Learning Objectives:After participating in this continuing education activity, the provider should be better able to:

1. Describe the pathophysiology and neuroanatomy of labor pain.

2. Explain how to assess laboring patients for epidural analgesia.

3. Outline the fundamental principles of labor epidural analgesia administration.

This article is the first of 2 parts.

Pain during labor profoundly affects women's birth experience. Labor pain induces the maternal stress response, which causes increased release of catecholamine and stress hormones. The activation of the sympathetic nervous system can elevate blood pressure, cardiac output, and vascular resistance. Also, pain leads to maternal hyperventilation and increased oxygen consumption.1 Epidural analgesia effectively relieves labor pain. Epidural analgesia blocks sympathetic and sensory nerves at the lower thoracic level down to the sacral level, mitigating the stress response related to labor pain.2

This is first of 2 consecutive review articles discussing labor epidural for obstetric patients. In this article, we will discuss the neurophysiology and neuroanatomy of labor pain, basic assessment of laboring patient for epidural analgesia and the principles of epidural analgesia administration.

Neurophysiology of Pain

Pain during the first stage of labor results from the distention of the cervix and lower uterine segment.2 Dilation of the endocervix and lower uterine segment results in the activation of afferent nerves to the T10 to L1 segments of the spinal cord. The painful stimulus passes through the paracervical plexus, the hypogastric plexus, and the lumbar sympathetic chain before entering the spinal cord. Somatic pain is transmitted by C and A-delta nerve fibers, which enter the spinal cord through dorsal root ganglion in the ipsilateral superficial laminae of the dorsal horn. Visceral pain travels via visceral C-fibers entering the dorsal horns and the ventral horn, before crossing to the contralateral dorsal horn. The visceral pain in the first stage of labor is due to stretching and distension resulting from uterine contractions. The mechanism of the action potential generation of the nerve terminals during labor involves multiple ion channels responding to the membrane distortion, neurotransmitters, local ischemia, and low pH. Epidural labor analgesia is based on the blockade of afferent nerve conduction by local anesthetics at the level of spinal nerve roots.

Pain during the second stage of labor is transmitted via the dorsal root ganglion at S2 to S4 spinal cord segments. Pain during the second stage is exclusively somatic pain, localized to the portio of the cervix, vagina, and perineum. To provide successful analgesia during the second stage of labor, the epidural blockade should extend the coverage from the T10 to S4 spinal cord segments.2

Assessment of the Laboring Patient for Epidural Analgesia

Epidural analgesia is used for relief of pain during the first and second stages of labor. The most common reason for providing epidural analgesia is maternal request. Contrary to previous thought, women who request early epidural analgesia are not at an increased risk of prolonged labor. The risk of cesarean delivery is also not increased when the epidural is placed in the first stage of labor. In a randomized study of over 12,000 laboring women, those who received an epidural during the latent phase of labor (at 1-cm cervical dilatation) or active (at least 4-cm dilatation) had similar outcomes. Outcomes measured were rate of cesarean delivery, instrument-assisted delivery, time to, and duration of the stages of labor.3

Laboring women at risk for a complicated labor course should be evaluated for early placement of an epidural. Examples include women with preeclampsia, twin gestation, or who are morbidly obese with a potentially difficult airway. Epidural analgesia can reduce maternal catecholamine surge and lower blood pressure and heart rate in patients with preeclampsia. In the event that a cesarean delivery is necessary, early placement of an epidural is desired rather than an emergent general anesthetic.

Assessment of Coagulation Status and Spinal Epidural Hematoma Risk in Pregnancy

The reported incidence of spinal epidural hematoma is 3 to 4 per 1,000,000 in obstetric cases.4 The alterations in coagulation that occur during pregnancy reduce the risk of spinal epidural hematoma in pregnancy. Risk factors for spinal epidural hematoma with unfractionated heparin (UFH) or low-molecular-weight heparin (LMWH) include traumatic needle placement, concomitant use of other anticoagulants, and inadequate interval between the time of heparin administration and needle placement or catheter removal.5,6

A summary of interval recommendation for heparin dosing and neuraxial procedures is presented in Table 1. The preparation of pregnant patients on a thromboprophylactic or therapeutic dose of anticoagulants should be approached in a multidisciplinary way. The care team should have a standardized protocol to assess the women who qualify for thromboprophylaxis, an alert system that identifies the record of the patients on anticoagulation, predelivery anesthesia consultation, and prompt communication among the obstetric, anesthesia, and nursing teams.

Table 1. Interval Between Unfractionated Heparin or Low-Molecular-Weight Heparin and Neuraxial Procedures

Anatomy of the Epidural Space and Techniques of Epidural Analgesia

The spinal cord extends from the medulla oblongata to the level of the first lumbar vertebral body. There are 3 meninges surrounding the spinal cord. The pia mater is the innermost layer of the meninges. The arachnoid mater is a nonvascular middle membrane, adherent to the outermost dura membrane. Cerebrospinal fluid (CSF) flows in the subarachnoid space between the pia and arachnoid membranes. The dura mater is a dense membrane made of collagen. The epidural space is located outside the dura mater. It is bound by the posterior longitudinal ligament (anterior), ligament flavum and lamina (posterior), and the vertebral pedicles and intervertebral foramina (lateral). The epidural space contains spinal nerve roots, venous plexus, fat tissue, lymphatics, and connective tissue.

The epidural space is identified by using "loss of resistance technique." When an epidural needle is advanced slowly through the spinous ligaments while applying pressure gently to the syringe plunger, resistance is suddenly lost with the entrance of the needle tip into the epidural space. Once the epidural space is identified with an epidural needle, an epidural catheter is threaded and left about 4 to 5 cm into the epidural space. In the epidural space, injected anesthetic solution spreads through the dural sac and nerve roots exiting the spinal canal. Instead of placing only the epidural catheter, combined spinal epidural (CSE) analgesia or dural puncture epidural (DPE) analgesia can be used.8

In both the CSE and DPE, an intentional dural puncture with a small-gauge spinal needle is performed. The small-gauge needle is passed through the epidural needle after identification of the epidural space using loss of resistance technique. A small dose of spinal anesthetic is given through the spinal needle in CSE analgesia; no spinal dose is injected if DPE analgesia is being performed. In DPE, a small dose of the epidural anesthetic enters the intrathecal space through the dural puncture to provide additional analgesic. CSE or DPE analgesia allows the reliable spread of the analgesia to the sacral nerve roots. CSE analgesia provides faster onset of analgesia compared with epidural or DPE analgesia, but it is associated with more maternal and fetal side effects such as hypotension, pruritus, and category II fetal heart rate tracing. DPE analgesia provides better quality of sacral analgesia compared with epidural analgesia and fewer maternal and fetal effects than CSE analgesia.8

Timing of Neuraxial Analgesia and Delivery Outcomes

Epidural analgesia does not increase the risk of cesarean delivery.9 Observational studies identified an association between epidural analgesia during the latent phase of labor and a higher rate of cesarean delivery in the 1990s. However, multiple randomized studies have not revealed any evidence of an increased rate of cesarean or instrumental vaginal delivery.10,11

Contraindications to Epidural Analgesia

Contraindications to epidural analgesia can be divided into absolute and relative contraindications. The most common absolute contraindication is patient refusal. Other absolute contraindications include coagulopathy, which can be seen in conditions such as preeclampsia or a primary coagulopathy. Sepsis, hemodynamic instability (hypovolemia), and infection at the lumbar insertion site are also absolute contraindications. Relative contraindications include prior lumbar surgery, mild-to-moderate aortic stenosis, or neurologic disease. Each patient needs to be evaluated on a case-by-case basis.

If a patient is on systemic anticoagulation, the anticoagulant should be held per guidelines before insertion of the epidural and should be restarted as advised. Anticoagulants include oral anticoagulants, subcutaneous (SQ) heparin, IV heparin, and SQ LMWH. Treatment for pulmonary arterial hypertension with IV prostanoids (eg, treprostanil sodium) can lead to platelet dysfunction. Continuous IV infusions of prostanoids in patients with severe pulmonary hypertension should not be abruptly discontinued. These patients may experience life-threating rebound pulmonary hypertension due to the short half-life time of IV prostanoids. Decisions in these cases should be made on a case-by-case basis using a multidisciplinary approach. Patients on low doses of IV prostanoids can be switched to inhaled prostanoids. Inhaled prostanoids have less effect on platelet function compared with IV infusion. However, patients with severe pulmonary hypertension on large doses of IV prostanoids should continue their infusion. Labor epidural analgesia is not contraindicated in these patients, but it may be associated with a higher risk of bleeding and hematoma. A multidisciplinary approach that includes cardiology specialists is pivotal to successful and safe labor epidural analgesia administration in these patients. Prudent assessment of the patient for any sign of abnormal clinical bleeding is essential. Minimizing the number of attempts and placement of the epidural catheter by an experienced anesthesiologist are recommended in these situations.

Epidural Drug Delivery Modes

Once an epidural is placed, medication is delivered through the epidural catheter. Intermittent bolus of medication by a clinician is one technique used to administer labor relief. In comparison to other delivery modes, bolus administration can be more labor intensive and does not provide a reliable steady state of analgesia.12 An infusion pump can be programmed to administer the desired dose and rate of epidural medication. An improvement to the infusion pump is the development of patient-controlled epidural analgesia. Patient-controlled epidural analgesia has been used in labor analgesia for over 30 years.13 It allows the patient to receive the continuous infusion while also being able to receive additional patient-controlled doses. The clinician sets the additional doses for a designated volume, time, and lockout interval.

Maintenance of labor analgesia by delivering local anesthetics as a programmed intermittent epidural bolus has been investigated. Programmed intermittent epidural bolus entails delivering programmed intermittent doses (boluses) with or without patient-controlled demand boluses. Multiple studies have suggested that programmed intermittent epidural bolus may have the benefit of improving analgesia with less local anesthetic consumption, decreasing the risk of breakthrough pain and motor blockade, and results in better maternal satisfaction compared with continuous epidural infusion.11 The possible mechanism of improved analgesia is a more extensive spread of anesthetic solution in the epidural space with the programmed intermittent epidural bolus technique. The potential complication with programmed intermittent epidural bolus is unintentional intrathecal local anesthetic administration and high spinal block if there is catheter migration intrathecally. Programmed intermittent epidural bolus may cause abrupt hemodynamic instability or motor blockade. Appropriate patient monitoring and a system to alert obstetric and anesthesia providers for possible complications should be implemented.11 An example of programmed intermittent epidural bolus is described in Table 2.14

Table 2. Example of Programmed Intermittent Epidural Bolus Regimen

Epidural Medications for Labor Analgesia

Medications used in epidurals for labor analgesia generally are local anesthetics, opiates, or both. Additional medications that can be administered as adjuncts to local anesthetics are epinephrine and sodium bicarbonate. Local anesthetics are effective in blocking somatic pain, whereas opioids are more effective in blocking visceral pain. Commonly used opiates are fentanyl and sufentenil. Opioids are more effective early in labor until cervical dilation of 7 cm is reached. Epinephrine prolongs the duration of action of the local anesthetic and reduces systemic absorption. Sodium bicarbonate allows for faster onset of the local anesthetic by increasing the pH of the solution closer to the ionization constant (pKa) of the local anesthetic. The closer the pKa of the local anesthetic is to the local pH, the faster the onset of the block due to the availability of more local anesthetics molecules in the nonionized form. Local anesthetic molecules in the nonionized form readily cross the nerve membrane to bind to local anesthetic receptor. The most commonly used local anesthetics for labor epidural analgesia are bupivacaine and ropivacaine because of their longer duration of action. A summary of the characteristic of local anesthetics used for labor epidural analgesia is described in Table 3.

Table 3. Common Local Anesthetics for Epidural Analgesia

Communication

Ineffective communication during handoff and among patient care disciplines is associated with poor patient outcomes. The labor and delivery floor is a dynamic clinical arena. Thus, effective communication among multiple disciplines involved in the care of the obstetric patient is quintessential to patient safety and statisfaction.15 The most successful approach to communication is that which is systematic and protocol driven. An existing protocol crafted by the anesthesiology and obstetric leadership to delineate the criteria of obstetric patients who need multidisciplinary assessment prior to placement of labor epidural analgesia is invaluable.

Medical problems such as pulmonary hypertension, cardiomyopathy, aortic stenosis, coagulopathy, hypercoagulability, arrhythmia, pulmonary embolism, mediastinal tumors, coronary artery disease, and congenital cardiac abnormalities are the most common conditions requiring multidisciplinary management. Patients on therapeutic doses of anticoagulants deserve special attention for safe administration of epidural anesthesia while the existing hypercoagulable pathologic state is treated. Epidural placement planning should focus on the two critical time points of placing and removing the catheter, during which bleeding can occur leading to serious neurological consequences.

Another layer of patient safety and quality assurance is participation of the anesthesiology, obstetric, nursing, social work, and perinatology teams in daily rounds, also known as "board rounds" or "multidisciplinary rounds." These rounds are usually held twice daily following the change of shifts. These rounds allow all clinical staff to communicate and discuss pertinent patient information and treatment plans. However, it important to note that the initial communication regarding a labor epidural analgesia request from a patient should proceed immediately upon the patient's request. Early consultation of anesthesiologist to allow for appropriate assessment and discussion, obtaining of informed consent, and placement of the epidural is prudent. In addition to the patient safety benefits of the aforementioned communication and team collaboration approaches, they serve as a powerful tool to share knowledge and leverage collective competence on the labor floor.

Conclusion

Epidural analgesia is the gold standard analgesic modality for labor pain management. Safe administration and maintenance of epidural analgesia relies on good communication, prudent evaluation, and assessment of patient coagulation status. In this article, we reviewed the fundamentals of epidural analgesia for obstetric patient. In the following article (labor analgesia for obstetric patients: part II), we will discuss identification and management of side effects associated with the administration of labor epidural analgesia in obstetric patients.

Practice Pearls

* Successful epidural analgesia during the second stage of labor requires blockage of the spinal roots from T10-S4 spinal cord segment.

* Labor epidural analgesia should be initiated upon request from the patient, regardless of cervical dilatation with little effect on delivery outcomes.

* High-risk cases such as patients with preeclampsia, twin gestation, and morbid obesity should be evaluated for early placement of epidural analgesia.

* Assessment of the patient's coagulation status and proper timing of medication labor epidural are essential to avoid a spinal hematoma.

* Local skin and/or subcutaneous infection at the site of epidural needle insertion is an absolute contraindication to labor epidural analgesia.

* DPE and CSE analgesia provide better quality of sacral analgesia compared with epidural analgesia.

* CSE analgesia provides faster onset of analgesia compared with epidural or DPE analgesia, but it is associated with more maternal and fetal side effects such as hypotension, pruritus, and category II fetal heart rate tracing.

REFERENCES