Keywords

Mechanisms of Pain During Labor

Pain during the first stage of labor arises from stretching and dilation of the lower uterine segment and cervix. Visceral afferent neurons accompany sympathetics through the paracervical region, hypogastric plexus, and lumbar sympathetic chain and eventually transmit the signal to the dorsal horn of the spinal cord at the level of T10-L1. During the second stage of labor, somatic afferent neurons arising in the cervix, vagina, and perineum convey signals via the pudendal nerve, entering the spinal cord at S2-4 [1].

Benefits of Labor Analgesia

The surge of catecholamines, particularly epinephrine, which can occur during contractions, can lead to detrimental effects for mother and fetus. Relief of pain during labor prevents this pain-induced activation of the sympathetic nervous system and is beneficial in many ways, including:

  1. 1.

    Elimination of beta-adrenergically mediated tocolysis, possibly normalizing the labor pattern.

  2. 2.

    Prevention of increased peripheral vascular resistance and its associated decrease in uteroplacental blood flow.

  3. 3.

    Prevention of pain-induced hyperventilation leading to respiratory alkalosis, a leftward shift of the oxyhemoglobin dissociation curve, and decreased unloading of oxygen to the fetus.

  4. 4.

    Prevention of compensatory hypoventilation between contractions and associated maternal and fetal hypoxemia.

  5. 5.

    Prevention of delayed gastric and bladder emptying.

Irrespective of the physiologic benefits of controlling labor pain, epidural analgesia has added benefit of facilitating rapid conversion to surgical anesthesia should the need for emergency cesarean delivery arise [1].

Consequences of Labor Analgesia

While the treatment of pain during labor has many positive effects, there are some consequences to consider. Abrupt decreases in epinephrine-mediated tocolysis can lead to transient uterine hyperstimulation and tetanic contractions causing fetal stress and bradycardia. If neuraxial analgesia is utilized, blockade of ascending sacral spinal tracts leads to decreased secretion of endogenous oxytocin and could theoretically prolong labor.

Management of Labor Pain: Inhalation Agents

Nitrous Oxide: Nitrous oxide has historically been used very infrequently in the United States (<1 % incidence). It is co-administered with oxygen typically in 50:50 mixture using a blender or premixed cylinder and via a mask or mouthpiece. It is believed to act by stimulating the release of endogenous opioids and inhibition of descending spinal pain pathways, but this has never been fully elucidated [2]. Pharmacologically, nitrous has rapid onset and offset due to its low solubility and undergoes minimal metabolism. Its physiologic effects are limited to slight reduction in tidal volume with some compensation through an increased respiratory rate. Nitrous oxide has little-to-no effect on cardiovascular or uterine functions. The most common side effects are nausea and vomiting [2]. A systematic review of 58 studies concluded that there is less pain relief with the use of nitrous oxide when compared to neuraxial analgesia, and Apgar scores were not significantly different between the two groups. Additional research is needed given the poor quality of most studies to date [3].

Anesthetic gases: Volatile halogenated agents have limited use secondary to concern for maternal amnesia, sedation, and loss of airway reflexes as well as environmental contamination. They are administered via facemask or mouthpiece. Sevoflurane is preferable to desflurane due to the latter’s irritation to upper airways. None are currently used within the United States for labor analgesia.

Management of Labor Pain: Parenteral

Opioids : Due to their high lipid solubility and low molecular weight, opioids easily cross the placenta and have the potential to lead to neonatal respiratory depression. They can be administered either intramuscularly or intravenously. Intravenous dosing is delivered by intermittent boluses from healthcare providers or patient-controlled analgesia (PCA) . Fentanyl PCA is one of the most ideally suited methods for use in obstetrics because of its rapid onset, short duration, and lack of active metabolites. Remifentanil is also used for labor analgesia given its pharmacologic profile—rapid hydrolysis by nonspecific plasma and tissue esterases leading to short elimination half-life, short context-sensitive half-life, extensive redistribution/metabolism by fetus, demonstrated by low umbilical artery:vein concentration ratio. Remifentanil has been shown in many randomized, double-blinded trials to offer superior pain control compared to fentanyl or meperidine [4] and nitrous oxide [5]. Studies comparing remifentanil PCA to epidural anesthesia are mixed, with either similar or lower pain scores in the epidural group and increased side effects with remifentanil including sedation, hypopnea, desaturation, and need for supplemental oxygen [6, 7].

Management of Labor Pain: Regional

Epidural : Of all methods listed, epidural is the most effective means of relieving pain during labor. It is associated with decreased pain scores and increased patient satisfaction when compared to nonpharmacologic techniques, parenteral, and inhaled medications [8]. It is also the most commonly utilized technique in the United States by laboring women [9]. Few absolute contraindications to its use exist, and these include: [1]

  1. 1.

    Patient refusal

  2. 2.

    Allergy to injectate

  3. 3.

    Intracranial lesions with associated increased intracranial pressure

  4. 4.

    Local infection at the site of needle insertion

  5. 5.

    Coagulopathy

  6. 6.

    Recent anticoagulant administration (see guidelines)

  7. 7.

    Uncorrected maternal hypovolemia

Following placement of an epidural catheter, local anesthetic (LA), opioid, or a combination of the two may be used for initial bolus and maintenance of analgesia. Long-acting amide local anesthetics are typically utilized, and there appears to be no clinically significant difference between Bupivacaine and Ropivacaine, despite numerous studies comparing the two [10]. Low concentration-high volume dosing decreases the total dose required and increases patient satisfaction compared to high concentration-low volume administration of the same LA [11]. Addition of a lipid-soluble opioid to the LA decreases the concentration of LA required to achieve adequate analgesia [12] and decreases the total dose of LA in a dose-dependent fashion [13]. It has also been shown to speed onset, lengthen duration, and increase the quality of the block achieved [14]. Fentanyl and Sufentanil are most commonly used for this indication, and no significant difference has been found between the two [15]. Adjuvants including epinephrine, clonidine, and neostigmine are also occasionally utilized in epidural anesthesia , but no absolute indications for their use exist.

In addition to choice of medication and dosing regimens, methods of administration have also been studied in regard to epidural anesthesia. Currently, patient-controlled epidural anesthesia (PCEA) with or without background continuous infusion is most commonly utilized. Studies have shown that while background infusion improves analgesia, it also leads to administration of higher total doses of local anesthetic. The significance of this is questionable, since no increased motor block or difference in adverse obstetric outcomes between the two has been shown [16]. Although not yet mainstream, the use of programmed intermittent epidural boluses is being used to replace continuous infusions. Studies investigating this method of drug delivery report decrease in the overall consumption of LA and incidence of motor block while increasing patient satisfaction [17, 18].

Combined Spinal-Epidural ( CSE ) : Advantages of this technique include its significantly faster onset to effective analgesia, faster onset to sacral analgesia, and decreased incidence of failed epidural catheter. Although the technique involves puncture of the dura, with use of a small-gauge pencil-point needle, the risk for post-dural puncture headache does not appear to be increased [19].

Dural Puncture Epidural : This technique involves puncture of the dura with a small-gauge spinal needle, but no intrathecal injection of medication. Sacral coverage following epidural injection of a LA and opioid has been shown to be superior to traditional epidural, likely secondary to spread of medications through the puncture site [20].

Paracervical Block : This technique can be used during the first stage of labor to relieve pain associated with cervical dilation. It involves injection of local anesthetic lateral to the cervix. Although uncommon, maternal complications include neuropathy, hematoma, abscess, and laceration. Fetal bradycardia and direct injection into the fetal scalp leading to systemic toxicity are both possible [1].

Pudendal block : Injection of local anesthetic into the bilateral vaginal wall can partially relieve pain associated with the second stage of labor. Maternal and fetal complications of this block are similar to those associated with paracervical block [1].

Management of Labor Pain: Nonpharmacologic

Many nonpharmacologic means of pain control are utilized during labor in an attempt to minimize fetal exposure. These include intradermal water injections, transcutaneous electrical nerve stimulation, acupuncture, hypnosis, biofeedback, and hydrotherapy.