Labor Analgesia: Pain Relief Options During Childbirth

Labor Pains

• Labor pains arise from different genital structures during childbirth and parturition. The factors responsible are:
  • It is similar to ischemic pain caused by hypoxia of contracted myometrium.
  • The stretching of the peritoneum overlying the fundus.
  • The compression of nerve ganglia in the cervix and lower uterus.
  • It is referred pain owing to:
    • The uterine contractions via the inferior hypogastric plexus from T10 to L1 segments. 
    • The cervical dilatation via the sacral plexus to the S2, S3, and S4 segments.
  • Localized perineal pain via the pudendal nerve.
  • Referred pain can occur due to compression of the bladder, urethra, and rectum.

Pharmacological Pain Relief

• Pharmacological pain relief management includes:
  • Regional blocks or local anesthesia, given by an obstetrician. These include paracervical blocks and pudendal nerve blocks.
  • Parenteral agents
  • Regional analgesia or epidural analgesia

Also read: Clinical Management of the Third Stage of Labor: Techniques and Best Practices

Principles Of Pain Relief

• Concerns with labor pains.
• Concerns due to physiological changes of pregnancy.
• Maternal factors that may prompt anesthetic consultation.
• If and what is an ideal analgesic?

Concerns With Labor Pains

• There is an increase in stress hormones called catecholamines.
• There is an increase in basal metabolic rates.
• There is an increase in oxygen consumption.
• Due to sympathetic stimulation, there is tachycardia, an increased cardiac output, and high blood pressure during labor.
• The catecholamines can lead to uterine artery vasoconstriction, leading to impaired oxygen delivery to the fetus.
• Because of pain, there can be hyperventilation, which can cause hypocarbia. Hypocarbia further decreases the ventilatory drive in between the contractions. 
• There is an increasing tendency for respiratory alkalosis, which leads to a left shift of the hemoglobin oxygen dissociation curve. These can lead to impaired oxygen delivery to the fetus.
• Furthermore, these concerns can be aggravated due to dehydration and maternal exhaustion.
  • Thus, it is crucial to maintain a safe environment for the patient and the fetus.
• Medical conditions or fetal compromise can add to these concerns.

Also read: Understanding and Managing Right Occiput Posterior (ROP) Position in Labor

Concerns Due To Physiological Changes Of Pregnancy

• If opioids are used for labor pain analgesia, these can cause side effects such as respiratory depression and decreased ventilation drive. 
• During pregnancy, there is a 20% decrease in functional residual capacity. As a result, there is a decreased supply of oxygen. This, coupled with increased consumption of oxygen, can lead to rapid desaturation in laboring women with periods of apnea and airway obstruction.
  • Obesity and smoking can worsen these physiological changes. In obese women, there can be airway closure while lying in a supine position.

• During labor, the gastric emptying time is delayed. The patient may also demand epidural analgesia, which is why the patient is kept on clear fluids. As a side effect, the opioids can also decrease the emptying time.
• Before anesthesia, H2 receptor antagonist drugs are given to decrease acid production. These are a must, as anesthesia increases the risk of aspiration; thus, these drugs act as aspiration prophylaxis. Opioids can increase the risk of aspiration by causing respiratory depression and delayed emptying time. Furthermore, metoclopramide is given to increase the lower esophagus sphincter tone to decrease the risk of aspiration.

Also read: Preconception Counseling: Optimizing Health for a Successful Pregnancy

Maternal Factors That May Prompt Anesthetic Consultation

• It is crucial to identify the maternal factors that may prompt anesthetic consultation as an obstetrician.
  • Anatomical abnormalities of the face, neck, or spine.
  • Thyromegaly
  • Cardiac, pulmonary, renal, hepatic, or neurological disease.
  • Bleeding disorders
  • History of prior anesthetic complications
  • Obesity, specifically morbid obesity with a BMI more than 40 kg/m2.
  • Anticipated operative delivery- It is crucial to anticipate any potential complications.
  • Severe hypertension
  • Obstetrical hemorrhage

Ideal Analgesic

• The ideal analgesia should:
  • Provide good analgesia.
  • Safe for the mother and baby.
  • Be predictable and constant in its response/effects. Inconsistency in pain relief will lead to dissatisfaction in the patient. For instance, opioids exert inconsistent pain relief. Some patients can feel drowsy, and on the contrary, some patients do not note any symptomatic relief.
• The analgesia should be reversible, if necessary.
• It should be easily administrable.
• There should be no loss of maternal consciousness.
• It should not interfere with the uterine contractions.
• It should not interfere with the mobility of the patient. Anesthesia that might render a patient bedridden contradicts the goals of providing labor care, as it should not interfere with the patient's mobility, considering the hypercoagulable state of pregnancy and the risk of deep vein thrombosis (DVT).

Also read: Effective Study Techniques for OBS-GYN Residents

Paracervical Block

• The paracervical block is given in 4 o'clock and 8 o'clock positions instead of 3 o'clock and 9 o'clock positions as it is the location of the descending cervical branch of the uterine artery. This way, systemic anesthetic toxicity can be prevented by avoiding giving local anesthetics in the blood vessels.
• It can give satisfactory pain relief during the first stage of the labor, as there will be no blockage of the pudendal nerve.
• It consists of 5-10 ml of 1-2% lidocaine solution.
• In clinical practice, the paracervical block is not given in the labor room. However, it is a good block in Medical Termination of Pregnancy (MTP) and Dilation and Curettage (D&C) procedures, where the goal is to get rid of the cervical dilatation pain. However, the use of this block in the labor room can lead to fetal bradycardia. This is a consequence of drug-induced vasospasm of the uterine artery-mediated uterine insufficiency.

Pudendal Nerve Block

• This route involves vaginal entry to identify the ischial spine. The target is to stay slightly medial and below the ischial spine. The local anesthetic is infiltrated after piercing the sacrospinous ligament. A crucial landmark is the ischial vessels present on the lateral side of the ischial spine, which should be avoided to prevent vascular injury. This nerve block requires expertise to prevent injury or systemic injection of the nerve block.
• There are specially designed introducers available that allow only 1-1.5 cm of the needle to poke out from the tip of the introducer; otherwise, spinal needles can be used. Stepwise precautions are required while aspirating the needle to check if it has pricked the vessel.
• It provides pain relief in the second stage of labor or after delivery for repair of perineal lacerations.
• This block is ineffective for the exploration of the cervix, upper vagina, and the uterine cavity.
• The pudendal nerve supplies to the perineal muscle and external anal sphincter; thus, the nerve block can cause motor blockade of these anatomical sites.

Also read: Placental Development And Circulation

Limitations Of Regional Nerve Blocks

• Expertise is needed to conduct these types of nerve blocks.
• It has the potential for systemic toxicity. For instance, these blocks can cause cardiovascular complications like an increase in heart rate and blood pressure within one minute of intravenous injection. On the other hand, in the central nervous system (CNS), the patient may complain of lightheadedness, dizziness, tinnitus, and metallic taste. System introduction of these nerve blocks will first lead to CNS excitation, followed by depression.
• These can cause hematoma formation. For instance, paracervical block can cause broad ligament hematoma, which if missed out can cause the patient to bleed. There could be associated abdominal pain, shock, and delayed identification. Similarly, pudendal nerve block-associated hematoma can happen in retroperitoneal space, which may cause delayed identification. This is why these are not so widely practiced by obstetricians.
• Other complications may include infection due to hematoma and allergic reactions. Lastly, these are short- acts and need to be repeated.
• Local anesthetics cause reversible blockage of nerve conduction by sodium channel blockage for 50-90 minutes.

Parenteral Agents For Pain Relief

• Some of the names include meperidine, morphine, butorphanol, fentanyl, and ramifentanyl. These are synthetic opioid analgesics.
• A Cochrane review has failed to find an ideal parenteral opioid agent.
• Our body has endogenous opioids, which act via delta receptors present in the CNS and dorsal horn of the spinal cord. On the contrary, synthetic opioids act via mu (μ), or kappa (κ) receptors. Thus, these synthetic opioids can cause side effects potentially similar for all agents. The difference lies in the duration of action, so labor room protocols are customized intra-institutionally, and whether or not to use these agents is a matter of choice.
• The most commonly used are meperidine or morphine. Other agents are less preferred due to limited practical exposure to these agents and limited availability. One thing to consider here is ultra short duration of action for butorphanol, fentanyl, and ramifentanyl. These days, patient-controlled analgesia is utilized, thus short-acting opioids can be used in such settings.

• The above table gives the pharmacological profile of meperidine and morphine. 
• The quality of pain relief is not optimal for meperidine; it has to be repeated.
• Opioids are given along with promethazine (Phenergan), an antihistamine with sedative, anti-anxiety, and antiemetic properties. This is given in the dose of 25mg IM. Promethazine can reduce opioid doses. The dose should not exceed 100 mg in 24 hours.
• These agents, although they provide pain relief, are unreliable.

• They are available, cheaper, and are easy to use.
• Maternal side effects include nausea, vomiting, sedation, respiratory depression, pruritus, bradycardia, and constipation (increased gastrointestinal transit time).
• Fetal side effects include:
  • Decreased fetal heart rate (FHR) variability and baseline FHR.
  • Neonatal respiratory depression.
  • Low Apgar scores.
  • Neurobehavioral alterations.
  • Decreased early breastfeeding, as the suckling reflex of the newborns is affected.
  • The fetal side effects are dose- and duration-dependent. These agents should not be used in the absence of naloxone (opioid antagonist). Naloxone can reverse neonatal respiratory depression and can be life-saving.

• Meperidine forms an active metabolite called normeperidine with a prolonged half-life in the neonatal period. It can be up to 72 hours with an intramuscular dosage.
• The neonatal half-life of morphine is about 7 hours.
• Neonates born 2-3 hours following meperidine administration are frequently affected by respiratory depression.
  • Thus, opioid analgesics are not preferred in the second and advanced period of labor.
• Neonates born after 4 hours of administration of these opioid analgesics have low chances of respiratory depression.
• Thus, considering the side effects of these analgesics, epidurals are considered safer analgesics. Epidurals are also associated with severe side effects, although they are rarely encountered. Opioid analgesics should be used in resource-constrained settings where epidurals are not available.

Also Read: Neuroendocrinology Of Female Reproduction And Steroid Hormones

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