Mechanisms of labor analgesia

Abstract

Labor pain is perceived as moderate to severe pain by the majority of parturients. Untreated labor pain can activate the sympathetic nervous system and increase catecholamine levels. Increased catecholamines can cause uterine vasoconstriction, tocolysis and cardiorespiratory stimulation. Untreated labor pains can have psychological consequences such as depression. Labor pain is transmitted by separate overlapping neural pathways during three stages of labor. Pain during the first stage of labor is predominantly visceral and transmitted by the sympathetic fibers originating from T10 to L1. Second-stage labor pain is predominantly somatic pain and transmitted by S2-S4 fibers of the pudendal nerve. Regional analgesia is the most effective mode of analgesia and is that which is received by the majority of women in labor. Epidural and combined spinal and epidural analgesia performed with either bupivacaine or ropivacaine are the most commonly utilized types of analgesia. Fentanyl is often used as an adjuvant medication which serves to improve the quality and duration of pain relief as well as to decrease the amount of local anesthetic required. A lower dose or volume of local anesthetic often translates into a decreased incidence of hypotension and motor blockade. Patient-controlled analgesia (PCA) with continuous intravenous infusion of remifentanil is an alternative to regional analgesia for women who have contraindications to the placement of an epidural catheter. Paracervical and lumbar plexus blocks are less commonly utilized and are useful only for pain in the first stage of labor. Pudendal nerve block and perineal infiltration are exclusively used for pain in the second stage of labor.