Double-blind Comparison of Morphine and Bupivacaine for Continuous Epidural Analgesia in Labor

Abstract

In a double-blind, randomized study 16 healthy parturients received epidural morphine, 0.025%, or bupivacaine, 0.25%, for continuous lumbar epidural analgesia in active labor. Morphine was dissolved in dextrose to avoid possible inhibition by saline of agonist-receptor binding. Adequacy of analgesia was assessed using a simple pain relief score and by observing maternal blood pressure, pulse rate and response to pin scratch. Skin temperature was measured to identify sympathetic blockade. Neonatal status was determined by Apgar scroes, umbilical cord blood gas value and neurobehavioral assessment at 2-4 and 20-26 h postpartum. Bupivacaine provided pain relief in all 8 subjects, while morphine produced acceptable relief in only 2 subjects given 2.5 mg and 3.5 mg, respectively (P < 0.05). Six parturients having unsatisfactory analgesia with morphine later achieved good pain relief with 2-chloroprocaine, 2%. Blood pressure decreased 5 min after bupivacaine (P < 0.05) but not after morphine. Changes in blood pressure were transient, with no patient requiring vasopressor therapy. Skin temperature increased after bupivacaine (P < 0.05) but not after morphine. Numbness to pin scratch was demonstrated in both morphine subjects with pain relief and in all bupivacaine subjects. Apgar scores and umbilical cord blood gas values were similar in both groups. Borderline status in the Scanlon neurobehavioral examination occurred in 6 neonates in the morphine group at the first assessment while all bupivacaine neonates were normal (P < 0.05). When 2-chloroprocaine was given after morphine, its mean duration of action was increased from a normal value of 40-65 min to 83 .+-. 5.5 min. Morphine in dextrase, in the low doses employed, is unsuitable for continuous epidural analgesia in labor. Insufficient occupancy of opiate receptors by morphine may be the explanation for the findings.