Local anesthetic requirements are greater in dystocia than in normal labor

Dystocia is characterized by abnormal progress of labor and is a common contemporary indication for cesarean delivery in the United States. There has been considerable controversy as to whether epidural analgesia causes dysfunctional labor leading to cesarean delivery for dystocia. The minimum local analgesic concentration (MLAC) is a clinical model used to determine the relative potencies of local anesthetics in the first stage of labor. In this article, the authors report a prospective study determining the MLAC of bupivacaine in early labor of parturients who eventually delivered either vaginally or via cesarean section.

An up-down sequential allocation technique was used to determine the MLAC of bupivacaine in 57 nulliparous parturients assigned to either vaginal delivery or cesarean section arms. In addition, patients were assigned to groups receiving or not receiving intravenous oxytocin at the time of epidural placement. Only patients who delivered by the assigned delivery mode were included in the MLAC analyses.

Parturients who later delivered vaginally had 25% and 31% lower MLAC values (0.078% and 0.085% wt/vol bupivacaine, receiving or not receiving intravenous oxytocin, respectively) than those who later delivered by cesarean section (0.102% and 0.106% wt/vol bupivacaine, receiving or not receiving intravenous oxytocin, respectively).

These data suggest that an increased local anesthetic requirement for epidural labor analgesia is associated with more intense pain related to dystocia. Women in early, clinically normal labor but who later develop dystocia require more local anesthetic and, by inference, are experiencing more severe pain than women who deliver vaginally. This association should be considered when studying the relation between the method of labor analgesia and the course of labor.