The important finding of our systematic review and meta-analysis was an increased risk for instrumental vaginal delivery in patients who used PCEA with a continuous background infusion compared with PCEA alone for maintenance of epidural labor analgesia. Although the risk for cesarean delivery was not different between the 2 groups, the confidence interval for the relative risk of cesarean delivery was wide and included a clinically significant difference.
The relative risks we calculated for instrumental vaginal and cesarean deliveries demonstrated different directions of effect. This discrepancy may be due to the low number of patients included in the analysis. Our risk estimate for instrumental vaginal delivery was heavily influenced by 1 study10; all other studies crossed the line of no effect with fairly large confidence intervals.
The addition of a background infusion to PCEA also led to a prolonged duration of the second stage of labor by approximately 12 minutes; the clinical significance of this prolongation is unclear. The rate of patients requiring physician-delivered boluses was lower when PCEA was combined with a CI; the number of PCEA boluses was also lower in this treatment arm. The consumption of ropivacaine equivalents was higher in the PCEA + CI group than in the PCEA-only group, but there was significant heterogeneity among studies.
One would expect that higher local anesthetic consumption would result in a greater incidence of motor block, but pooling of data from the studies we reviewed showed no difference in the number of patients with no motor weakness. In vitro studies28 show that a constant infusion leads to higher extraneural levels of local anesthetics than intermittent boluses, which allow extraneural levels to decline in the interval between 2 boluses. The high extraneural concentration of local anesthetics may cause high intraneural concentration, and these levels may become sufficiently high to cause motor blockade. There are several possible reasons that could explain our finding no differences in motor weakness when adding a background infusion to PCEA. First, there may be subtle changes in the degree of motor block that were not assessed. Second, the weakness or block of peripheral muscles, usually of the lower limb, may not correlate with the effect on muscles involved in childbirth. Third, despite differences between groups, the amount of drug may be too low to cause a motor block, even in the higher-dose group. Some support for this hypothesis comes from a meta-analysis that compared continuous epidural infusions with PCEA-only.1 The trials included in that meta-analysis also reported on the number of parturients without motor weakness and found a higher percentage in the PCEA-only group. The risk for instrumental vaginal delivery, however, was not increased, suggesting that there may be no association between instrumental delivery and motor block.
There are some limitations to our systematic review. Heterogeneity among studies in our analysis could have been caused by differences in techniques among studies, including the concentration of the local anesthetic used in the study arms, early versus late initiation of epidural analgesia, and initiation of analgesia with a combined spinal-epidural versus a traditional epidural analgesia technique. In a recent meta-analysis, Sultan et al.30 showed that low concentrations of local anesthetics for labor epidural analgesia reduce the risk for instrumental vaginal delivery compared with high concentration solutions. Bupivacaine concentrations below 0.1% and ropivacaine below 0.17% were considered low in this context. Of the studies in our review, 6 trials used a concentration that met the above definition. The trial by Ferrante et al.21 administered 0.125% bupivacaine. The concentration of the local anesthetic, therefore, is unlikely to have a major impact on the association between CI and the risk of instrumental vaginal delivery in our meta-analysis. Because low-concentration solutions are now the norm, the conclusion from our meta-analysis appears generalizable to current practice.
Another parameter that varied among studies included in our analysis is the timing of the initiation of labor analgesia. Early initiation was performed in 2 studies,21,26 and at a cervical dilation of <5 cm in the study by Lim et al.25 Women at >5 cm cervical dilation were excluded in the study by Haydon and colleagues.27 No details were given in 3 studies.22–24 The effect of the type of neuraxial analgesia should also be considered. Combined spinal-epidural (CSE) analgesia was used in 3 studies23,25,27 and epidural analgesia in 4 studies included in our meta-analysis. A Cochrane review meta-analysis31 found a lower incidence of assisted vaginal delivery in women randomly assigned to receive CSE analgesia compared with traditional (high-dose) epidural analgesia, but CSE analgesia did not perform better compared with low-dose epidural regimens. A further limitation of our review was that not all included studies reported on all outcome variables of interest.
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