In Reply:—

We appreciate the interest in our work and the comments by Drs. Polley and Columb. The exact determination of the relative potency of ropivacaine when compared with bupivacaine is an important issue. It directly questions the advantages associated with the use of this new local anesthetic, i.e. , reduced incidence of motor blockade and lower central nervous system or cardiac toxicity.

We agree with the comments of Drs. Polley and Columb. Our study design was clearly in favor of ropivacaine: relatively important doses were used, sufentanil was added, and early stage of labor was considered. Nevertheless, even in these conditions, our conclusion was clear: ropivacaine 0.125% with sufentanil affords reliable analgesia with minimal motor blockade when compared with bupivacaine. However, this benefit is not of major clinical importance because it becomes obvious only after the third epidural injection and has no influence on the rate of instrumental delivery.

With regard to the relative potency of ropivacaine when compared with bupivacaine, when the study was designed (end of 1997/beginning of 1998), there were not many studies that clearly quantified the lower potency of ropivacaine. This is obvious when considering studies performed in obstetrics 1,2or using intrathecal ropivacaine in human volunteers. 3We also studied intrathecal ropivacaine in a clinical situation 4and were able to confirm the 50% difference in potency reported by McDonnald et al.  3 

Peres da Silva, Abboud TK, Lee J: Comparison of 0.07% ropivacaine plus sufentanil and 0.07% bupivacaine plus sufentanil for epidural anesthesia during labor and delivery (abstract). A NESTHESIOLOGY 1997; 87:A882
Capogna G, Celleno D, Fusco P, Lyons G, Columb M: Relative potencies of bupivacaine and ropivacaine for analgesia in labor. Br J Anesth 1999; 82:371–3
Mc Donnald, Liu S, Kopacz D, Stephenson C: Hyperbaric spinal ropivacaine: A comparison to bupivacaine in volunteers. A NESTHESIOLOGY 1999; 90:971–7
Gautier P, De Kock M, Van Steenberge A, Poth N, Lahaye-Goffart B, Fanard L, Hody JL: Intrathecal ropivacaine for ambulatory surgery: A comparison between intrathecal bupivacaine and intrathecal ropivacaine for knee arthroscopy. A NESTHESIOLOGY 1999; 91:1239–45