To the Editor:—
We read with interest the study by Gautier et al. 1that compared bolus administration of epidural bupivacaine 0.125% wt/vol and epidural ropivacaine 0.125% wt/vol in combination with 7.5 μg sufentanil for labor analgesia in 90 patients. Given the relatively high concentrations used, it is not surprising that both groups experienced effective analgesia. After the third epidural injection, the ropivacaine group demonstrated significantly less motor block. Recognizing that the two initial study groups may not have received equipotent concentrations, an additional 40 patients were randomized to receive bupivacaine 0.125% wt/vol with 7.5 μg sufentanil or bupivacaine 0.100% wt/vol with 7.5 μg sufentanil. The authors are to be commended in attempting to adjust their methodology to consider possible potency differences. We assume the aim was to determine if ropivacaine 0.125% wt/vol appeared clinically similar to 0.100% wt/vol bupivacaine in combination with sufentanil.
Bupivacaine concentrations of 0.125% wt/vol are at the top of the analgesic concentration–response curve, 2and the addition of sufentanil results in significant reductions in bupivacaine requirements. 3All four of the tested combinations are likely in the flat upper portion of the curve, where differences in analgesic potency are obscured. The authors concluded that patients in the 0.100% wt/vol bupivacaine–sufentanil group experienced inferior analgesia. This conclusion seems to be based on patient feedback elicited during the postpartum period and the greater number of epidural injections, because there was no significant difference in prospective visual analog pain scores. It should also be noted that this group was studied closer to the time of delivery, and because bupivacaine requirements increase with progression of labor, 4this may account for the more frequent bolus doses.
In addition, the authors state that “most clinical studies suggest that ropivacaine is approximately 20% less potent than bupivacaine” and reference our work. 5In fact, our study demonstrated that ropivacaine was approximately 40% less potent than bupivacaine with a potency ratio of 0.6. Therefore, in the study by Gautier et al. , the 0.125% wt/vol ropivacaine group would be approximately equipotent with a 0.075% wt/vol additional bupivacaine group. We fully agree with the authors that equipotent concentrations must be determined before making comparisons of local anesthetic side effects.