We read with interest the articles by Polley et al.  1and Benhamou et al.  2in which they simultaneously published the same study in terms of objective and design performed on different continents. Minimum local analgesic concentration results, including the potency ratios, were surprisingly different from expectations. In a similar study comparing bupivacaine and ropivacaine minimum local analgesic concentrations, Polley et al.  3in the United States and Capogna et al.  4in Italy found different results although the exact same potency ratios. This makes us wonder if the current inconsistencies in results were just intercontinental differences or something else? The possible explanations we propose that were not commented in either study are as follows: 1) No real difference between the drugs. There were no statistical differences in either study, so we should accept the fact as it is. Why did this happen, having in mind all the evidence gathered so far that there is a potency rank? No clue, except for: 2) Chance, which is always a possibility. Here is our greatest concern about the method: When we study subtle differences (i.e. , 15 or 20 percent), is the minimum local analgesic concentration method powerful enough to detect these differences without changing the number of subjects to be studied? As this issue is one of the strengths of the up and down sequential allocation method (to detect this clinically small difference), we might need more patients to be included in the design. We encourage the authors to deepen in the interpretation of the “negative” results. We are sure they are as surprised as we are. 3) End point: Is ED50for pain relief in first-stage labor in the vicinity of the lower part of the dose-response curve for both drugs? Can this be the same kind of effect, but on the opposite part of the curve, when we use supramaximal doses and the potency relationship is lost? Maybe if we analyze another end point, namely motor block, a higher dosage requirement and potency differences will become more apparent.5 

Finally, we agree with Benhamou et al.  that further studies are required to verify this hypothesis.

* Pontificia Universidad Catolica de Chile, Santiago, Chile, and Duke University Medical Center, Durham, North Carolina. lacas001@mc.duke.edu

Polley LS, Columb MO, Naughton NN, Wagner DS, van de Ven CJ, Goralski KH: Relative analgesic potencies of levobupivacaine and ropivacaine for epidural analgesia in labor. Anesthesiology 2003; 99:1354–8
Benhamou D, Ghosh C, Mercier FJ: A randomized sequential allocation study to determine the minimum effective analgesic concentration of levobupivacaine and ropivacaine in patients receiving epidural analgesia for labor. Anesthesiology 2003; 99:1383–6
Polley LS, Columb MO, Naughton NN, Wagner DS, van de Ven CJ: Relative analgesic potencies of ropivacaine and bupivacaine for epidural analgesia in labor: Implications for therapeutic indexes. Anesthesiology 1999; 90:944–50
Capogna G, Celleno D, Fusco P, Lyons G, Columb M: Relative potencies of bupivacaine and ropivacaine for analgesia in labour. Br J Anaesth 1999; 82:371–3
Lacassie HJ, Columb MO: The relative motor blocking potencies of bupivacaine and levobupivacaine in labor. Anesth Analg 2003; 97:1509–13