To the Editor:
Meister et al. (1) compared epidural analgesia with 0.125% ropivacaine/fentanyl versus 0.125% bupivacaine/fentanyl during obstetric labor. The local anesthetics used were of equal concentration but were not equianalgesic. Minimum local analgesic concentration studies have demonstrated that ropivacaine has only 0.6 the potency of bupivacaine (2,3), and therefore, equiconcentration but not equipotent drugs have been used. The concentrations used, 0.125% local anesthetic, are both above their calculated 95% effective doses, and therefore, both would be expected to provide effective analgesia, as demonstrated in the study.
That significantly less motor block was seen in the ropivacaine/fentanyl group is not surprising in view the lower potency of ropivacaine. The authors state that ropivacaine cannot be less potent than bupivacaine as less supplemental analgesia was needed in the ropivacaine group. This simply reflects the longer half-life of ropivacaine compared with bupivacaine, a result of its S-enantiomer form and its greater vasoconstrictor action. The conclusion that the drugs appear to be equipotent at clinically used concentrations only reflects the higher than 95% effective dose concentrations being used.
Amanda J. Pinder BSc, MBChB, FRCA
Martin Dresner MBBS, FRCA
1. Meister GC, D’Angelo R, Owen M, et al. A comparison of epidural analgesia with 0.125% ropivacaine with fentanyl versus 0.125% bupivacaine with fentanyl during labor. Anesth Analg 2000; 90: 632–7.
2. Polley LS, Columb MO, Naughton NN, et al. Relative analgesic potencies of ropivacaine and bupivacaine for epidural analgesia in labor: implications for therapeutic indexes. Anesthesiology 1999; 90: 944–50.
3. Capogna G, Celleno D, Fusco P, et al. Relative potencies of bupivacaine and ropivacaine for analgesia in labour. Br J Anaesth 1999; 82: 371–3.