Minimum local anesthetic volume blocking the femoral nerve in 50% of cases: A double-blinded comparison between 0.5% ropivacaine and 0.5% bupivacaine

Recent studies demonstrated that ropivacaine was nearly 40% less potent tha n bupivacaine in the first stage of labor, but contrasting results have bee n reported. We, therefore, conducted a prospective, randomized, double-blin ded study to determine the effects of the ropivacaine/bupivacaine potency r atio on the minimum volume of local anesthetic required to produce effectiv e block of the femoral nerve in 50% of patients. Fifty adults premedicated with TV midazolam, 0.05 mg/kg, undergoing elective knee arthroscopy receive d femoral nerve blocks with a multiple-injection technique with a nerve sti mulator (contractions of vastus medialis, vastus intermedius, and vastus la teralis were elicited with a 0.5-mA stimulating current). Patients randomly received either 0.5% ropivacaine (n = 25) or 0.5% bupivacaine (n = 25). Th e anesthetic volume was decided according to Dixon's up-and-down method, st arting from 12 mt and being equally divided among the three elicited twitch es. Successful nerve block was loss of pinprick sensation in the femoral ne rve distribution with concomitant block of the quadriceps muscle within 20 min after injection, as assessed by a blinded observer. Positive or negativ e responses determined a 3-mL decrease or increase for the next patient, re spectively. According to the up-and-down sequences, the minimum local anest hetic volume providing successful nerve block in 50% of cases was 14 +/- 2 mt in the ropivacaine group (95% CI: 12-16 mt) and 15 +/- 2 mt (95% CI: 13- 17 mt) in the bupivacaine group (P = 0.155). We conclude that the volume of 0.5% ropivacaine required to produce effective block of the femoral nerve in 50% of patients is similar to that required when using 0.5% bupivacaine.