The effects of single or multiple injections on the volume of 0.5% ropivacaine required for femoral nerve blockade

We compared the effects of using a single- or multiple-injection technique on the volume of 0.5% ropivacaine required to block the femoral nerve, in a prospective, randomized, blinded fashion in which 50 premedicated patients received a femoral nerve block with 0.5% ropivacaine by use of a nerve sti mulator and either a single (n = 25) or multiple- (n = 25) injection techni que. Muscular twitches were elicited at less than or equal to0.5 mA before anesthetic injection. The designated volume of local anesthetic was equally divided among contraction of the vastus medialis, vastus intermedius, and vastus lateralis for the multiple injections, or it was injected at the con traction of the vastus intermedius with motion of the patella for the singl e injection. The local anesthetic volumes were varied for consecutive patie nts by using an up-and-down staircase method; a blinded observer determined the adequacy of nerve blockade (loss of pinprick sensation in the medial, patellar, and lateral portions of the knee, with concomitant block of the q uadriceps muscle) 20 min after injection. The mean (95% confidence interval ) volume required for blocking the femoral nerve with the multiple-injectio n technique (14 [12-16] mL) was significantly smaller than that observed wi th the single injection (23 [20-26] mL) (P = 0.001). According to logistic regression analyses, the 95% effective volumes of ropivacaine required to b lock the femoral nerve within 20 min after injection were 29 and 21 mL with a single or multiple injection, respectively. We conclude that searching f or multiple muscular twitches reduces the volume of 0.5% ropivacaine requir ed to produce blockade of the femoral nerve.