The effects of the single or multiple injection technique on the onset time of femoral nerve blocks with 0.75% ropivacaine

We evaluated the effect of the injection technique on the onset time and efficacy of femoral nerve block performed with 0.75% ropivacaine. A total of 30 patients undergoing arthroscopic knee surgery were randomly allocated to receive femoral nerve blockade with 0.75% ropivacaine by using either a single injection (Single group, n = 15) or multiple injection (Multiple group, n = 15). Nerve blocks were placed by using a short-beveled, Teflon-coated, stimulating needle. The stimulation frequency was set at 2 Hz, and the intensity of stimulating current, initially set at 1 mA, was gradually decreased to <0.5 mA after each muscular twitch was observed. In the Single group, 12 mL of 0.75% ropivacaine was slowly injected, as soon as the first muscular twitch was observed. In the Multiple group, the stimulating needle was inserted and redirected, eliciting each of the following muscular twitches: contraction of vastus medialis, vastus intermedius, and vastus lateralis. At each muscular twitch, 4 mL of the study solution was injected. Placing the block required 4.2 +/- 1.7 min (median, 5 min; range, 2-8 min) in the Multiple group and 3.4 +/- 2.2 min (median, 3 min; range, 1-5 min) in the Single group (P = 0.02). Onset of nerve block (complete loss of pinprick sensation in the femoral nerve distribution with concomitant inability to elevate the leg from the operating table with the hip flexed) required 10 +/- 3.7 min in the Multiple group (median, 10 min; range, 5-20 min) and 30 +/- 11 min in the Single group (median, 30 min; range, 10-50 min) (P < 0.0005). Propofol sedation was never required to complete surgery; although 0.1 mg fentanyl at trocar insertion was required in two patients of the Multiple group (13%) and nine patients of the Single group (60%) (P = 0.02). We conclude that searching for multiple muscular twitches shortened the onset time and improved the quality of femoral nerve block performed with small volumes of 0.75% ropivacaine.