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REGIONAL ANESTHESIA AND PAIN MEDICINE

The Effects of the Single or Multiple Injection Technique on the Onset Time of Femoral Nerve Blocks with 0.75% Ropivacaine

Department of Anesthesiology, University of Milan and IRCCS San Raffaele Hospital, Milan, Italy

Address correspondence and reprint requests to Andrea Casati, MD, Department of Anesthesiology, IRCCS San Raffaele Hospital, Via Olgettina 60, 20132 Milan, Italy. Address e-mail to casati.andrea{at}hsr.it

	Abstract

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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.

Implications: This prospective, randomized, blinded study was conducted to evaluate the effect of searching for multiple muscular twitches when performing femoral nerve block with small volumes of 0.75% ropivacaine. Our results demonstrated that multiple injections markedly shortened the onset time and improved the quality of nerve blockade. This technique-related effect must be carefully considered when different clinical studies evaluating the use of new local anesthetic solutions for peripheral nerve blocks are compared.

	Introduction

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When evaluating new local anesthetic solutions for peripheral nerve blocks, physicians usually compare onset time, efficacy, and duration of the nerve block with those of previous anesthetic solutions. However, various factors markedly affect the efficacy outcomes of peripheral nerve blocks, including the concentration and volume of the injected anesthetic solution (1,2), the use of additives (3,4), and the technique used for both nerve location and anesthetic injection (5). At our institution, peripheral nerve blocks are routinely performed by using a multiple injection technique with the aid of a nerve stimulator (6). This technique provides adequate nerve block by using volumes of local anesthetic solution markedly less than those usually reported, with no increase in the incidence of nerve injury (7).

Evaluating the use of 0.75% ropivacaine for different peripheral nerve blocks, we recently reported onset time characteristics similar to those of a short-acting, intermediate duration anesthetic, such as mepivacaine, with successful anesthesia after injection of very small volumes of a local anesthetic (8,9). However, other authors using different injection techniques and volumes have reported different results (10,11). We therefore conducted a prospective, randomized, blinded study to evaluate the effects of the single or multiple injection technique on the onset time and efficacy of femoral nerve block performed with small volumes of 0.75% ropivacaine according to our clinical practice.

	Methods

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With ethical committee approval and written informed consent, we studied 30 inpatients of ASA physical status I and II, ages 18–65 yr, scheduled to have arthroscopic knee surgery under combined sciatic-femoral nerve block. Patients with contraindications to regional anesthesia, as well as those with respiratory or cardiac disease, diabetes, peripheral neuropathy, and those receiving chronic analgesic therapy were excluded. The sample size of the study population was calculated to detect a 5-min difference in the time required to achieve adequate surgical anesthesia between the two injection techniques, accepting a one-tailed error of 5%, and a ß error of 10% (12), according to previous findings (8).

After an 18-gauge IV cannula had been placed at the forearm, all patients received standard IV premedication with 0.05 mg/kg midazolam 10 min before placement of the block. First we performed the sciatic nerve block with 15 mL of 2% mepivacaine by using the classic Labat’s approach (7–9). Afterward, the patient was turned to the supine position and the femoral nerve block was performed by using 12 mL of 0.75% ropivacaine. A nerve stimulator and a short-beveled, Teflon-coated stimulating needle (needle length, 3.5 cm; diameter, 25-gauge) were used for block placement. The stimulation frequency was set at 2 Hz, although the intensity of stimulating current, initially set to deliver 1 mA, was gradually decreased to <0.5 mA after each muscular twitch was observed.

Patients were randomly allocated to receive a single injection (Single group, n = 15) or a multiple injection (Multiple group, n = 15) technique. After the skin of the inguinal region had been disinfected, the stopwatch was started. The stimulating needle was inserted lateral to the femoral artery at the intersection between the femoral artery and a line connecting the anterior superior iliac spine to the pubic tubercle. Paresthesias were never sought. In the Single group as soon as the first muscular twitch was observed, after aspiration had excluded intravascular needle placement, 12 mL of 0.75% ropivacaine was slowly injected. 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. If the injection of 1 mL of the study solution immediately stopped the muscular twitch elicited at 0.5 mA, the needle location was considered adequate and the remaining 3 mL of 0.75% ropivacaine was injected (6–9). At the end of local anesthetic injection, the stopwatch was stopped and block performance time was recorded. The anesthesiologist performing the nerve block then, reset the stopwatch and left the room. All blocks were performed by two anesthesiologists with substantial expertise in regional anesthesia, and were assessed by an independent, blinded observer.

The onset of sensory and motor blocks was evaluated every 2 min for the first 10 min and then, every 5 min, until surgical block was achieved. The patient was judged ready for surgery when he/she showed complete loss of pinprick sensation in the femoral nerve distribution with concomitant inability to elevate the leg from the operating table after the observer had passively flexed the patient’s hip. The stopwatch was then stopped and the time from the completion of femoral nerve injection to the achievement of surgical anesthesia, as defined previously, was recorded and surgery started. The total time required from skin disinfection to surgical anesthesia was also calculated by adding the performance time to the onset time.

The degree of pain when trocars were inserted in the knee was assessed with a 5-point verbal rating scale score (VRS: 0 = no pain, 1 = mild pain, 2 = moderate pain, 3 = severe pain, 4 = unbearable pain). If VRS 2 was reported by the patient, 0.1 mg supplemental fentanyl was given IV. If this did not provide adequate analgesia, a 1 mg/kg IV propofol bolus was given, followed by a continuous IV infusion (2–4 mg · kg^-1 · h^-1). The quality of femoral nerve block was evaluated, as follows, according to the need for supplementary IV analgesia: satisfactory nerve block = no analgesia required to complete surgery; unsatisfactory nerve block = fentanyl administration required to complete surgery; failed nerve block = fentanyl and propofol administration required to complete surgery.

Patient acceptance was evaluated 24 h postoperatively at discharge from the orthopedic ward by using a two-point score: 1 = "Good, if necessary, I will repeat it"; 2 = "Bad, I will never repeat it again." Patients were also questioned by the anesthesiologist about the occurrence of neurological complications at discharge from the orthopedic ward, and one week after hospital discharge by the orthopedic surgeon during the routine postoperative examination.

Statistical analysis was performed by using the program Systat 7.0 (SPSS, Chicago, IL). The Mann-Whitney U-test was used to compare demographic data, performance time, and time for readiness to surgery. Categorical variables were analyzed by using the contingency table analysis with Fisher’s exact test. The time from skin disinfection to readiness for surgery was also compared by a log-rank test by using the Kaplan-Meier’s survival analysis. P < 0.05 was considered significant. Results are presented mean (± SD) and median (range), or as number (%).

	Results

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No differences were observed between the two groups with respect to age (Single group: median, 44 yr; range, 24–65 yr; Multiple group: median, 41 yr; range, 25–65 yr), weight (Single group: median, 63 kg; range, 50–95 kg; Multiple group: median, 70 kg; median, 45–83 kg), height (Single group: median, 165 cm; range, 155–180 cm; Multiple group: median, 170 cm; range, 154–183 cm), and male to female ratio (Single group: 5 men/10 women; Multiple group: 7 men/8 women).

Performance time required 4.2 ± 1.7 min (median, 5 min; range, 2–8 min) with the Multiple group and 3.4 ± 2.2 min (median, 3 min; range, 1–5 min) in the Single group (P = 0.02). The time from the completion of femoral injection to achieve surgical anesthesia was 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). Figure 1 shows survival analysis of the total time required from skin disinfection until patients were judged ready for surgery with femoral nerve block placed by using either a single injection or the multiple twitches technique. Although block placement took less time with the single injection technique, total preoperative time was shorter in patients receiving the multiple injection technique, and the log-rank curves were significantly different (P < 0.0005).

View larger version (17K): [in this window] [in a new window]   Figure 1. Survival analysis of the total time required from skin disinfection to achievement of surgical block. Percentage of patients judged ready for surgery by using either single twitch (Single group, n = 15) or multiple twitch (Multiple group, n = 15) techniques. Complete loss of pinprick sensation in femoral nerve distribution and inability to extend the leg with the hip passively flexed, constituted the end point of the study for the patient concerned. The log-rank curves representing the two groups studied are significantly different (P = 0.0005).

Patient acceptance was similarly good in the two groups, and no patient stated he/she would never repeat the anesthetic procedure again. No complications were observed at hospital discharge or during routine postoperative orthopedic examination.

	Discussion

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This prospective, randomized, blinded study demonstrated that performing femoral nerve block with small volumes of 0.75% ropivacaine, using a multiple injection technique, provided a significant reduction in the time required to achieve readiness for surgery compared with the single injection technique, and provided a clinically relevant improvement in the efficacy of nerve block itself. These results are consistent with previous investigations evaluating the onset time of nerve block with 0.75% ropivacaine in different nerve blocks (8,9,13–15) and confirm findings about the usefulness of multiple injections when performing peripheral nerve blocks (6,7). In agreement with our findings, other authors (5,16,17) have reported clinically relevant improvement in onset time and quality of nerve block when using multiple injections.

Searching for multiple muscular twitches during block placement prolonged the performance time compared with injection after one single twitch; this finding was not surprising, because of the insertion and redirection of the stimulating needle during the procedure. However, this extra time for the multiple injections was compensated for by the shorter onset time of nerve block and better spread of analgesia compared with the single injection, which lead to a nearly 60% reduction in total preoperative time (the time lasting from skin disinfection until patients were judged ready for surgery). Similar results have been reported by other authors, who demonstrated longer performance time for axillary brachial plexus block when using multiple injections, but reported shorter preoperative time in patients receiving the multiple injections compared with those receiving a single injection (5).

Achievement of surgical block after injection of a local anesthetic solution depends on the concentration gradient between the injection site and the nerve, the distance between the two, and the rate of absorption from the injection site. Because volume, dose, and site of injection of 0.75% ropivacaine were the same in the two groups, a shorter distance for diffusion of local anesthetic molecules to the different branch of femoral nerve can explain these findings.

According to previous studies and our daily practice (6–9), we used smaller volumes of local anesthetic solution than those usually reported for lower limb nerve blocks (11,18). This resulted in an unacceptably high rate of fentanyl supplementation in the Single group; although increasing the dose up to 20–40 mL results in a success rate similar to what we observed in the Multiple group with only 12 mL (18). If the single injection technique is used to perform femoral nerve block, larger doses should be advocated to improve the quality of nerve block. However, the advantages of providing adequate nerve block with a minimal dose of local anesthetic solution should be carefully considered by physicians when choosing the injection technique during block placement, considering that the main morbidity risk associated with peripheral nerve blocks is systemic toxicity (19). These considerations further confirm that searching for multiple muscular twitches produces successful peripheral nerve blocks with a markedly reduced volume of local anesthetic solution.

In a large multicenter study on peripheral nerve blocks with the multiple injection technique, poor patient acceptance was reported in up to 26% of patients (7); on the contrary, in this randomized controlled trial, patient acceptance was similarly good in the two groups, and no patient refused the same anesthetic procedure for the future. Sample size of the current study does not provide adequate power to accurately evaluate patient acceptance; however, these results suggest that, if adequate sedation and amnesia are provided before block placement, inserting and redirecting the stimulating needle to elicit various muscular twitches does not worsen acceptance of the anesthetic procedure as compared with a single injection.

Despite the clear clinical advantages reported with the multiple injection technique, most anesthesiologists are concerned about the increased theoretical risk of needle trauma or intraneural injection as compared with the single injection. The aim of this study was not to evaluate the risk for nerve injury with the two injection techniques; although sufficiently powered, randomized studies should be advocated to solve this problem. However, no evidences of an increased risk for postoperative neurologic dysfunction has been reported in the literature with the multiple injection technique (7), and the incidence of neurological injury after peripheral nerve blocks ranges between 0.5/10000 and 4.8/10000 (19).

Finally, peripheral nerve blocks are associated with a decreased morbidity compared with central neuraxial blocks, such as spinal or epidural anesthesia (19). This consideration, together with the improved efficiency in total preparation times, could suggest that the use of peripheral nerve blocks with multiple injections should be advocated, rather than central nerve blocks, when appropriate for the surgical procedures.

In conclusion, this prospective, randomized, blinded study demonstrated that searching for multiple muscular twitches when performing femoral nerve block with small volumes of 0.75% ropivacaine shortened the onset time and improved the quality of nerve blockade. This effect was related only to the injection technique, and must be carefully considered when different clinical studies evaluating the use of new local anesthetic solutions for peripheral nerve blocks are compared.

	References

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Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins with the assistance of Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999