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REGIONAL ANESTHESIA

The Effects of Single or Multiple Injections on the Volume of 0.5% Ropivacaine Required for Femoral Nerve Blockade

University of Milan, Department of Anesthesiology, IRCCS H. San Raffaele, Milan, Italy

Address correspondence to Dr. A. Casati, Department of Anesthesiology, IRCCS H. San Raffaele, Via Olgettina 60, 20132 Milan, Italy. Address e-mail to casati.andrea{at}hsr.it

	Abstract

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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 stimulator and either a single- (n = 25) or multiple- (n = 25) injection technique. Muscular twitches were elicited at 0.5 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 contraction of the vastus intermedius with motion of the patella for the single injection. The local anesthetic volumes were varied for consecutive patients 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 quadriceps muscle) 20 min after injection. The mean (95% confidence interval) volume required for blocking the femoral nerve with the multiple-injection technique (14 [12–16] mL) was significantly smaller than that observed with the single injection (23 [20–26] mL) (P = 0.001). According to logistic regression analyses, the 95% effective volumes of ropivacaine required to block the femoral nerve within 20 min after injection were 29 and 21 mL with a single or multiple injection, respectively. We conclude that searching for multiple muscular twitches reduces the volume of 0.5% ropivacaine required to produce blockade of the femoral nerve.

Implications: We evaluated the effects of using a single- or multiple-injection technique onthe volume of 0.5% ropivacaine required to block the femoral nerve. The 95%effective concentration values for producing the same degree of sensory andmotor blockade of the femoral nerve within 20 min after injection were 29 mLafter elicitation of a patella twitch and 21 mL when the three main branchesof the femoral nerve were identified, potentially leading to an importantbenefit for patients receiving peripheral nerve blocks.

	Introduction

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Although using large volumes of local anesthetic solution improves the success rate and predictability of peripheral nerve blocks (1), it may also increase the risk for local anesthetic-related systemic toxicity (2,3). Systemic reactions induced by local anesthetic solutions can be related to accidental intravascular injection and are influenced by the total dose (3); using the minimum effective volume of local anesthetic required to successfully block a peripheral nerve could theoretically decrease the risk of a systemic reaction (3). The clinical relevance of this assumption further increases when a combination of different nerve blocks is used, such as for lower-limb procedures.

Nerve stimulators allow a multiple-injection technique by eliciting different muscular twitches during block placement (4–6); this technique provides effective peripheral nerve blocks with volumes of local anesthetic solution markedly less than those usually reported, reportedly without increasing the risk for nerve injury (2,7). However, no studies have evaluated the effects of using a multiple-injection technique on the volume of local anesthetic solution required to block a peripheral nerve.

The aim of this prospective, randomized, blinded study was to compare the effects of using a single- or multiple-injection technique on the volume of 0.5% ropivacaine required to block the femoral nerve.

	Methods

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With ethical committee approval and written informed consent, we studied 50 ASA physical status I and II inpatients, aged 18–65 yr, scheduled for elective knee arthroscopy under combined sciatic/femoral nerve block. Patients with contraindications to regional anesthesia; patients with respiratory or cardiac disease, diabetes, or peripheral neuropathy; and patients receiving chronic analgesic therapy were excluded.

After an 18-gauge IV cannula had been placed at the forearm, all patients received IV midazolam premedication (0.05 mg/kg) 10 min before we placed the block. First we performed a sciatic nerve block with 10 mL of 2% mepivacaine by using the classic Labat’s approach with a nerve stimulator (Plexival, Medival, Italy) and a short, beveled, Teflon-coated, 12-cm, 22-gauge stimulating needle (Locoplex, Vygon, France). The volume of local anesthetic solution was equally divided between the plantar flexion of the foot and toes and the dorsiflexion of the foot and toes (7,8). Afterward, the patient was turned to the supine position and a femoral nerve block was placed with a 3.5 cm, 25-gauge stimulating needle. The stimulation frequency was set at 2 Hz, and the intensity of stimulating current, initially set to deliver 1 mA, was gradually decreased to <0.5 mA after each designated muscular twitch had been observed. The stimulating needle was inserted lateral to the femoral artery at the level of the inguinal crease. Paresthesias were never sought.

All femoral nerve blocks were performed with a 0.5% ropivacaine solution (Naropin; AstraZeneca, Sodertalje, Sweden).

By use of a computer-generated number sequence, patients were randomly allocated to receive either a single-injection (Single-Injection group, n = 25) or a multiple-injection (Multiple-Injection group, n = 25) femoral block according to the technique previously published (4,8). Because the femoral nerve divides into its branches at or above the level of the inguinal ligament (9), in the multiple-injection group the total volume of local anesthetic solution was the sum of volumes injected after appropriate identification of the three main branches of the femoral nerve: the vastus medialis nerve (contraction of the vastus medialis muscle), the vastus intermedius nerve (contraction of the vastus intermedius muscle with patella movement), and the vastus lateralis nerve (contraction of the vastus lateralis muscle) (9). In each case, the stimulating needle was inserted perpendicular to the skin until contraction of vastus medialis was elicited with a stimulating intensity 0.5 mA; then, after aspiration excluded intravascular placement and the injection of 1 mL of the study solution immediately stopped the muscular twitch, the needle location was considered adequate, and the remaining volume of local anesthetic solution was injected (7,8). Afterward, the needle was advanced in the same direction until movement of the patella was observed. If this did not occur, the stimulating needle was withdrawn to the skin and redirected slightly laterally, increasing the angle by nearly 5° until eliciting the contraction of the vastus intermedius muscle, where the designated volume of 0.5% ropivacaine was injected as previously described. Finally, the stimulating needle was again withdrawn to the skin and redirected laterally, increasing the angle by nearly 5°, and the maneuver was repeated until successful elicitation of the contraction of the vastus lateralis. In case it was not possible to elicit one of the three muscular twitches within 10 min of skin disinfection, the patient was excluded from the study, and the next patient received the same anesthetic volume. In the Single-Injection group, the designated volume of local anesthetic solution was slowly injected after elicitation of the contraction of the vastus intermedius muscle with movement of the patella at 0.5 mA.

The same two anesthesiologists performed all blocks. Sensory and motor blocks were evaluated 20 min after injection by an independent blinded observer, who was not present during injection and was unaware of both the injection technique and the volume of local anesthetic solution injected. Sensory block was defined as complete loss of pinprick sensation (20-gauge hypodermic needle) on the medial, patellar, and lateral portions of the knee. Motor block was defined as the inability to extend the leg of the operated limb against gravity with the hip passively flexed at 90°.

We sought, by using a staircase method, to determine the volume of 0.5% ropivacaine required to produce sensory and motor block of the femoral nerve within 20 min after injection (10,11). Each trial started with an arbitrary volume of local anesthetic as determined from our clinical experience (4,7,8). The initial volume of 0.5% ropivacaine was 12 mL in the Single-Injection group and 4 mL per injection (total of 12 mL) in the Multiple-Injection group. The outcome of each patient’s response determined the volume of local anesthetic for the next patient. When sensory and motor blocks were present 20 min after local anesthetic injection, the volume used for the next study patient was decreased by 3 mL in the Single-Injection group or by 1 mL per site of injection in the Multiple-Injection group. Conversely, when sensory and motor blocks were not simultaneously present 20 min after local anesthetic injection, the volume used for the next study patient was increased by 3 mL in the Single-Injection group or by 1 mL per site of injection in the Multiple-Injection group.

Statistical analysis was performed with the program Systat 7.0 (SPSS, Inc., Chicago, IL). Data are presented as mean (SD or 95% confidence interval [CI₉₅]). The volumes of 0.5% ropivacaine providing adequate block of the femoral nerve in 50% of patients (ED₅₀) were calculated from the midpoints of pairs of the volumes from consecutive patients in whom an inadequate nerve block was followed by an adequate nerve block (10–12). The data were further analyzed with a probit transformation and a logistic regression model to calculate the volume of 0.5% ropivacaine required to produce the defined sensory and motor block of the femoral nerve within 20 min after injection in 95% (ED₉₅) of subjects (13). A value of P 0.05 was considered adequate.

	Results

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There were no differences in age (50 ± 12 yr in the Multiple-Injection group and 49 ± 13 yr in the Single-Injection group), weight (70 ± 19 kg in the Multiple-Injection group and 70 ± 10 kg in the Single-Injection group), height (167 ± 8 cm in the Multiple-Injection group and 166 ± 7 cm in the Single-Injection group), sex (13 men and 12 women in both groups), and ASA physical status (17 ASA I and 8 ASA II in the Multiple-Injection group and 19 ASA I and 6 ASA II in the Single-Injection group) between the two groups.

In one multiple-injection patient, the elicitation of vastus lateralis contraction was not achieved within 10 min after skin disinfection; this patient was excluded from the study, and the next patient received the same volume of local anesthetic solution.

Eleven patients in the Single-Injection group and 13 in the Multiple-Injection group achieved the defined sensory and motor block of the femoral nerve within 20 min after injection. The total volumes for adequate nerve block according to the up-and-down staircase method were 14 mL in the Multiple-Injection group (CI₉₅: 12–16 mL) and 23 mL in the Single-Injection group (CI₉₅: 20–26 mL) (P = 0.001) (Fig. 1). The ED₉₅ values for adequate block of the femoral nerve, calculated with the logistic regression analyses, were 29 mL in the Single-Injection group and 21 mL in Multiple-Injection group.

View larger version (22K): [in this window] [in a new window]   Figure 1. The volume of 0.5% ropivacaine for consecutive subjects in each group. Error bars represent 95% confidence intervals; the testing interval was 3 mL; and filled and empty marks represent negative and positive responses, respectively, as defined in the text.

	Discussion

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Local anesthetics are responsible for 5% to 10% of all reported adverse reactions to anesthetic drugs (15). Local anesthetic-related systemic toxicity is strictly dose dependent (3,14), and it is not influenced by the experience of the physician placing the block (16). The presence of a negative blood aspiration before the injection does not prevent the risk of severe systemic toxicity (17–19). Because all strategies minimizing the risk for developing local anesthetic-related systemic toxicity should be undertaken (3), use of a multiple-injection technique with a nerve stimulator may increase the safety of peripheral nerve blocks by reducing the required volumes of local anesthetic solution, as well as the volume of anesthetic injected at each site.

Achievement of surgical block after local anesthetic solution has been injected 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 (20). Because both the injection site and concentration of the local anesthetic solution were the same in the two groups, a shorter distance for diffusion of the local anesthetic molecules to the different branches of the femoral nerve can probably explain these results.

The determination of the ED₅₀ was based on the use of an arbitrary time end point. This was a conditio sine qua non of the up-and-down staircase method, which requires an objectively defined positive or negative response. This statistical method is widely used in pharmacologic and clinical research when comparing relative potencies of different treatments (10,11), and it has also been applied to determine dose-effect relationships in different fields of anesthesia research (21,22). Furthermore, it should be pointed out that, as compared with the traditional random group assignment design, the up-and-down staircase method has the advantage of minimizing the number of patients with a negative response, thus addressing the ethical concerns about determining a dose-response curve on patients (12).

Testing the blockade of motor function by changes in the ability to elevate the limb against gravity and loss of pinprick sensation represent well established methods for assessing the intensity of nerve block (23,24). Other methods have been described, such as transcutaneous electrical stimulation or force dynamometry. However, frequent use of painful stimulations before the onset of nerve block is uncomfortable for the patient.

Searching for multiple muscular twitches is a simple and easy method of optimizing the advantages of using nerve stimulators for peripheral block placement. Various authors have demonstrated that this technique provides shorter onset time with improved quality of nerve blockade as compared with the single-injection technique (4–6,25). Nonetheless, many anesthesiologists are concerned with use of the multiple-injection technique because of the theoretically increased risk for direct nerve injury by moving the stimulating needle toward a partially anesthetized nerve. In fact, the assumption that moving the stimulating needle and eliciting different muscular twitches increases the risk for nerve injury is not supported by clinical evidence. A prospective observational study of nearly 4000 nerve blocks performed with the multiple-injection technique clearly demonstrated that the risk for permanent nerve injury with the multiple-injection technique is similar to that reported by Auroy et al. (2) in their prospective evaluation of severe complications associated with regional anesthesia (7). Properly designed randomized studies should be done to clarify this crucial and controversial question.

In conclusion, results of this prospective, randomized, blinded study demonstrate that using a multiple-injection technique reduced the total volume, as well as the volume injected to block each branch of the femoral nerve. This most likely has the potential for adding to the safety of peripheral nerve blocks, especially for lower-extremity surgery, for which multiple blocks are required (20).

	Acknowledgments

 
We thank Prof. Jacques Chelly, University of Texas–Houston, for his helpful suggestions in writing the paper, as well as the staff of anesthesia nurses (University Department of Anesthesiology, IRCCS San Raffaele Hospital), without whose help and cooperation this study would not have been possible.

	References

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