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REGIONAL ANESTHESIA

What Is the Minimum Effective Volume of Local Anesthetic Required for Sciatic Nerve Blockade? A Prospective, Randomized Comparison Between a Popliteal and a Subgluteal Approach

Manuel Taboada, MD^*, Jaime Rodríguez, MD PhD^*, Cristina Valiño, MD^*, Javier Carceller, MD^*, Begoña Bascuas, MD^*, Juan Oliveira, MD, Julian Alvarez, MD, PhD^*, Francisco Gude, MD, and Peter G. Atanassoff, MD

Department of *Anesthesiology, Clinical Epidemiology Unit, University of Santiago de Compostela, Hospital Clínico Universitario de Santiago, Spain; and Department of Anesthesiology, Yale University School of Medicine, New Haven, Connecticut

Address correspondence and reprint requests to Manuel Taboada Muñiz, MD, Department of Anesthesiology, Hospital Clínico Universitario de Santiago, Travesía da Choupana s/n. 15706 Santiago de Compostela, Spain. Address e-mail to manutabo{at}yahoo.es.

	Abstract

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For sciatic nerve blockade, no study has defined the optimal volume of local anesthetic required to block the nerve. The current, prospective, randomized investigation was designed to find a minimum volume of 1.5% mepivacaine required to block the sciatic nerve using the subgluteal and posterior popliteal approaches. A total of 56 patients undergoing foot surgery were randomly assigned to receive sciatic nerve block by means of a posterior subgluteal (group subgluteal, n = 28) or a posterior popliteal (group popliteal, n = 28) approaches. All blocks were performed with the use a nerve stimulator (stimulating frequency, 2 Hz, intensity 1.5-0.5 mA) and a perineural stimulating catheter. In all patients, plantar flexion of the foot was elicited at <0.5 mA, to maintain consistency among groups. The volume of local anesthetic used in each patient was based on the modified Dixon’s up-and-down method. Complete anesthesia was defined as complete loss of pinprick sensation in the sciatic nerve distribution with concomitant inability to perform plantar or dorsal flexion of the foot 20 min after injection. The mean volume of local anesthetic required to block the sciatic nerve was 12 ± 3 mL in the subgluteal group and 20 ± 3 mL in the popliteal group (P < 0.05). The ED₉₅ for adequate block of the sciatic nerve was 17 mL in the subgluteal group and 30 mL in the popliteal group. The authors conclude that a larger volume of local anesthetic is necessary to block the sciatic nerve at a more distal site (popliteal approach) as compared with a more proximal level (subgluteal approach).

	Introduction

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Various factors markedly affect the outcome of peripheral nerve blocks, including the intensity of the current at which peripheral nerve stimulation is achieved (1), the number of injections (2), the evoked motor response (3–5), the approach (6–7), and the volume and concentration of the local anesthetic solution (8). Several approaches to the sciatic nerve have been described, each using different volumes of local anesthetic (9–12). However, no study has determined the optimal volume of local anesthetic solution required to block the sciatic nerve.

The primary objective of this prospective randomized study was to compare the required amount of 1.5% mepivacaine necessary to block the sciatic nerve at a proximal (subgluteal) and a distal (popliteal) injection site.

	Methods

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The study protocol was approved by the Hospital’s Ethical Committee of the University of Santiago de Compostela, and written informed consent was obtained from all participants. Fifty-six ASA physical status I–II patients, aged 18–80 yr, and scheduled for elective hallux valgus repair under sciatic nerve blockade were included. Exclusion criteria consisted of patient refusal, pregnancy, neurologic or neuromuscular disease, anticoagulation, or skin infection at the site of needle insertion.

Before nerve blockade, IV access was established. Continuous electrocardiogram, noninvasive arterial blood pressure, and pulse oximetry were monitored during block insertion and throughout the surgical procedure. All patients received 1–2 mg midazolam IV as premedication. Patients were randomized using sealed envelopes that were opened just before block performance. Participants received one of two sciatic nerve blocks: a subgluteal approach previously described by di Benedetto (9) (subgluteal group, n = 28) or a classic posterior popliteal approach (Popliteal Group, n = 28). As all surgeries required a tourniquet below the knee, a femoral nerve block with 15 mL of 1.5% mepivacaine was also performed.

All peripheral nerve blocks were performed by the same two senior anesthesiologists. A 10-cm, 19-gauge short-beveled stimulating needle (Pajunk, Medizintechnologie, Geisingen, Germany) and a 22-gauge stimulating catheter (Stimulong-set, Pajunk, Medizintechnologie, Geisingen, Germany) were both connected to a nerve stimulator (Pajunk, Medizintechnologie, Germany). The stimulation frequency was set at 2 Hz, and the duration of the stimulating pulse was set at 0.1 ms. Initially, the stimulating current delivered 1.5–2.0 mA; this was progressively decreased to <0.5 mA while maintaining the appropriate motor response. A plantar flexion of the foot identified the tibial nerve. This was the evoked motor response elicited in all patients to maintain consistency among groups. In case of a peroneal nerve stimulation, the needle was withdrawn and redirected 2–3 mm more medially.

Patients in the subgluteal group were placed in the lateral decubitus position, with the leg to be blocked uppermost and rolled forward with the knee flexed at a 90º angle (Sim’s position). A line was drawn from the midpoint of the greater trochanter to the ischial tuberosity. A second perpendicular line was drawn from the midpoint of the previous line and extended caudally for 4 cm (9). This point represented the site of needle insertion. After local skin infiltration, the stimulating needle with a stimulating catheter inside was inserted. The long axis of the needle was maintained at a 30–45º angle to the skin and its bevel was directed cephalad. After identification of the tibial nerve with a stimulating intensity <0.5 mA, the stimulating catheter was advanced 4–5 cm past the needle tip while maintaining plantar flexion with an intensity <0.5 mA with the goal to position the stimulating catheter close the tibial nerve. The needle was then withdrawn over the catheter while continuously eliciting plantar flexion with the stimulating catheter to ensure proper catheter positioning during needle withdrawal. Local anesthetic was then slowly injected after intermittent aspirations.

Patients in the popliteal group were placed in the prone position. After local skin infiltration, the stimulating needle with the stimulating catheter inside was inserted, 9 cm above the popliteal crease and 1 cm lateral to the midline. The needle bevel was oriented cephalad at an angle of 30–45º to the skin. Catheter advancement was performed as previously described. Local anesthetic was then slowly injected after intermittent aspirations.

Final catheter position was considered acceptable when plantar flexion was elicited at a current output via the catheter <0.5 mA. The lowest current via the needle and the catheter was recorded. If the catheter could not be placed, the patient was withdrawn from the investigation.

Mepivacaine 1.5% was administered for sciatic nerve blockade in all patients using the modified Dixon’s up-and-down method (13–14). The volume of local anesthetic received by a particular patient in either group was determined by the response (success or failure of blockade 20 min after local anesthetic injection) of the previous patient in that group to a larger or smaller volume by using an up-down sequential allocation technique. The initial amount of local anesthetic injected in the 2 groups was 20 mL. Depending on success or failure blockade to the previous volume, the amount for the next patient would be adjusted up or down in increments of 2 mL. For example, if a patient developed complete sensory and motor block in all distributions of the sciatic nerve within 20 min after local anesthetic injection, the volume used for the next patient would decrease by 2 mL. If anesthesia was incomplete after 20 min, the volume used for the next patient would then increase by 2 mL.

Sensory and motor blockade on the operated limb was evaluated every 5 min after injection of the local anesthetic for 20 min. Data collection was performed by an independent observer who was not involved in the regional block performance. Time required for onset of motor and sensory block was recorded. The extent of sensory blockade of each nerve (deep and superficial peroneal, calcaneus, lateral and medial plantar, and sural nerves) was classified as follows: 0 = normal sensation within the nerve distribution (no block), 1 = blunted sensation within the nerve distribution (hypoalgesia), and 2 = absence of sensation within the nerve distribution (anesthesia). Sensory block was considered complete when each sensory testing by pinprick test reached a score of 2. If the score was <2 in any of the nerve distributions at the end of the 20 min, the nerve block was considered incomplete. Motor block was assessed for voluntary motor responses by asking the patient to plantarflex or dorsiflex the foot and was classified as follows: 0 = normal movement, 1 = decreased movement and 2 = no movement. Motor block was considered complete when motor response for both plantar flexion and dorsiflexion had a score of 2; otherwise it was considered incomplete. The success rate was defined as a complete sensory and motor block 20 min after local anesthetic injection. Patients who did not have complete sensory and motor block after 20 min were given a supplemental 5 mL of mepivacaine 1.5% through the catheter and then an additional 5 mL if necessary to obtain a complete sciatic block. The additional bolus of local anesthetic was not included in data analysis. The degree of pain during surgery was assessed on a 4-point verbal rating scale with 0 = no pain, 1 = mild or moderate pain, 2 = severe pain, 3 = unbearable pain. If a verbal rating scale of more than 1 was reported by the patient, 50-100 µg of supplemental fentanyl was given IV. If this did not provide adequate conditions, general anesthesia was induced.

After the surgery, the stimulating catheter was connected to an electronic patient-controlled analgesia pump (CADD-Legacy 6300; Deltec, Inc. St. Paul, MN), for postoperative analgesia, with a continuous infusion of 0.0625% levobupivacaine and the possibility of an incremental bolus every 20 min (basal infusion rate of 3 mL/h, patient-controlled bolus dose of 3 mL, lockout time of 20 min). At 24 h, the acceptance of the anesthetic technique was evaluated using a 3-point score: 1 = very satisfactory, 2 = satisfactory and 3 = unsatisfactory.

To calculate the required number of participating patients, previous volumes for sciatic nerve blockade after single injection served as a reference (6,15). To detect a 5-mL difference in the amount of local anesthetic necessary to block the sciatic nerve at the 2 injection sites with a two-tailed error of 5% and a ß error of 20%, a study size of 24 patients per group was calculated. Four more patients were included in each group for possible dropouts.

Statistical analysis was performed by using the Statistical Package for the Social Sciences (SPSS for Windows, version 10.0; SPSS Inc., Chicago, IL). The volumes of 1.5% mepivacaine providing adequate blockade in 50% of patients (ED₅₀) was calculated from the midpoints of pairs of the volume from consecutive patients in whom an inadequate nerve block was followed by an adequate nerve block. The data were further analyzed with a logistic regression model to calculate the volume of 1.5% mepivacaine required to produce the defined sensory and motor block of the sciatic nerve within 20 min after injection in 95% of subjects (ED₉₅). Data distribution was first evaluated using the Kolmogorov-Smirnov test. Continuous variables between groups were compared using either two-sampled Student’s t-test or the Mann-Whitney U-test according to the data distribution. Discrete variables between groups were compared using a ² or Fisher’s exact test when numbers were small. A P value of <0.05 was considered statistically significant. Continuous variables are presented as mean ± sd; qualitative data are presented as numbers (percentage).

	Results

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There were no significant differences between groups in terms of demographic data (age, weight, and height) (Table 1). The depth of catheter position and the lowest current for stimulation were similar in both groups (Table 2).

The mean volume of local anesthetic required to block the sciatic nerve based on the modified Dixon’s up-and-down method was 12 ± 3 mL in the subgluteal group and 20 ± 3 mL in the popliteal group (P < 0.05). The sequences of effective and ineffective blockade of the sciatic nerve are shown in Figures 1 and 2. The ED₉₅ values for adequate block of the sciatic nerve, calculated with the logistic regression analyses, were 17 mL in the subgluteal group and 30 mL in the popliteal group (P < 0.05).

View larger version (12K): [in this window] [in a new window]   Figure 1. Volume of 1.5% mepivacaine administered in 28 consecutive patients with the subgluteal approach, based on the modified Dixon’s up-and-down method. Filled and empty marks represent success and failure of blockade, respectively. The mean volume of local anesthetic required to block the sciatic nerve was 12 ± 3 mL.

View larger version (13K): [in this window] [in a new window]   Figure 2. Volume of 1.5% mepivacaine administered in 28 consecutive patients with the popliteal approach, based on the modified Dixon’s up-and-down method. Filled and empty marks represent success and failure of blockade, respectively. The mean volume of local anesthetic required to block the sciatic nerve was 20 ± 3 mL.

Patients’ overall satisfaction with the anesthetic procedure is displayed in Table 2. No differences were found between groups. No severe untoward event was reported in any patient.

	Discussion

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The volume of local anesthetic administered is one of the major determinants for the success of a regional block procedure. The present investigation demonstrated that, following a proximal subgluteal injection site, the volume of 1.5% mepivacaine required to block the sciatic nerve was less than after a distal posterior popliteal approach.

Various factors are responsible for the success of peripheral nerve blockade. These include the intensity of the current with which peripheral nerve stimulation is achieved (1), a multiple injection technique (2), the type of evoked motor response obtained (3–5), the type of approach used (6–7), and the concentration of the injected anesthetic solution (8). Basically, all these variables were maintained the same in the present investigation. Only the injection site used was different in the two groups. Therefore, an explanation for the different amount of local anesthetic necessary to block the sciatic nerve must be sought in the sciatic anatomy.

The sciatic nerve derives its fibers from the L4 to S3 spinal segments; it leaves the pelvis through the sacrosciatic foramen, and from there it courses down the posterior aspect of the thigh to the popliteal fossa. Anatomical studies revealed that the nerve divides into its terminal portions, the tibial and common peroneal nerves, at highly variable distances above the popliteal fossa crease. In some cases, the two trunks are separated by several centimeters at the popliteal level (16–17). Using computed tomographic scanning, Floch et al. (18) observed that 2 separate trunks existed in 72% of subjects at 25 cm and in 90% at 30 cm distal to the greater trochanter. The cross-sectional areas of the perineural space measured at 25 and 30 cm were 3.9 and 5.6 cm², and the space between the trunks was filled with adipose tissue and blood vessels. This anatomy may explain the larger volume of local anesthetic necessary to block the sciatic nerve with the distal approach as compared with the more proximal injection site, where the two trunks are very close together and separated by only a negligible amount of adipose tissue. Schirmek and Deusch (19) introduced a catheter into the popliteal fossa and found an increasing number of patients with complete anesthesia after successive injections of local anesthetic.

To determine the volume of local anesthetic necessary to block the sciatic nerve, we used the modified Dixon’s up-and-down method (13–14). Depending on the efficacy of sciatic nerve blockade with the previous volume of local anesthetic, the volume for the next patient would then be adjusted up or down in increments of 2 mL. This method is often used to determine relationships between dose and volume effects in different fields of anesthesia research (14,20). The Dixon’s up-and-down method for determining the minimum effective volume to block a peripheral nerve has the drawback that many patients might have an incomplete blockade. To minimize this problem in the present study, we placed a stimulating perineural catheter maintaining an acceptable motor response with a current output of <0.5 mA. Patients who did not have complete anesthesia at the end of 20 minutes were then given a supplemental 5-10 mL of the local anesthetic through the catheter. Only two patients in the present study needed additional general anesthesia during surgery.

In conclusion, the results of the present randomized study demonstrate that a larger volume of local anesthetic was necessary for blockade of the sciatic nerve at a more distal site (posterior popliteal approach) as compared with a more proximal injection site (subgluteal approach) when a stimulating catheter was used. The anatomy at the subgluteal level appears to favor the administration of smaller amounts of local anesthetic.

	Footnotes

 
Support was provided solely from institutional and departmental sources.

Accepted for publication September 7, 2005.

	References

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