Reducing the local anesthetic volume required to obtain successful peripheral nerve blocks is crucial to improve the safety of regional techniques.1 The possibility of blocking peripheral nerves with a smaller amount of local anesthetics is attractive for surgery of the lower extremities, where a combination of regional techniques is often required to achieve surgical anesthesia, and systemic toxicity can become a relevant clinical concern.2,3 Nevertheless, in addition to the local anesthetic concentration,4 the use of additives, single or multiple injection technique, and the approach used,5,6 local anesthetic volume2 also may affect the success rate and onset time of peripheral nerve blocks.
Ultrasound (US) guidance may theoretically offer an advantage over conventional techniques (anatomical landmarks and neurostimulation [NS]) because it allows direct visualization of nerve structures, needle pathway, and local anesthetic spread in real time.7,8
We therefore tested the hypothesis that US guidance may reduce the minimum effective anesthetic volume of 1.5% mepivacaine required to block the sciatic nerve in 50% of patients (MEAV50) with a subgluteal approach as compared with NS.
With ethical committee approval and written informed consent, we enrolled 60 ASA physical status I–II in-patients, aged 18–80 yr, undergoing elective knee arthroscopy with sciatic and femoral nerve blocks.
Patients with clinically significant coagulopathy, infection at the injection site, allergy to local anesthetics, severe cardiopulmonary disease, body mass index >35, diabetes mellitus, neuropathies, and patients receiving major opioids for chronic pain were all excluded. After arrival in the operating room, a 20-gauge IV catheter was placed in the opposite forearm to the surgical site. Standard monitoring was applied throughout the procedure, including noninvasive arterial blood pressure, heart rate, and pulse oximetry. Standard premedication was administered IV (midazolam 0.03 mg/kg).
All blocks were performed by the same investigator (GD), who has substantial expertise in regional anesthesia techniques. First, we performed a femoral nerve block with 20 mL of mepivacaine 1.5% using the anterior approach with US guidance.
Afterward, using a computer-generated sequence of random numbers, patients were randomly allocated, via sealed envelopes, to receive a posterior sciatic nerve block using either NS (n = 30) or US (n = 30).
After femoral block, each patient in both groups was placed in the lateral decubitus position with the leg to be blocked uppermost and rolled forward with the knee flexed at a 90° angle (Sims position). Sciatic nerve block was performed with a 1.5% solution of mepivacaine in both groups.
In the NS group, a line was drawn from the midpoint of the greater trochanter to the ischial tuberosity. After local skin infiltration, a stimulating, 100-mm-long, 18-gauge, short-beveled, Teflon-coated Tuohy needle (Locoplex®, Vygon, Padova, Italy) was inserted in the middle point of this line.9 A nerve stimulator (Plexygon®, Vygon) set with a pulse duration of 0.1 ms, current intensity of 1.5 mA, and frequency of 2 Hz was used for nerve location. The long axis of the needle was maintained at a 30°–45° angle to the skin, and its bevel was directed cephalad. If the specific tibial nerve-mediated muscular response (plantar flexion of the foot and toes or foot inversion) was not observed, the needle was withdrawn at the level of the skin and reintroduced with a different angle until the proper tibial nerve-mediated response was obtained, which has been shown to be associated with improved onset time and success rate compared with a common peroneal-mediated response.6,10 After identification of the tibial nerve with a stimulating intensity of >0.2 mA but <0.5 mA, local anesthetic was slowly injected after intermittent aspirations. The 20-gauge perineural catheter was then introduced through the Tuohy needle and advanced 4 cm beyond the needle tip. The needle was then withdrawn over the catheter, and the catheter was secured with Steri-Stripes™ (3M Italia S.P.A., Milan, Italy) and covered with a transparent dressing.
In the US group, nerve location was performed using a semiconvex probe (2–5 mHz) (LOGIQ e®, GE Healthcare, Milan, Italy). After localization of the ischial tuberosity and the greater trochanter, the sciatic nerve was located with an in-plane approach.9,11,12 A stimulating, 100-mm-long, 18-gauge, short-beveled, Teflon-coated Tuohy needle (Locoplex) was then inserted. Direct visualization of the needle tip was maintained with US while inserting the needle, until optimal positioning of the needle tip was achieved within the fascial space as close as possible to the sciatic nerve avoiding intraneural injections. Then, the designated volume of 1.5% mepivacaine was injected slowly under direct sonographic visualization with 1-mL incremental aliquots allowing subtle corrections of the needle tip to obtain the major circumferential spread of the local anesthetic within the fascial space around the nerve. The 20-gauge perineural catheter was then introduced through the Tuohy needle and advanced just beyond the needle tip. The needle was then withdrawn over the catheter, and the catheter was secured with Steri-Stripes and covered with a transparent dressing.
After completion of local anesthetic injection, sensory and motor blocks were evaluated every 2 min for the first 10 min after injection, and then every 5 min until 20 min after injection, by an independent observer who was not present during block placement and blinded to both the nerve location technique and the injected volume. Sensory and motor blocks of the sciatic nerve were assessed using the following 4-point scale:
- Loss of a sharp sensation with pinprick testing (20-gauge hypodermic needle) delivered on the plantar aspect of the foot (the tibial nerve) = +1 point.
- Loss of a sharp sensation with pinprick testing (20-gauge hypodermic needle) delivered on the dorsolateral aspect of the foot (peroneal nerve) = +1 point.
- Patient’s inability to perform foot dorsiflexion and eversion of the operated limb against gravity (peroneal branch) = +1 point.
- Patient’s inability to perform foot plantar flexion and inversion of the operated limb against gravity (tibial branch) = +1 point.
Effective sciatic nerve block was defined as complete loss of a sharp sensation at pinprick testing in the central sensory area of the peroneal and tibial nerves with complete inability to move the foot of the operated limb (total score = 4). A score <4 points was considered a failed block.
According to previous investigations,13 the initial volume of local anesthetic solution (1.5% mepivacaine) was 12 mL, and the outcome of each patient’s response determined the dose for the subsequent patient. Mepivacaine 1.5% was administered for sciatic nerve blockade in all patients using the modified Dixon’s up-and-down method.14,15 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.
When effective sciatic nerve block, as defined above (total score = 4), was achieved within 20 min after injection, the volume of local anesthetic solution for the next patient was decreased by 2 mL. Conversely, when effective sciatic nerve block was not observed (total score <4), the volume of local anesthetic solution for the next patient was increased by 2 mL.
In those patients with ineffective block of the sciatic nerve, supplemental anesthesia with a 1.5% mepivacaine bolus of 5 mL through the perineural catheter was used to complete surgery. If this was not adequate to complete surgery, fentanyl IV bolus ≤200 μg or propofol infusion and a laryngeal mask airway were administered.
Each patient’s procedure-related pain during anesthesia was assessed at the end of the block performance using a verbal numerical rating scale (NRS) from 0 (= no pain) to 10 (= worst imaginable pain). Blood aspirations from accidental vascular punctures and needle redirections during block performance were also registered. A needle redirection was defined as any needle withdrawal of at least 10 mm with subsequent forward movement. The day after surgery, complete recovery of neurological function on the operated limb was checked by a blinded investigator, and the occurrence of untoward events, including paresthesia, dysesthesia, or motor deficits, was recorded.
The main outcome variable was the MEAV50. To calculate the MEAV50 of mepivacaine 1.5% with the sequential up-and-down allocation technique in each group, we predetermined a minimum a priori number of independent negative-positive up-and-down deflections of ≥5.
Power calculations were based on the mean and sds of MEAV50 of mepivacaine 1.5% required to block the sciatic nerve determined in previous investigations.13
We considered a 2-mL difference in the main outcome variable with an effect size of 0.75. A minimum of 28 patients per group was required to detect the designated difference in the MEAV50 of mepivacaine 1.5% providing adequate surgical block of the sciatic nerve within 20 min after the injection, accepting a 2-tailed α-error of 5%, and a β-error of 10%.
Statistical analysis was performed using the program SPSS 15 (SPSS, Chicago, IL). The MEAV50 (95% confidence interval [CI]) of mepivacaine 1.5% providing adequate block of the sciatic nerve was calculated from the midpoints of pairs of the concentrations from consecutive patients in which a negative response (inadequate nerve block within 20 min after injection) was followed by a positive one (adequate nerve block within 20 min after injection).
The data were further analyzed with a probit transformation and a logistic regression analysis to calculate the volume of mepivacaine 1.5% required to produce sciatic nerve block within 20 min after injection in 50% effective dose (ED50) and 95% effective dose (ED95) in subjects. Comparisons of continuous variables between the 2 groups were performed using the Mann-Whitney U-test. Categorical variables were analyzed using a contingency table analysis with Fisher’s exact test. Continuous variables are presented as mean (sd or 95% CI) or median (range) according to data distribution. Categorical variables are presented as number (%). A value of P ≤ 0.05 was considered significant.
No differences in anthropometric variables were observed between the 2 groups (Table 1). No patient was discontinued from protocol interventions and/or monitoring (Fig. 1).
The sequences of positive and negative responses recorded in consecutive patients of both groups are demonstrated in Figure 2. The MEAV50 of 1.5% mepivacaine required to block the sciatic nerve using the subgluteal approach according to the Dixon’s up-and-down method was 12 (1) mL (95% CI, 10–13 mL) in Group US and 19 (2) mL (95% CI, 15–23 mL) in Group NS (P < 0.01). The ED50 and ED95 calculated with the probit transformation and logistic regression analysis were 10 mL (95% CI, 5–15 mL) and 14 mL (95% CI, 12–17 mL) in Group US, and 18 mL (95% CI, 10–27 mL) and 29 mL (95% CI, 25–40 mL) in Group NS, respectively (P = 0.008).
There were no differences in the duration of block performance (3 [1–20] min in Group US versus 4 [1–20] min in Group NS), vascular punctures (0 [0–1] in Group US versus 0 [0–1] in Group NS, P = 0.305) or in needle redirections during block performance (3 [0–9] in Group US versus 3 [0–15] in Group NS, P = 0.851). The NRS score for procedure-related pain during block performance was not significantly different between groups: 5 (0–9) in Group US versus 3 (0–8) in Group NS (P = 0.133).
Fifteen patients in Group US (50%) and 17 patients in Group NS (56%) showed a negative response 20 min after block placement, with an incomplete sensory and/or motor peroneal and/or tibial nerve block at the end of the study (P = 0.796). After recording the presence of a negative response according to the up-and-down sequence, these patients received a 5-mL bolus of 1.5% mepivacaine in the perineural sciatic catheter and a bolus of IV fentanyl ≤200 μg. This supplementation was sufficient to complete surgery uneventfully in all patients except 8 in Group US and 4 in Group NS (P = 0.366) who required propofol infusion and placement of a laryngeal mask airway to complete surgery. The median (range) fentanyl consumption did not differ between groups: 100 (100–200) μg in Group US versus 100 (100–200) μg in Group NS (P = 0.628).
No severe side effects were reported in either group. No neurological complications were reported at the 24-h follow-up, and complete recovery of sensory and motor functions was observed in all patients.
US guidance allows the clinician to identify peripheral nerves and to directly visualize the spread of local anesthetic solution. Several studies16–18 have proven that regional techniques are optimized by US guidance. Better quality nerve blocks and shorter onset times were reported when using US guidance rather than NS. These studies concluded that there is a possibility of reducing the local anesthetic dose required to achieve adequate surgical block.
This prospective, randomized, single-blinded study showed that US guidance allows reduction of the MEAV50 required to produce an effective surgical block of the sciatic nerve within 20 min after injection compared with NS alone. Minimizing the amount of local anesthetic required to achieve an adequate surgical nerve block may improve the safety of peripheral blocks, because systemic toxicity has been reported as the main risk factor for severe complications during peripheral nerve blocks,2 especially for lower extremity surgery in which a combination of 2 different blocks is usually required. As previously stated by Casati et al.,2 the observed reduction in the MEAV is similar to that reported when comparing a multiple-injection technique with the single-injection technique.19 Moreover, US guidance confers the advantage of directly visualizing the needle’s movements in real time.2,16 Nevertheless, in this study, no difference was found between groups in terms of vascular punctures, which might be biased by the absence of vessels in the sciatic region, or in number of needle redirections during block performance. Thus, the NRS score for procedure-related pain during block performance was comparable between groups.
Consistent with previous reports, the ED50 and ED95 calculated with the probit transformation and logistic regression analysis were 18 and 29 mL when using NS to block the sciatic nerve at a proximal level (posterior subgluteal approach). In fact, Taboada et al.13 reported that a larger volume of local anesthetic is necessary to block the sciatic nerve at a more distal site (popliteal approach) compared with a more proximal level (subgluteal approach). Our study demonstrates that the MEAV50 of the local anesthetic (mepivacaine 1.5%) may be further reduced by using US guidance instead of NS.
To determine the volume of local anesthetic necessary to block the sciatic nerve, we used the modified Dixon’s up-and-down method.14 Depending on the efficacy of sciatic nerve blockade with the previous volume of 1.5% mepivacaine, the volume for the next patient would then be adjusted up or down in increments of 2 mL. When applied to determine the MEAV to block peripheral nerves, the up-and-down method has the drawback that many patients may have incomplete block. To optimize surgical block, we placed a perineural catheter to administer a supplemental bolus of 1.5% mepivacaine when necessary to complete surgery.
In conclusion, US guidance for the subgluteal sciatic approach provided a 37% reduction of the MEAV50 required to produce a surgical block compared with NS guidance. Consequently, US guidance may reduce the total amount of local anesthetic and thus systemic toxicity, which is a relevant clinical concern, especially with lower extremity blocks because more than 1 block is usually needed for surgical anesthesia.
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