Various factors affect the success rate and onset time of peripheral nerve blocks, including the concentration and volume of the injected anesthetic solution (1), the use of additives (2), a double injection technique (3–5), the type of evoked motor response obtained (6–8), or the intensity of the current at which peripheral nerve stimulation is achieved(9). One recent article focused on an additional factor: the type of approach used (10).
Using a single-injection technique with 30 mL of local anesthetic, 3 different injection sites to block the sciatic nerve (classic Labat, subgluteal, and lateral popliteal) were evaluated in terms of onset time and quality of blockade (10). A clinically relevant improvement in onset time was achieved with a more proximal access to the nerve (10). The sciatic anatomy may play an important role in this result. In their course down the lower limb, the two sciatic nerve components become more distant from one another with the presence of more layers of fat and connective tissue between them (11,12). Floch et al. (13), using computed tomography (CT) scans of the thighs, demonstrated that there were 2 separate trunks in 27% of subjects at 20 cm and in 90% at 30 cm distal to the greater trochanter (GT). These radiological results may have clinical implications if a small volume of local anesthetic is used with a single-injection technique.
To confirm clinically what had been demonstrated by CT scans previously (13), this investigation was designed to compare onset times and success rate of two different injection sites to the sciatic nerve, located 20 cm and 30 cm distal to the GT, using a single injection of 20 mL of mepivacaine 1.5%.
The study protocol was approved by the Hospital Ethical Committee of the University of Santiago de Compostela, and written informed consent was obtained from all participants. Fifty patients, ASA physical status I or II, aged 18–80 yr old, and scheduled for foot surgery under lateral sciatic nerve blockade were included in the study. Exclusion criteria were patient refusal, pregnancy, neurologic or neuromuscular disease, coagulation disorders, and skin infection at the site of needle insertion.
IV access was established. Continuous electrocardiogram, noninvasive arterial blood pressure, and pulse oximetry were monitored during needle insertion and throughout the surgical procedure. All patients received 1–2 mg of midazolam IV as premedication. Patients were randomized by means of sealed envelopes to receive a lateral sciatic nerve blockade at 1 of 2 injection sites: 20 cm (proximal group) and 30 cm (distal group) distal to the GT.
Patients were positioned supine with their legs extended at the knee joint. The long axis of the foot was positioned at a 90-degree angle to the table. A line was drawn from the upper border of the GT to the lateral femur epicondyle at the knee joint, parallel to the femur. On this line, the site of needle insertion was 20 cm or 30 cm distal to the GT. After local skin infiltration, an 8-cm, 22-gauge short-beveled stimulating needle (Pajunk, Medizintechnologie, Geisingen, Germany) attached to a nerve stimulator (Pajunk, Medizintechnologie) was inserted perpendicularly to the skin or with a slight posterior direction and advanced until either plantar flexion or dorsiflexion of the foot was obtained. The stimulating current was set initially between 1.5 and 2 mA (frequency, 2 Hz; time, 100 μs). The intensity of the stimulating current was gradually decreased as the needle approached the targeted nerve. In all patients, the targeted evoked motor response was plantar flexion of the foot (8) at a current intensity of ≤0.5 mA. In case of a dorsiflexion motor response, the needle was redirected more medially. Twenty milliliters of 1.5% mepivacaine was injected slowly after careful intermittent aspirations. Patients received an additional femoral inguinal nerve blockade with 10–15 mL of 1.5% mepivacaine because all surgeries were performed with an arterial tourniquet below the knee. All blocks were performed by one of two senior anesthesiologists.
Arterial blood pressure, heart rate, pulse oximetry, and progress of both sensory and motor blockade on the operated limb were evaluated by an independent blinded observer every 5 min after local anesthetic injection for a total of 45 min. Sensory blockade assessments were performed in the distributions of the common peroneal and tibial nerves, i.e., the superficial and deep peroneal nerves, the sural, lateral plantar, medial plantar, and calcaneus plantar nerves (a total of six peripheral nerves). The time required for onset of motor and sensory blockade was recorded. The extent of sensory blockade of each nerve was classified as follows: 0 = normal sensation in the respective nerve distribution (no block), 1 = blunted sensation (analgesia), and 2 = absence of sensation (anesthesia). Sensory blockade was considered complete when each sensory testing by pinprick with a 22-gauge hypodermic needle in all sciatic nerve distributions yielded a score of 2. With a score of <2, the block was considered incomplete. Motor blockade was assessed for voluntary motor responses by asking the patient to plantarflex or dorsiflex the foot. It was classified as follows: 0 = normal movement, 1 = decreased movement, and 2 = no movement. Motor blockade was considered complete when motor responses after 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 blockade associated with a pain-free surgery. Patients who did not have complete anesthesia at the surgical site by the end of a 45-min period were given a supplemental lateral popliteal sciatic nerve blockade (4) before surgery. The degree of pain during surgery was assessed with a 4-point verbal rating scale score (0 = no pain, 1 = mild or moderate pain, 2 = severe pain, and 3 = unbearable pain). If a verbal rating scale of more than 1 was reported by the patient, 50–100 μg of supplemental IV fentanyl was given. If this did not provide adequate conditions, general anesthesia was induced.
To calculate the required number of patients to be included in the study, we considered previous findings on sciatic blockade with single injection technique (10). A power analysis estimated that 24 patients per group would be required to detect a 5-min difference in the onset time of sciatic nerve blockade after a proximal lateral approach compared to a distal access, with a two-tailed α error of 5% and a statistical power of 80%. One additional patient was 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). Data distribution was first evaluated using the Kolmogorov-Smirnov test. Continuous variables were compared between groups using either two-sampled Student’s t-test or the Mann-Whitney U-test, depending on data distribution. Discrete variables were compared between groups using a χ2 or Fisher’s exact test when numbers were small. A P value <0.05 was considered statistically significant. Continuous variables are presented as mean ± sd, and qualitative data are displayed as numbers (percentage).
Fifty patients were enrolled in the study (25 in each group). There were no significant differences in patient demographics between groups (Table 1).
The onset of complete sensory and motor blockade was faster after a proximal injection than after the distal approach (Table 2, P < 0.05). Patients receiving the nerve blockade at 30 cm of the GT had a slower onset time of sensory and motor blockade in the distribution of the common peroneal nerve than patients with a proximal approach. However, no significant differences were found between the two injection sites in terms of onset time of sensory and motor blockade in the distribution of the tibial nerve (Table 2).
Surgical anesthesia (success rate) was achieved in a larger percentage of patients with the proximal than the distal approach, i.e., 88% versus 56%, respectively (P < 0.05). An additional lateral popliteal nerve blockade became necessary in 10 patients in group distal and in 2 patients in group proximal (P < 0.05). General anesthesia was administered to three patients after the distal approach and in one patient after the proximal approach (P = ns).
In the present prospective, randomized, double-blind investigation, using a lateral approach to the sciatic nerve with a single injection of 20 mL of mepivacaine 1.5%, a faster complete anesthesia and a more frequent success rate were achieved with a more proximal injection compared with a distal approach.
Different factors affect the onset time and success rate of peripheral nerve blocks. These include the concentration and volume of the injected local anesthetic, a multiple-injection technique, the type of evoked motor response, and the intensity of the current. All of these were similar in the two groups of patients studied. Therefore, the two different injection sites used and the sciatic anatomy may explain the results obtained. The sciatic nerve derives its fibers from the L4 to S3 spinal segments; the nerve then leaves the pelvis through the sacrosciatic foramen and from there courses down the posterior aspect of the thigh to the popliteal fossa. Floch et al. (13), using CT scans of the thighs, observed that there were 2 separate trunks in 27% of subjects at 20 cm and in 90% at 30 cm distal from the GT. The cross-sectional areas of the perineural space measured at 20 and 30 cm were 1.8 and 5.6 cm2, respectively. This may explain the less frequent success rate and slower onset of nerve blockade after the distal approach as compared with the proximal one. It is likely that when a single injection of a small amount of local anesthetic is administered at 30 cm from the GT, exposure of one of these 2 separate trunks to the local anesthetic may be limited. After a distal injection, only 56% of the patients had complete sensory anesthesia. In contrast, a single injection of the same volume of local anesthetic injected more proximally (20 cm from the GT) seemed sufficient to block the 2 components of the sciatic nerve. The present results demonstrate clinically what had been shown previously by CT scans (13).
Previously, the effects of three injection sites, two proximal ones—the classic Labat and subgluteal approach—and one distal one—the lateral popliteal approach—were evaluated. Onset times and efficacy of sciatic nerve blockade after a single injection of 30 mL of 0.75% ropivacaine were compared (10). A clinically relevant improvement in blockade onset time was achieved with the two most proximal injection sites. A similar study (14) investigated a proximal mid-femoral (15) and a distal lateral popliteal approach (16) to the sciatic nerve using 30 mL of 0.5% ropivacaine. In most of the patients, a double-injection technique was used. Shorter onset times were observed with the more proximal approach (14). However, in contrast to the present study, no differences in the success rate of nerve blockade were found between proximal and distal approaches (10,14). The amount of local anesthetic (30 mL) administered in these 2 studies(10,14) versus 20 mL in the present investigation may explain the difference in results.
Different volumes of local anesthetic (15–50 mL) have been used to block the sciatic nerve (17–24). To prevent large dosages of local anesthetic when sciatic and femoral nerve blocks are combined, small volumes of local anesthetic were administered. However, 20 mL with a single-injection technique was not sufficient to block all components of the sciatic nerve when a distal approach was used. Larger volumes may improve the success rate in a distal approach yet may be associated with toxic plasma concentrations.
Besides volume, a double-injection technique may lead to more frequent success (4,14,24). Paqueron et al.(4) compared single- and double-injection techniques in lateral popliteal sciatic nerve blockade with 20 mL of local anesthetic and reported a more frequent success rate when the common peroneal and tibial nerves were identified and anesthetized separately (88%) than after a single injection (54%). The 54% of success rate with a single-injection technique at the distal site is similar to our results (56%).
In conclusion, a lateral sciatic nerve blockade performed at a more proximal site generated a shorter onset time and a more frequent success rate as compared with an injection administered more distally when a single injection of 20 mL of mepivacaine 1.5% was used. These results confirm clinically what has formerly been demonstrated radiographically (13).
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