The electrophysiological effect of dextrose 5% in water on single-shot peripheral nerve stimulation

When performing regional anesthesia, a small volume of local anesthetic or normal saline abolishes a motor response induced by a low current (0.5 mA). In this case series we describe the electrophysiological effect of a nonconducting (dextrose 5% in water, D5W) injectate on a motor response elicited by low current electrical stimulation. Twenty-nine peripheral nerve blocks were performed in 20 patients using insulated needles. Each needle was primed with D5W. The needle was advanced towards the target nerve until corresponding motor responses were observed using a current of 0.5 mA or less. Once the needle position was optimally placed, 1 mL of D5W was injected followed by a predetermined dose of local anesthetic. The effects of the injectates (D5W and local anesthetic) on the motor response were observed at all needle insertion sites. In all cases, the motor response was at least maintained or augmented (96%) immediately after the injection of D5W. All motor responses diminished after the injection of local anesthetic (100%). All blocks were considered clinically successful.     

Patients' perception of pain during axillary and humeral blocks using multiple nerve stimulations

BACKGROUND AND OBJECTIVES: Axillary or humeral blocks by multiple nerve stimulation (MNS) are used for ambulatory hand surgery. This double-blind study identified which of the three main components of the procedure (repeated needle passes, local anesthetic injections, or electrical stimulations) is most painful, quantified its intensity, and recorded patients' preferences for a future anesthetic. METHODS: Eighty unsedated ambulatory patients were randomized to 2 equal groups: axillary (A) and humeral (H). In each patient, 4 terminal motor nerves (musculocutaneous, median, ulnar, and radial) were electrolocated by use of an initial current of 2 mA, 0.1 ms and a target current of 0.1 to 0.5 mA. After block placement and before the start of surgery, patients were requested to identify which of the 3 main components of the block was most unpleasant and to quantify its intensity on a visual analog scale (VAS) of 0 to 100. Twenty minutes after completion of the block, the unblocked nerves were electrolocated at the elbow and supplemented. Patients were declared ready for surgery when they had complete analgesia of the hand and forearm. Before discharge from the hospital, patients indicated which anesthetic method (block alone, block plus sedation, or general anesthesia) they would choose for future hand surgery. RESULTS: Twenty-seven patients in group A vs. 17 patients in group H reported electrical stimulations as the most unpleasant block component (P =.03). No significant differences occurred in any of the VAS scores. Patients' request for the same anesthetic, 35 in group A and 37 in group H, were similar. Group A patients were ready for surgery sooner than group H patients (mean 26 minutes vs. mean 30 minutes for group H patients; P =.04). No serious complications were observed. CONCLUSIONS: This study found that more axillary-block patients compared with humeral-block patients reported electrical stimulation as the most unpleasant part of the block but failed to detect significant differences in the intensity of the 3 block components (repeated needle passes, local anesthetic injections, and electrical stimulations). Most patients in both groups would accept the same block for future hand operations. Patients were ready for surgery sooner after axillary block, but the clinical importance of this finding is doubtful.     

Neurostimulation in ultrasound-guided infraclavicular block: a prospective randomized trial

Ultrasound guidance (USG) for infraclavicular blocks provides real time visualization of the advancing needle and local anesthetic distribution. Whether visualization of local anesthetic spread can supplant neurostimulation as the end point for local anesthetic injection during USG block has never been formally evaluated. Therefore, for this prospective randomized study, we recruited 72 patients scheduled for hand or forearm surgery and compared the speed of execution and quality of USG infraclavicular block with either USG alone (Group U) or USG combined with neurostimulation (Group S). In Group U, local anesthetic was deposited in a U-shaped distribution posterior and to each side of the axillary artery using as few injections as possible (1, 2, and 3 injections in 29, 6, and 3 patients, respectively). In Group S, a single injection was made after obtaining a distal motor response with a stimulating current between 0.3 and 0.6 mA. The anesthetic solution consisted of 0.5 mL/kg of lidocaine 1.5%, bupivacaine 0.125%, and epinephrine 1:200 000 (final concentrations). Procedure times were significantly shorter in Group U compared with Group S (3.1 +/- 1.6 min and 5.2 +/- 4.7 min, respectively; P = 0.006). In Group S, anesthetic spread was mainly anterior to the axillary artery in 37% of patients and mainly posterior in 63% of patients. Thirty minutes after the injection, 86% of patients in Group U had complete sensory block in the musculocutaneous, median, radial, and ulnar nerve territories compared with 57% in Group S (P = 0.007). Patients blocked in Group U with a single injection had the same rate of complete block (86%) as those blocked with more than one injection (86%). Block supplementation rates were 8% in Group U versus 26% in Group S (P = 0.049). Block failure occurred in one patient in Group S because of an inability to obtain a distal stimulation after 20 min. We conclude that USG infraclavicular block is more rapidly performed and yields a higher success rate when visualization of local anesthetic spread is used as the end point for injection. Posterolateral spread of local anesthetic around the axillary artery predicts successful block, circumventing the need for direct nerve visualization.     

神经刺激术在超声引导锁骨下臂丛神经阻滞中的应用：一项前瞻性随机研究

超声引导（ultrasound guidance，USG）锁骨下臂丛神经阻滞可提供实时的进针程度和局麻药分布图像。但在USG下行臂丛神经阻滞时，局麻药扩散情况能否取代神经刺激反应作为局麻药的注药指征，目前尚无正式的研究报道。因此，我们进行了此项前瞻性随机研究实验：以72例择期行手和前臂手术的患者作为观察对象，将患者分为单独应用超声引导组（U组）和超声引导与神经刺激联合应用组（S组），比较两组完成神经阻滞操作的时间以及锁骨下阻滞的效果。U组患者采用尽可能少的注药次数注入局麻药，在腋动脉的后方及两侧呈U形分布（其注射次数为1、2和3次的患者数分别为29、6和3例）。S组患者则应用0．3—0．6mA的刺激电流产生远端运动反应之后，行单次注药。注入的麻醉药液为1．5％的利多卡因与0．125％的布比卡因混合液内含1：200000的肾上腺素（最终浓度），药物容量为0．5ml／kg。与S组患者比较，U组患者的阻滞操作时间明显缩短（分别为3．1±1．6分钟及5．2±4．7分钟；P=0．006）。S组患者中，37％的患者局麻药液向腋动脉前方扩散，而63％的患者局麻药液向腋动脉后方扩散。注药后30分钟产生肌皮神经、正中神经、桡神经和尺神经区域完全性感觉阻滞效果的U组患者为86％，而S组患者中仅占57％（P=0．007）。U组患者单次注药与多次注药所产生的完全阻滞率相同，均为86％。U组患者需要追加给药率为8％，而S组为26％（P=0．049）。S组中有1例患者由于20分钟之后仍不能获得远端刺激反应而阻滞失败。由此，我们得出如下结论，在超声引导行锁骨下臂丛神经阻滞时，以观察药液分布情况作为注入局麻药的指征，可缩短操作时间，提高麻醉成功率。局麻药围绕腋动脉的后一侧方向扩散可预示阻滞成功，并免除了直接观察神经的必要。     

Perivascular brachial plexus block. Ultrasound versus nerve stimulator

Background

Optimizing the needle position using ultrasound (US) instead of electrical nerve stimulation (NSt) is increasingly common for perivascular brachial plexus block. These two methods were compared in a prospective, randomized, single-blinded controlled trial regarding effectiveness and time of onset of peripheral nerve blockade.

Methods

After puncture (penetration of neurovascular sheath and complete insertion of needle) 56 patients were randomly assigned to either the US group (finding the needle tip in transpectoral section, short axis, correction of needle position if local anesthetic spread was insufficient) or the NSt group (target impulse reaction in median, ulnar or radial nerve of 0.3?mA/0.1?ms, if necessary correction of position before injection of local anesthetic) to verify the needle position. All patients received 500?mg 1% mepivacaine. Sensory and motor blocks were tested by single nerve measurements (SNM) 5, 10 and 20?min after finishing the injection, where 0 represents minimal and 2 maximal success of the block.

Results

Single nerve measurements were analyzed using repeated measures ANOVA. The mean results of cumulative SNMs were significantly higher in the US group at all measurement times. Sensitivity US/NSt: 5?min: 3.36±2.32/2.63±1.87; 10?min: 5.45±2.41/4.21±2.45; 20?min: 7.30±2.02/6.43±2.43, p=0.015, motor function US/NSt: 5?min: 3.91±1.81/3.02±1.67; 10?min: 5.27±1.66/4.05±1.70; 20?min: 6.64±1.37/5.50±1.90, p<0.001. At the beginning of surgery complete nerve blockade was achieved in 89% in the US group and 68% in the NSt group (p=0.006), 3 (US) versus 7 (NSt) patients needed supplementation and 3 (US) versus 11 (NSt) patients needed general anesthesia (p=0.022). To achieve the nerve block took approximately 1?min more in the US group (p=0.003).

Conclusion

The use of ultrasound in perivascular brachial plexus blocks leads to significantly higher success rates and shorter times of onset.     

Painful paresthesiae are infrequent during brachial plexus localization using low-current peripheral nerve stimulation

BACKGROUND: Considerable controversy exists over the relationship of paresthesia to nerve stimulation. The purpose of this study was to determine the frequency with which patients report paresthesia at the point that an acceptable motor response is obtained to low-intensity current electrical stimulation. METHODS: Low-intensity current nerve stimulation (0.6 mA, 200 microseconds, 2 Hz) was used to identify the brachial plexus in 64 consecutive patients having shoulder or arm surgery with an interscalene block. During nerve localization and while maintaining a motor response (0.20 mA-0.40 mA), the patients were queried regarding any radiating sensation or pain (paresthesia) in the shoulder or extremity on the side of the blockade. Sensory distribution of the block, motor strength of the arm muscles, and adequacy of anesthesia were used to assess the extent of blockade. RESULTS: Ninety-five percent of patients had satisfactory surgical anesthesia. None of the patients spontaneously reported having a paresthesia during nerve stimulation. However, on careful questioning, half of the patients (55%) reported electrical paresthesia, defined as dull tingling sensation traveling down to their hands and coinciding with the motor response. In addition, most patients (71%) spontaneously reported having a mild, radiating paresthesia on initial injection of local anesthetic. CONCLUSIONS: Painful paresthesiae should be infrequent when a low-stimulating current is used to identify the neural components of the brachial plexus and when the block needle is advanced slowly. Low-current intensity nerve stimulation can be used to achieve successful interscalene block with minimal discomfort to the patient.     

Spread of injectate associated with radial or median nerve-type motor response during infraclavicular brachial-plexus block: an ultrasound evaluation

BACKGROUND AND OBJECTIVES: We have compared ultrasound characteristics of spread during infraclavicular brachial-plexus blocks by use of electrically evoked radial-nerve- or median-nerve-type distal motor responses to guide the injection of 30 mL of 1.5% mepivacaine. METHODS: Consecutive patients who required surgery distal to the upper arm were prospectively included in this study. With radial- or median-evoked distal motor response at a stimulating current intensity of less than 0.5 mA, patients were distributed into 2 equal groups. An independent investigator blinded to the evoked response described ultrasound characteristics of the spread of local anesthetic and assessed block quality 30 minutes after placement. A quality diffusion score proportional to the extent and intensity of spread around the axillary artery was used, and dynamic movements during injection were noted. RESULTS: Thirty-two patients were included. With radial-nerve-type motor response, the success rate of infraclavicular plexus block was 100%, but 3 supplemental axillary blocks were requested with median-nerve-type motor response. Quality diffusion scores were significantly higher with radial-nerve-type as compared with median-nerve-type motor response (P = .03). Injection after radial-nerve-type motor response resulted in a typical and reproducible ultrasound feature of posterior local-anesthetic spread associated with medial and upper movement of the axillary artery. With median-nerve-type motor response, failed blocks were associated with a specific posterior displacement of the axillary artery that resulted from superficial spread. CONCLUSION: We have demonstrated that as compared with median-nerve-type motor response, injection performed after a radial-nerve-type motor response promoted reproducible and remarkable ultrasound spread characteristics associated with complete sensory block of the 3 cords at 30 minutes.     

Elektrische Nervenstimulation bei peripheren Nervenblockaden

Background

The use of nerve stimulation is a common standard procedure for peripheral nerve blocks. However, ultrasound guidance is increasingly being used as an alternative. This study explored the relationship between needle positioning defined by ultrasound guidance and the electrical nerve stimulation before and after injection of 5% glucose solution (G5%).

Patients and methods

After obtaining permission from the ethics committee, 60 patients were enrolled in the study and the results from 51 patients could be analyzed. For sonographically defined correct needle placement the lowest electrical threshold of the elicited motor responses before and after injection of 1 ml G5% was determined.

Results

In 76% of cases nerve structures could be visualized with high quality and 90% of the blocks were successful. Only 29% of patients with a successful block showed a motor response with a stimulation current ≤0.5 mA. There was a relationship only between the quality of the visualization and the success of the blockade. Addition of G5% did not result in significant changes in stimulation thresholds.

Conclusion

With the protocol used the success of a blockade depends only on the quality of visualization. With correct ultrasound-guided needle tip positioning the electrical information seems to be skewed and doubtful.     

Needle placement and injection posterior to the axillary artery may predict successful infraclavicular brachial plexus block: a report of three cases

PURPOSE: The combined use of ultrasound and nerve stimulation for localization of the brachial plexus during infraclavicular block has not been evaluated. We describe three cases of infraclavicular block where we used ultrasound to place the needle and catheter, observe type of muscle twitch obtained and local anesthetic spread after injection. CLINICAL FEATURES: Injection of local anesthetic after obtaining proximal muscle stimulation was associated with local anesthetic spread between the axillary artery and pectoral muscle. This resulted in block failure (case 1).In case 2, proximal stimulation was associated with anterior spread after a test injection. The needle and subsequently the catheter were repositioned posterior to the axillary artery and distal muscle stimulation obtained. Injection through the catheter resulted in local anesthetic spread posterior to the artery and successful block.In case 3, no distal twitch could be obtained but in light of previous experience the needle and then the catheter were placed posterior to the axillary artery. Posterior local anesthetic spread was observed and successful block ensued despite absence of any muscle stimulation. CONCLUSION: Ultrasound guidance during infraclavicular brachial plexus block enables direct visualization of needle/catheter tip location and confirmation of appropriate local anesthetic spread. Our early experience suggests that spread of injectate posterior to the second part of the axillary artery is associated with successful block.     

Use of a nerve stimulator does not improve the efficacy of ultrasound-guided supraclavicular nerve blocks

OBJECTIVE: To evaluate the efficacy of nerve stimulation as an adjunct to ultrasound-guided supraclavicular nerve blocks. DESIGN: Prospective database review. SETTING: Tertiary-care medical center. MEASUREMENTS: The records of 94 consecutive adult patients requiring surgery below the elbow and consenting to receive regional anesthesia were studied. The focus of this study was on supraclavicular nerve block using ultrasound guidance for nerve identification and needle localization. A nerve stimulator with a motor response lower than 0.5 mA was used for confirmation of findings. An ultrasound image was considered adequate if two trunks of the brachial plexus were visualized and if the needle was completely seen on the long axis. A successful block was defined as one that sufficed as the sole anesthetic without conversion to general anesthesia. Motor and sensory examination findings on the upper extremity were also evaluated. RESULTS: 74 patients had an adequate ultrasound image. Of the 64 patients with a positive motor response, 88% had a successful block, as compared with 90% of the 10 patients without a motor response (relative risk, 1.09; 95% confidence interval, 0.79-1.51; P = 0.52). Neither multivariate correction for baseline characteristics nor inclusion of the 20 patients with inadequate ultrasound images changed the results. CONCLUSION: For adequately imaged ultrasound-guided supraclavicular nerve blocks, a positive motor response to nerve stimulation does not increase the success rate of the block. In addition, the high false-negative rate suggests that these blocks are usually effective, even in the absence of a motor response. Nerve stimulation as an adjunct to ultrasound guidance may have a limited role.     

The threshold current in the intrathecal space to elicit motor response is lower and does not overlap that in the epidural space: a porcine model

Purpose

Using electrical epidural stimulation, a current of 1 to 10 mA is required to confirm the presence of the tip of an epidural catheter in the epidural space. The purpose of this study was to examine the hypothesis that the threshold current required to elicit a motor response in the intrathecal space is significantly lower than that in the epidural space in a porcine model.

Methods

Four 20-kg pigs were used in this experiment. Eighteen gauge, insulated, Tuohy needles were advanced into the epidural space using the loss of resistance technique at five different spinal levels in each pig. When the epidural space was entered, an electrical current was applied to the needle and increased progressively until a motor response was elicited. The needle was then further advanced until cerebrospinal fluid (CSF) was observed or until the needle had been advanced a maximum of 1 cm. At this point, the current was reapplied and increased until motor activity was evident.

Results

A total of 20 needles were inserted in four pigs. The current required to produce a motor response in the epidural space was 3.45 ± 0.73 mA (mean ± SD). The current required to produce a motor response in the intrathecal space (entry confirmed by the presence of CSF) was 0.38 ± 0.19 mA (mean ± SD). Two needles were advanced 1 cm without obtaining CSF but the current thresholds were similar to those obtained when CSF was evident (0.4 mA and 0.3 mA, respectively).

Conclusion

The threshold current of an insulated needle required to elicit a motor response in the intrathecal space, was significantly (P < 0.01) lower than that in the epidural space in a porcine model.     

Threshold current of an insulated needle in the intrathecal space in pediatric patients

A threshold current of <1 mA has been suggested to be sufficient to produce a motor response to electrical stimulation in the intrathecal space. We designed this study to determine the threshold current needed to elicit motor activity for an insulated needle in the intrathecal space. Twenty pediatric patients aged 7.3 +/- 3.9 yr scheduled for lumbar puncture were recruited. After sedation with propofol, patients were turned to the lateral position and an 18-gauge or 20-gauge introducer needle was placed at the L4-5 level through which an insulated 24-gauge Pajunck unipolar needle (with a Sprotte tip and stylet) was inserted. The needle was advanced into the intrathecal space as suggested by the presence of a "pop." At this point, a nerve stimulator was attached to the insulated needle and the current was gradually increased until motor activity was evident. The needle hub was checked for cerebrospinal fluid. If cerebrospinal fluid was not present, the needle was advanced further until cerebrospinal fluid was present. The threshold current was retested. The mean current in the intrathecal space required to elicit a motor response was 0.6 +/- 0.3 mA (range, 0.1-1 mA). In 19 patients, the twitches were observed at the L4-5 myotomes and 1 patient had twitches at L2. Twitches were observed unilaterally in 19 children and bilaterally in one child. This confirms the hypothesis that the threshold current in the intrathecal space is <1 mA and that it differs significantly from the threshold currents reported for electrical stimulation in the epidural space.     

Inability to Consistently Elicit a Motor Response following Sensory Paresthesia during Interscalene Block Administration

Background: Two methods of nerve block based on eliciting neural feedback with the block needle currently exist. The paresthesia technique uses sensory feedback to ascertain that the needle tip is close to the nerve. By contrast, a peripheral nerve stimulator makes use of motor responses to electrical stimulation. The relation of motor responses to an electrical peripheral nerve stimulator and sensory nerve contact (paresthesia) had not been studied.

Methods: Thirty consecutive unpremedicated patients who presented for shoulder surgery with interscalene block anesthesia were prospectively studied. Interscalene block was performed by the single paresthesia method of Winnie, using an insulated or noninsulated needle connected to a peripheral nerve stimulator with the power off. At the precise point of paresthesia, the peripheral nerve stimulator was turned on, and the current was slowly increased to 1.0 mA with a pulse width of 0.2 ms. Presence and location of any motor responses were observed and recorded.

Results: All patients had easily elicited paresthesias. The site of first paresthesia was to the shoulder in 73% of patients. Only 30% of patients exhibited any motor response to electrical stimulation up to 1.0 mA. There was no relation between site of paresthesia and associated motor nerve response.     

Patients' experiences with multiple stimulation axillary block for fast-track ambulatory hand surgery

BACKGROUND: Ambulatory axillary block by multiple nerve stimulation (MNS) is effective and time efficient, but may be rejected by patients because of block pain. This prospective study assessed patients' anxiety and acceptance of this block, identified which of the components of blocking procedure is most painful (i.v. line insertion, repeated needle passes, local anesthetic injection, or electrical stimulation) and recorded patients' anesthetic preferences for the future hand surgery. METHODS: Upon arrival at the day unit, 100 unpremedicated adult patients without previous experience of peripheral nerve stimulation indicated on the visual analog scale (VAS; 0-100) their anxiety about the block. The blocking procedure was then explained step-by-step. After inserting the i.v. line and freezing the skin in axilla, four terminal nerves (musculocutaneous, median, ulnar, radial) were electrolocated using an initial current of 2 mA and a target current of 0.1-0.5 mA. Pain caused by the individual components of blocking procedure was assessed on VAS before the start of surgery. On the day after the operation, the patients reassessed their anxiety for the next axillary block and indicated which anesthetic method (block alone, block plus sedation, or general anesthesia) they would prefer for the future hand surgery. RESULTS: Before the block, 59 patients admitted being anxious about regional block (median anxiety VAS=27), compared with 42 patients (anxiety VAS=10) postoperatively: P<0.01. Median intensity of electrical stimulation pain was significantly higher (VAS=16) than pain of local anesthetic injections (VAS=8), i.v. line insertion (VAS=6) and multiple needle passes (VAS=5). However, only 53 patients categorized electrolocation as painful. Twenty-seven reported discomfort but not pain, and 20 patients described the sensation as 'funny' or 'strange'. None of the patients had surgical pain during operation. Mean duration of surgery was 77 min, and of hospital stay 166 min. Ninety-eight patients would choose the same block for the future hand surgery, 13 of which would like sedation before the block, and two patients did not wish to be awake during any surgery. Ninety-five patients were satisfied with fast-tracking. CONCLUSIONS: Fear of block pain is diminished after experiencing the blocking procedure. Electrical stimuli was perceived as painful by 53% of patients, and this pain was more intense than with other block components. The majority of our patients would choose axillary block without sedation for future hand surgery and are satisfied with fast-tracking.     

Dextrose 5% in water: fluid medium for maintaining electrical stimulation of peripheral nerves during stimulating catheter placement

It is well documented that a higher electrical current is required to elicit a motor response following a normal saline (NS) injection during the placement of stimulating catheters for peripheral nerve block. We present three cases of continuous brachial plexus catheter placement in which Dextrose 5% in water (D5W) was used to dilate the perineural space instead of NS. Three brachial plexus blocks (two interscalene and one axillary) were performed in three different patients for pain relief. In each case, an insulated needle was advanced towards the brachial plexus. A corresponding motor response was elicited with a current less than 0.5 mA after needle repositioning. A stimulating catheter was advanced with ease after 3–5 ml of D5W was injected to dilate the perineural space. A corresponding motor response was maintained when the current applied to the stimulating catheter was less than 0.5 mA. Local anesthetic was then injected and the motor response immediately ceased. All blocks were successful and provided excellent pain relief with the continuous infusion of local anesthetics.     

Determination of epidural catheter placement using nerve stimulation in obstetric patients

Background and Objectives. Peripheral nerve and spinal cord stimulation techniques have been used for many years. However, electrical stimulation methods rarely have been used to confirm epidural catheter placement. This study examines the practicality of this technique to confirm epidural catheter placement in obstetric patients. Methods. Thirty-nine obstetric patients in labor were studied. An electrocardiography (ECG) adapter (Arrow-Johans) was attached to the proximal end of the epidural catheter (19-gauge Arrow Flextip plus). Then, the lowest milliamperage (1–10 mA) necessary to produce a motor response (truncal or limb movement) was applied using this setup. A positive motor response indicated that the catheter was in the epidural space. This test was performed initially after catheter insertion and repeatedly after incremental local anesthetic doses. Results. The sensitivity and specificity of the new test was 100% and 100%, respectively, with 38 true positive tests and 1 true negative test. The threshold current required to produce a positive test increased after incremental doses of local anesthetic. A case of intravascular epidural catheter migration was detected using this new test. In this case, the patient's increased threshold milliamperage returned to the baseline value within 1 hour, despite having received local anesthetic. Intravascular epidural catheter migration was subsequently confirmed by a positive epinephrine test dose. Thus, this test appears to be a potentially useful method to detect intravascular catheter placement. Conclusion. This study demonstrates that this test may have a role in improving the success rate of epidural anesthesia.     

Ultrasound-guided supraclavicular brachial plexus block

In this study, we evaluated state-of-the-art ultrasound technology for supraclavicular brachial plexus blocks in 40 outpatients. Ultrasound imaging was used to identify the brachial plexus before the block, guide the block needle to reach target nerves, and visualize the pattern of local anesthetic spread. Needle position was further confirmed by nerve stimulation before injection. The block technique we describe aligned the needle path with the ultrasound beam. The block was successful after one attempt in 95% of the cases, with one failure attributable to subcutaneous injection and one to partial intravascular injection. Pneumothorax did not occur. Our preliminary data suggest that a high-resolution ultrasound probe can reliably identify the brachial plexus and its neighboring structures in the supraclavicular region. The technique of real-time guidance during needle advancement can quickly localize nerves. Distinct patterns of local anesthetic spread observed on ultrasound can further confirm accurate needle location. IMPLICATIONS: Real-time ultrasound imaging during supraclavicular brachial plexus blocks can facilitate nerve localization and needle placement and examine the pattern of local anesthetic spread.     

Ultrasound-guided subgluteal sciatic nerve blocks with stimulating catheters in children: a descriptive study

We describe our clinical experience of combining ultrasound guidance and nerve stimulation for the insertion of subgluteal sciatic catheters in children. Ten children scheduled for lower limb surgery with a combined general anesthetic and a subgluteal sciatic catheter placement for both operative anesthesia and postoperative pain relief were studied. Under ultrasonographic guidance the sciatic catheter was placed using an 17-gauge 50-mm Arrow continuous peripheral nerve block needle and a 19-gauge stimulating catheter (Stimucath). The minimal electrical current required for muscle contraction on the stimulating needle and catheter differed widely among patients. Based on the visualization of the spread of local anesthetic during injection through the catheter, a successful prediction for the sciatic block was made in all patients. All catheters were successfully placed and provided excellent postoperative pain relief without complications.     

Estimation of equivalent threshold currents using different pulse widths for the epidural stimulation test in a porcine model

Background

The epidural stimulation test can help detect if a catheter is correctly positioned in the epidural space. Previous studies showed that a current of up to 16 mA was required to elicit a motor response, but few peripheral nerve stimulators can produce a current this high. Manipulating pulse width can produce a positive response at a lower current. To clarify the effects of pulse width on the epidural stimulation test, we performed a single-blinded study in a porcine model to estimate the equivalent current needed at varying pulse widths.

Methods

After obtaining local ethics approval, an 18G insulated Tuohy needle was advanced into the epidural space at the lower lumbar spinal level, and a 20G stimulating epidural catheter was advanced 30 cm cephalad. A gradually increasing electrical current was applied, and a motor response was elicited at pulse widths of 0.1, 0.2, 0.3, 0.5, and 1 msec. This was followed by a 1-2 cm catheter withdrawal, and the process was repeated for a total of 15 locations per pig.

Results

Recorded threshold currents ranged from 0.36-9.5 mA at a pulse width of 0.2 msec. Our results show a linear relationship between threshold current and pulse width.

Conclusions

In situations where different pulse widths are needed, the nomograms presented here may be useful to estimate the equivalent threshold current which is required to elicit a motor response according to previously published criteria for epidural stimulation tests.