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.     

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 non-insulated 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. CONCLUSION: Elicitation of paresthesia does not translate to an ability to elicit a motor response to a peripheral nerve stimulator in the majority of patients.     

Use of sequential electrical nerve stimuli (SENS) for location of the sciatic nerve and lumbar plexus

BACKGROUND AND OBJECTIVES: Conventional electrical stimulation has been done by continuous adjustment of current amplitude at a single, set pulse duration (conventionally, 0.1 ms). This study evaluated a novel technique for nerve location by utilization of a peripheral-nerve stimulator (PNS) programmed to deliver sequential electrical nerve stimuli (SENS). A repeating series of alternating sequential pulses of 0.1, 0.3, and 1.0 ms at 1/3-second period intervals between pulses were generated so that at a greater distance from the nerve, only higher-duration pulses would stimulate the targeted nerve and result in 1 or 2 motor responses (MR) per second. Three MR per second at 0.5 mA or less signified the conventional endpoint for nerve location (相似文献   

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.     

What is the relationship between paresthesia and nerve stimulation for axillary brachial plexus block?

BACKGROUND AND OBJECTIVES: To quantify the motor threshold current of a needle following elicitation of paresthesia during axillary brachial plexus block (ABPB). METHODS: This is a prospective, observational study of ABPB in 72 patients. Having elicited paresthesia, the minimum current required to produce a motor response was noted. The development and success of the block were subsequently followed. RESULTS: Nineteen blocks were excluded (18 because of arterial puncture and 1 blocked needle). Of the remaining 53 blocks, 41 (77%) produced a motor response at 0.5 mA or less. The median current was 0.17 mA (range, 0.03 to 3.3 mA). The site of initial paresthesia and subsequent motor response were related in 43 (81%) of cases. CONCLUSIONS: A needle position causing paresthesia produced a motor response at 0.5 mA or less in 77% of cases studied. This current may, therefore, be a reasonable threshold to aim for when performing an ABPB.     

Nerve localization techniques for interscalene brachial plexus blockade: a prospective, randomized comparison of mechanical paresthesia versus electrical stimulation

Postoperative neurologic symptoms (PONS) are relatively common after upper extremity orthopedic surgery performed under peripheral neural blockade. In this study, we prospectively compared the incidence of PONS after shoulder surgery under interscalene (IS) block using the electrical stimulation (ES) or mechanical paresthesia (MP) techniques of nerve localization. For patients randomized to the MP group, a 1-in, 23-g long-beveled needle was placed into the IS groove to elicit a paresthesia to the shoulder, arm, elbow, wrist, or hand. For patients randomized to the ES group, a 5-cm, 22-g short-beveled insulated needle was placed into the IS groove to elicit a motor response including flexion or extension of the elbow, wrist, or fingers or deltoid muscle stimulation at a current between 0.2 and 0.5 mA. Each IS block was performed with 50-60 mL of 1.5% mepivacaine containing 1:300,000 epinephrine and 0.1meq/L sodium bicarbonate. Two-hundred-eighteen patients were randomized between the two groups. One patient was lost to follow-up. Twenty-five patients (23%) in the ES group experienced paresthesia during needle insertion. The incidence of PONS using the ES technique was 10.1% (11/109), whereas the incidence with the MP technique was 9.3% (10/108) (not significant). The PONS lasted a median duration of 2 mo, and symptoms in all patients resolved within 12 mo. The success rate, onset time, and patient satisfaction were also comparable between groups. We conclude that the choice of nerve localization technique can be made based on the patient's and anesthesiologist's comfort and preferences and not on concern for the development of PONS.     

三种常用定位方法肌间沟臂丛阻滞的比较

目的 比较筋膜突破(facial pop,FP)、异感(paresthesia,PAR)、外周神经刺激(peripheral nerve stimulation,PNS)3种定位法应用于肌间沟臂丛阻滞的临床效果以及局麻药在臂丛鞘内的分布与扩散状况.方法 90例拟行上肢手术的患者,采用随机数字表法分为筋膜突破组(FP组)、异感组(PAR组)和外周神经刺激组(PNS组),每组30例.评估感觉和运动神经的阻滞程度、手术过程中的麻醉效果,观察和记录并发症的发生情况,每组6例行C_4～T_3的横断面及注药侧肌问沟的冠、矢状面计算机体层摄影(computed tomography,CT).结果 3组患者中腋神经、肌皮神经、正中神经、桡神经的感觉及运动评分差异均无统计学意义;前臂内侧皮神经的感觉评分和尺神经的感觉及运动评分FP组明显高于PAR组(P<0.05)和PNS组(P<0.01),PAR组明显高于PNS组(P<0.05);损伤血管发生率PAR组明显高于FP组(P<0.01)和PNS组(P<0.05). CT结果显示局麻药在臂从鞘内呈不均匀扩散,仅在肌间沟水平似乎有鞘的特征,其以下有明显的分隔及囊袋,并有伪足.PNS组的总体麻醉效果高于FP组(P<0.05);尺侧手术麻醉效果FP组低于PAR组(P<0.05)和PNS组(P<0.01).结论 上臂及前臂桡侧手术采用3种定位均可,但FP法更安全简便;偏向尺侧的手术宜采用PAR或PNS定位法,以PNS定位为佳.     

Using the nerve stimulator for peripheral or plexus nerve blocks

Conventional methodology for nerve location utilizes anatomical landmarks followed by invasive exploration with a needle to a suitable endpoint. An appropriate endpoint can be either anatomical in nature (e.g. transaterial technique) or functional (paresthesia or motor response to electrical stimulation). Ability to electrically stimulate a peripheral nerve or plexus depends upon many variables, including; 1) conductive area at the electrode, 2) electrical impedance, 3) electrode-to-nerve distance, 4) current flow (amperage), and 5) pulse duration. Electrode conductive area follows the equation R = rhoL/A, where R = electrical resistance, p = tissue resistivity, L = electrode-to-nerve distance, and A = electrode conductive area. Therefore resistance varies to the inverse of the electrode's conductive area. Tissue electrical impedance varies as a function of the tissue composition. In general, tissues with higher lipid content have higher impedances. Modern electrical nerve stimulators are designed to keep current constant, in spite of varying impedance. The electrode-to-nerve distance has the most influence on the ability to elicit a motor response to electrical stimulation. This is governed by Coulomb's law: E = K(Q/r2) where E = required stimulating charge, K= constant, Q = minimal required stimulating current, and r = electrode-to-nerve distance. Therefore, ability to stimulate the nerve at low amperage (e.g. < 0.5 mA), indicates an extremely close position to the nerve. Similarly, increasing current flow (amperage) increases the ability to stimulate the nerve at a distance. Increasing pulse duration increases the flow of electrons during a current pulse at any given amperage. Therefore, reducing pulse duration to very short times (e.g. 0.1 or 0.05 ms) diminishes current dispersion, requiring the needle tip to be extremely close to the nerve to elicit a motor response. The above parameters can be varied optimally to enhance successful nerve location and subsequent blockade. Unlike imaging modalities such as ultrasonography, electrical nerve stimulation depends upon nerve conduction. Similarly, percutaneous electrode guidance (PEG) makes use of the above variables to allow prelocation of the nerve by transcutaneous stimulation.     

Continuous peripheral nerve blocks with stimulating catheters

BACKGROUND AND OBJECTIVES: This study evaluated the efficacy of stimulating catheters used for continuous peripheral nerve blocks as a means of immediate verification and confirmation of correct catheter position. METHODS: This observational study presents our experience with 130 stimulating catheters used in 40 intersternocleidomastoid, 24 axillary, 47 femoral, and 19 lateral midfemoral sciatic nerve blocks. Placement characteristics (amperage, depth of introducer needle or catheter insertion, elicited motor responses), subsequent postoperative analgesia, and catheter position evaluated with the radiopaque dye analysis were all studied. RESULTS: Except in femoral blocks, characteristics of motor responses elicited (1 Hz, 0,1 ms) by the introducer assembly and catheter differed. The amperage required to elicit motor responses typically was higher with the catheter than with the introducer needle (1.6 [0.2 to 4 mA] v 0.5 [0.4 to 1 mA] P <.0001). The ability to elicit a motor response with the stimulating catheter correlated with successful clinical anesthesia in 124 cases. Opacified radiography showed no aberrant position in these cases. Three catheters for upper limb block failed to stimulate, provided poor anesthesia, and had radiologic evidence of aberrant position. Even though they failed to stimulate, 3 catheters for sciatic block functioned well, and the opacified radiography showed correct position. CONCLUSION: The ability to electrostimulate nerves using an in situ catheter increases success rate in catheter placement for continuous peripheral nerve blocks. Further controlled investigations are necessary to compare this technique with more conventional methods in terms of cost and utility for various peripheral nerve blocks.     

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.     

儿科患者全麻下行周围神经阻滞成功率与神经电流刺激强度的相关性研究

背景本研究对产生运动反射的最小电流刺激强度与患儿全麻时行周围神经阻滞（PNB）的成功率，及其神经系统并发症发生率之间的关系进行了探讨。方法回顾费城儿童医院2002年10月至2006年7月的区域麻醉资料，将所有全麻时借助周围神经刺激器行单次PNB注射的儿科患者纳入研究范围。分析资料包括年龄、性别、阻滞类型、刺激阈值、感觉阻滞和运动阻滞程度以及神经系统并发症。结果研究期间共有660例患儿接受PNB。患儿平均年龄为13．8岁（范围为2-18岁）。采用的电流刺激强度为0．2～1mA（中位数为0．5mA，四分位间距为0．45～0．55mA）。成功率为96％。采用刺激阈≤0．5或〉0．5mA（96．3％vs95．9％；P=0．793）的两组间成功率差异无统计学意义。研究认为成功率与性别、阻滞类型、电流强度无明显关联。2例患儿在行坐骨神经阻滞后出现长达72小时的拇趾和足背的神经阻滞，但无长期后遗症。结论本研究发现弱刺激阈（≤O．5mA）和强刺激阈（〉0．5mA）可以取得相近的PNB成功率。所以，或许没必要在针刺操作时强求达到弱刺激阈（≤0．5mA），因为这可能会增加神经内注射的风险。     

Interscalene brachial plexus block for shoulder surgery: a proximal paresthesia is effective.

This study was designed to determine whether the location of paresthesias is related to the success of interscalene blocks in providing anesthesia for shoulder surgery. Interscalene blocks were performed in 45 patients presenting for elective shoulder surgery. Interscalene injections of 33-55 mL of 1.5% mepivacaine with epinephrine were performed after the first elicited paresthesia to the shoulder, arm, forearm, or hand. In 20 patients (45%), the initial elicited paresthesia was to the shoulder, whereas in 25 patients (55%), the first paresthesia was reported as distal to the shoulder. All patients developed brachial plexus anesthesia adequate for shoulder surgery. The time-course of onset of motor block as evaluated at the shoulder and elbow was not different between patients with shoulder paresthesias and those with more distal paresthesias. Handgrip strength was quantitatively evaluated with a dynamometer, and both paresthesia groups showed similar decrements in hand strength except at the end of the measurement period, when patients with distal paresthesias had a significantly weaker handgrip than patients with shoulder paresthesias. We recommend that paresthesias to the shoulder be accepted in performing interscalene blocks for patients undergoing shoulder surgery.     

Paresthesia in various spinal anesthesia techniques for cesarean section

OBJECTIVE: To determine the incidence of paresthesia with different spinal puncture techniques using a 27-gauge Whitacre needle. MATERIAL AND METHODS: Spinal puncture was performed in 224 elective cesarean sections using different techniques in this single-blind, prospective trial. Patients were randomized to 4 groups: group 1, combined epidural and subarachnoid puncture using an introducer needle; group 2, combined epidural and subarachnoid puncture without an introducer; group 3, subarachnoid puncture with an introducer; and group 4, subarachnoid puncture with an introducer to within a few millimeters of the dural sac, at which point the introducer was withdrawn. RESULTS: Paresthesia developed in 23, 11, 16 and 5 patients in groups 1, 2, 3 and 4, respectively. Various nerve roots were affected. In 2 and 11 cases the fourth and fifth lumbar nerve roots were affected; in 29 and 13 cases, the first and second sacral nerve roots were involved. In comparison with group 4, the risk of paresthesia was 7, 2.5 and 4 times greater in groups 1, 2 and 3, respectively. CONCLUSIONS: Combined epidural-subarachnoid puncture leads to a higher incidence of paresthesia in comparison with simple spinal puncture, probably because the lumbar puncture is performed on a dural sac that has been previously deformed due to the "tent effect" caused by the epidural needle. Fewer cases of paresthesia occur when the subarachnoid puncture is slow and steady and the introducer needle is withdrawn millimeters before it reaches the dural sac.     

三角肌运动反应用于小儿经肌间沟臂丛神经阻滞的临床观察

目的为三角肌运动反应作为在外周神经刺激器定位下小儿经肌间沟臂丛神经阻滞终点的临床应用提供参考。方法60例拟行上肢手术患儿,在外周神经刺激器定位下行经肌间沟臂丛神经阻滞,随机分为3组:A组(20例),三角肌运动反应终点组;B组(20例),肱二头肌运动反应终点组;C组(20例),三角肌和肱二头肌运动反应终点组。比较各组在刺激域电流、获得终点时间、合作患儿运动阻滞起效时间及神经阻滞效果的差异。结果组间刺激域电流、运动阻滞起效时间、神经阻滞效果差异无统计学意义(P>0.05);A、B组获得终点时间大于C组(P<0.05)。结论三角肌运动反应终点可以作为小儿经肌间沟臂丛神经阻滞穿刺针正确定位的标志。     

运动反应阈电流强于或弱于0．5mA在超声引导下行肌问沟神经阻滞均可产生良好效果

背景对超声引导下行肌间沟阻滞时神经电刺激的运动反应进行定量，并比较运动反应阈电流强于或弱于0．5mA时的阻滞情况。方法选择61例在肌间沟阻滞和全身麻醉下行非住院肩部手术的患者，进行前瞻性观察研究。手术前超声引导下在肌问沟放置一绝缘穿刺针，测定能诱发运动反应的最小电流值，并注射含肾上腺素的0．5％布比卡因30ml。测定上干（uppertrunk）支配区感觉和运动神经阻滞情况15分钟，然后开始全麻诱导。手术后记录麻醉后恢复室（PACU）中上干阻滞成功率和疼痛评分，并记录阻滞持续时间和手术后当晚镇痛药使用剂量。诱发运动反应的电流阈值≤0．5mA者编入A组，〉0．5mA者编入B组，并比较2组上述指标间的差异。结果诱发运动反应的电流阂值范围在0．14～1．7mA间，其中≤0．5mA者即A组病例占42％。所有患者均获得满意的上千感觉和运动阻滞，并且在PACU中无一例患者需要应用阿片类药物。两组的平均阻滞时问为17．8±4．9小时，手术后镇痛药物用量无差异。两组感觉阻滞起效时间相同，但15分钟时运动神经阻滞不全率B组（37％）高于A组（12％）（P=0．03）。结论超声引导下使用神经刺激仪行肌间沟神经阻滞时，出现运动反应的电流强度大于或小于Q5InA对上干阻滞成功率和持续时间无影响。     

Prilocaine in lumbosacral plexus block – general efficacy and comparison of nerve stimulation amplitude

The significance of the threshold amperage of peripheral nerve stimulation (PNS) for the efficacy and latency of sciatic block is shown in a controlled randomized study of stimulation amplitude. In all cases the block was complete within a short time when the threshold amperage was 0.3 mA or less. Incomplete motor and sensory blocks occurred with higher stimulation amplitudes of 0.5 and 1.0 mA. In view of these results a prospective study of the clinical efficacy of 852 combined sciatic/3-in-1 blocks using prilocaine, and performed by means of peripheral nerve stimulation was carried out. No CNS or cardiovascular complications, no problems resulting from methaemoglobinaemia and, above all, no nervous lesions were observed. The limiting factor for surgery of the lower limb with this method of anaesthesia is the tolerance of the femoral tourniquet which depends mainly on the efficacy of the 3-in-1 block. Ninety-one per cent of the combined blockades were primarily successful when there was no tourniquet at all, and 87% when the tourniquet was placed on the lower leg. In the course of surgery with a femoral pneumatic tourniquet, only 55% of the blocks did not require supplement when 20 ml of 1% prilocaine was used for the 3-in-1 block, while 72% and 74% were efficacious with 30 ml and 35 ml, respectively. The efficacy of the sciatic block proved to be extremely high (> 95%), its success depending on the dosage of the local anaesthetic and correct execution of the peripheral nerve stimulation.     

Can continuous low current electrical stimulation distinguish insulated needle position in the epidural and intrathecal spaces in pediatric patients?

BACKGROUND: Muscle twitches elicited with electrical stimulation (6-17 mA) during epidural insertion indicate correct epidural needle placement while muscle twitches at a lower current (<1 mA) may indicate intrathecal needle placement. This study examined whether applying continuous electrical stimulation at 6 mA could indicate needle entry into the epidural space without inadvertently penetrating the intrathecal space. METHODS: After institutional review board (IRB) approval, 10 pediatric patients scheduled for lumbar puncture were studied. Following sedation with propofol, an insulated 24-gauge Pajunck unipolar needle was inserted through an 18-gauge introducer needle placed at the L4-5 interspace. The needle was first connected to a nerve stimulator (6 mA) and advanced. At the first sign of muscle twitching, needle advancement was stopped and the threshold current for motor activity was determined. The current was then turned off, the stylet was removed and the needle checked for cerebrospinal fluid (CSF). If CSF was not present, the needle was advanced into the intrathecal space (as confirmed by the presence of CSF). Ten pediatric patients (ASA II or III) aged 7.8 +/- 4.3 years (2.8-16.0 years) were studied. RESULTS: All patients had two distinguishable threshold currents as the needle advanced. The mean threshold current to elicit muscle twitch in the presumed epidural space was 3.84 +/- 0.99 mA. CSF was not present in any of the patients at this location. The mean threshold current in the intrathecal space was 0.77 +/- 0.32 mA. The average estimated distance from the first threshold location to the intrathecal space was 3 mm. All muscle twitches were at the L3-5 myotomes. Nine muscle twitches were unilateral and one was bilateral. CONCLUSIONS: Monitoring with an insulated needle with electrical stimulation at 6 mA may prevent unintentional placement of epidural needles into the intrathecal space.     

成人肌间沟臂丛神经阻滞运动反应终点的研究

目的研究胸大肌、三角肌运动反应作为外周神经刺激器(PNS)定位下成人经肌间沟臂丛神经阻滞终点的可行性。方法择期成人上肢手术240例,PNS定位下行经肌间沟臂丛神经阻滞,随机均分为四组:胸大肌运动反应终点组(A组)、三角肌运动反应终点组(B组)、肱二头肌运动反应终点组(C组)和胸大肌、三角肌或肱二头肌运动反应终点组(D组)。比较组间刺激域电流、获得终点时间、运动阻滞起效时间、各神经分支感觉阻滞起效时间、运动阻滞效果、各神经分支感觉阻滞效果及神经阻滞综合效果。结果D组获得终点时间短于A、B、C组(P<0.05),组间刺激域电流、运动阻滞起效时间、各神经分支感觉阻滞起效时间、运动阻滞效果、各神经分支感觉阻滞效果及神经阻滞综合效果差异无统计学意义。神经阻滞综合优良率90.8%。结论胸大肌、三角肌运动反应终点可作为PNS定位下成人经肌间沟臂丛神经阻滞穿刺针正确定位的满意、安全标志。     

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.     

Threshold current for an insulated epidural needle in pediatric patients

We designed this study to determine the threshold current for nerve stimulation of an insulated needle in the epidural space. The intended dermatome was identified using the bony landmarks of the spine. An 18-gauge insulated Tuohy needle was inserted perpendicularly to the skin and advanced until "loss of resistance" was felt. A nerve stimulator was then connected to the insulated needle. Twenty patients were studied using an insulated Tuohy needle and one patient was studied using a noninsulated Tuohy needle. Muscle twitch was elicited with a current of 11.1 +/- 3.1 mA (mean +/- sd) in all patients in which an insulated needle was used. Muscle twitches were within 2 myotomes of the intended level (based on bony landmarks). Muscle twitch was not elicited with a noninsulated needle. After catheter threading, positive stimulation tests were elicited via epidural catheters in all patients (4.9 +/- 2.3 mA). Postoperative radiograph confirmed all catheter placements within 2 myotomes of the muscle twitches. Electrical stimulation may be a useful adjuvant tool to loss of resistance for confirming proper thoracic epidural needle placement. The threshold current criteria for an insulated needle (6-17 mA) would be higher than the original Tsui test criteria described for an epidural catheter (1-10 mA) in the epidural space.