Magnesium enhances local anesthetic nerve block of frog sciatic nerve

The present study was designed to examine the effects of increased magnesium concentration on the nerve block produced by lidocaine, benzocaine, and QX 572 (a quaternary derivative of lidocaine). Desheathed whole sciatic nerves from northern Rana pipiens frogs were placed in a sucrose gap chamber and stimulated at various frequencies at supramaximal intensity for the activation of the compound action potential. After control studies, the nerve was bathed by a calcium-free frog Ringer's solution containing magnesium concentrations of 3.0, 10.0, or 20.0 mM with or without 0.5 mM lidocaine, 0.5 mM benzocaine, or 0.75 mM QX 572. Compound action potentials were measured, and nonfrequency and frequency dependent blocks were compared in each solution. Increased magnesium ion concentration, in the absence of local anesthetics, enhanced the nonfrequency dependent block but did not change the frequency dependent block. All three local anesthetics enhanced both types of block. Increased magnesium concentrations enhanced only the nonfrequency dependent benzocaine block. In contrast, increased magnesium enhanced both types of block produced by QX 572 and lidocaine. These results suggest a potentially important interaction between high magnesium concentrations and local anesthetic nerve blocks.     

患者不同浓度局麻药配伍坐骨神经阻滞的效果

目的比较患者不同浓度局麻药配伍坐骨神经阻滞（经典后路法）的效果。方法择期行单侧膝关节及膝以下手术患者60例，随机分为2组（n=30）：高浓度组（1．5％利多卡因＋0．5％罗哌卡因）和低浓度组（1％利多卡因＋0．375％罗哌卡因）。依次给予腰丛阻滞和坐骨神经阻滞（经典后路法），以周围神经刺激器引出足跖屈为坐骨神经阻滞的目标运动反应。自注入所有局麻药后的45min内，每隔5分钟评价坐骨神经分支支配区域的感觉、运动阻滞情况。记录2组感觉阻滞和运动阻滞的起效时间、作用持续时间以及术中和术后并发症。结果2组坐骨神经阻滞的成功率差异无统计学意义（P〉0．05）。与高浓度组比较，低浓度组感觉、运动阻滞起效时间延长（P〈0．05），但2组感觉、运动阻滞作用时间比较差异无统计学意义（P〉0．05）。2组术中和术后均未出现并发症。结论采用低浓度的局麻药（1％利多卡因＋0．375％罗哌卡因）配伍即可提供满意的坐骨神经阻滞效果。     

Evidence that spinal segmental nitric oxide mediates tachyphylaxis to peripheral local anesthetic nerve block

BACKGROUND: Tachyphylaxis to sciatic nerve blockade in rats correlates with hyperalgesia. Spinal inhibition of nitric oxide synthase with N(G)nitro-L-arginine methyl ester (L-NAME) has been shown to prevent hyperalgesia. Given systemically, L-NAME also prevents tachyphylaxis. The action of L-NAME in preventing tachyphylaxis therefore may be mediated at spinal sites. We compared systemic versus intrathecal potency of L-NAME in modulating tachyphylaxis to sciatic nerve block. METHODS: Rats were prepared with intrathecal catheters. Three sequential sciatic nerve blocks were placed. Duration of block of thermal nocifensive, proprioceptive and motor responses was recorded. We compared spinal versus systemic dose-response to L-NAME, and examined effects of intrathecal arginine on tachyphylaxis. An additional group of rats underwent testing after T10 spinal cord transection. In these rats duration of sciatic nerve block was assessed by determining the heat-induced flexion withdrawal reflex. RESULTS: L-NAME was 25-fold more potent in preventing tachyphylaxis given intrathecally than intraperitoneally. Intrathecal arginine augmented tachyphylaxis. Spinalized rats exhibited tachyphylaxis to sciatic block. CONCLUSION: The increased potency of intrathecal versus systemic L-NAME suggests a spinal site of action in inhibiting tachyphylaxis. Descending pathways are not necessary for the development of tachyphylaxis since it occurs even after T10 spinal cord transection. Thus tachyphylaxis, like hyperalgesia, is mediated at least in part by a spinal site of action.     

Differential effect of nerve fiber structure on block by local anesthetic

The incidence of conduction block by lidocaine 0.3 mmol/l (8.1 mg/dl) in several successive lengths of individual afferent axons of rabbit was compared. The conduction velocity of the axons was either "slow," "intermediate" (1.3-4 m/s), or "fast." The "intermediate" group showed a higher incidence of proximal acceleration of conduction (P less than 0.001) and a greater incidence of block (P less than 0.001) than the "slow" and "fast" fiber groups. The results were interpreted as indicating that the fibers of the "intermediate" group had an unmyelinated peripheral and a myelinated proximal length, with a junctional heminodal region that was the seat of the high sensitivity to block. The potential clinical significance of the observation is discussed in terms of the known distribution of heminodes in the peripheral nervous system.     

Duration of experimental nerve block by combinations of local anesthetic agents

The effects of bupivacaine-prilocaine and meperidine-lidocaine combinations (as compared with those of the agents used alone) on the duration of peripheral sensory nerve block were studied with the infraorbital nerve block model (IONB) in the rat, and those on motor block with spinal anesthesia (SA) in the mouse. The duration of bupivacaine-induced IONB was invariably prolonged when prilocaine was included in the solution. When included in 0.125% bupivacaine, 1.0% prilocaine had a slightly less pronounced enhancing efTect than 0.5% prilocaine (24–57% vs. 74%-104%, respectively). The duration of IONB with 1.0% prilocaine was significantly reduced (14–37%) by inclusion of 0.125% bupivacaine. In SA, inclusion in 0.125% bupivacaine of prilocaine (0.5% or 1.0%) prolonged motor block by 128% and 192%, respectively. When included in 0.25% bupivacaine, both 0.5% and 1.0% prilocaine significantly reduced the duration of SA, by 42% and 37%, respectively. With one exception, the duration of IONB by meperidine was significantly shortened (<44%) when lidocaine was included in the solution. In SA, inclusion of 2% lidocaine with 2% meperidine did not affect the duration of meperidine-induced motor block. The duration of SA obtained with the combination of 4% lidocaine and 4% meperidine was 45% shorter than that induced by 4% meperidine alone. The reasons for these variable effects are not clear, but may be due to interaction or antagonism at any of multiple sites.     

Ultrasound as the only nerve localization technique for peripheral nerve block

Ultrasound facilitates the performance of peripheral nerve blocks and may increase block quality parameters. In this report, we show that ultrasonographic guidance makes peripheral nerve blocks possible in patients in whom the traditional methods of nerve localization are limited. Four cases are described in which conventional end points for successful blocks would have been impossible to use, whereas ultrasound guidance was successful and safe. The latter method increases applicability in a larger group of patients.     

Alkalinizing local anesthetic does not decrease pain during injection for dorsal penile nerve block

STUDY OBJECTIVE: To evaluate whether alkalizing local anesthetic with sodium bicarbonate reduces pain related to infiltration of local anesthetic during dorsal penile nerve block for circumcision. DESIGN: Prospective, randomized, double-blind, controlled study. SETTING: E. Wolfson Medical Center, Holon, Israel. PATIENTS: 144 patients aged 15 to 54 years (mean +/- SD = 22.7 +/- 8.8 years), randomized before the block into four groups of 36 patients each. INTERVENTIONS: Group A (control) received a standard solution of 1 mg/kg lidocaine 2% mixed with 0.5 mg/kg bupivacaine 0.5% without epinephrine (pH = 6.3). Patients in Group B, Group C, and Group D received this mixture added with 8.4% sodium bicarbonate for a mean final pH of 6.90, 7.16, and 7.43, respectively. Penile block was performed using a 23-gauge needle by an anesthetist who was blinded to the pH of the solution. MEASUREMENTS AND MAIN RESULTS: Patients were questioned regarding pain suffered during injection of the anesthetic solutions by another physicians who also was blinded to the drug used. Pain was rated by a previously reported subjective pain scale. Mean pain scales were 2.0 +/- 0.17, 2.3 +/- 0.15, 2.2 +/- 0.16, and 1.94 +/- 0.15 for Groups A to D, respectively. The differences between these results were not statistically significant. CONCLUSION: Alkalinizing the acidic local anesthetic solution by sodium bicarbonate does not decrease pain related to infiltration during penile nerve block for circumcision.     

Differential peripheral nerve block by local anesthetics in the cat

Controversy still surrounds the differential susceptibility of nerve fibers to local anesthetic conduction block. In order to help resolve this controversy, we developed an in vivo model of peripheral nerve blockade in the cat that closely reproduced the clinical state. Using this model, differential rate of nerve blockade of A-alpha, A-delta, and C fibers by 2-chloroprocaine, lidocaine, bupivacaine, and etidocaine was observed and quantitated. C fibers were blocked first by 2-chloroprocaine, lidocaine and bupivacaine. Etidocaine blocked A-delta fibers first. A-alpha fibers always were blocked last. Of the four local anesthetics tested, 2-chloroprocaine produced the greatest differential rate of block of the nerve fibers, and etidocaine produced the least.     

The local anesthetic properties and toxicity of saxitonin homologues for rat sciatic nerve block in vivo

BACKGROUND AND OBJECTIVES: Saxitoxin and its homologues are naturally occurring compounds that block the sodium channel with high potency. They have the potential for providing prolonged duration local anesthesia when coinjected with vasoconstrictors or conventional local anesthetics and are devoid of local neurotoxicity. Here, we compare sciatic nerve block with saxitoxin to those with neosaxitoxin, decarbamoyl saxitoxin, and tetrodotoxin (TTX), in a search for even safer compounds. METHODS: Rats received percutaneous sciatic nerve block with toxins. The compounds were compared in terms of lethality, onset and duration of action for thermal analgesia (hot-plate testing), and motor block (weight-bearing). Data were expressed as medians with 25th and 75th percentiles, and median effective concentrations were determined. RESULTS: The median concentrations at which analgesia of 60 minutes duration was achieved were neosaxitoxin, 34+/-2 micromol/L; saxitoxin, 58+/-3 micromol/L; TTX, 92+/-5 micromol/L; and decarbamoyl saxitoxin, 268+/-8 micromol/L. Similar trends were observed for other measures of effectiveness (block duration of 90 minutes, maximal block), and for lethality so that the therapeutic indices were similar. No toxin had a marked predominance of sensory or motor block. The potency of TTX was intermediate between those of the saxitoxins, and its therapeutic index was slightly better. No difference was observed in time to onset of nerve blockade among the toxins. CONCLUSIONS: Substitutions on the saxitoxin nucleus result in large differences in incidence and duration of block, and toxicity. The therapeutic indices of the saxitoxins are similar; that of TTX is slightly better.     

Role of local anesthetic spread pattern and electrical stimulation in ultrasound-guided musculocutaneous nerve block

Study ObjectiveTo evaluate local anesthetic spread on the frequency of success of musculocutaneous nerve block, and to determine needle-to-target-nerve distance by ultrasound imaging and electrical stimulation.DesignObservational study.SettingsPrivate hospital.Patients48 ASA physical status I and II adults (16 men and 32 women) scheduled for elective carpal tunnel release or wrist ganglion cyst surgery in an outpatient setting.InterventionsThe musculocutaneous nerve (MCN) was identified by ultrasound. An insulated needle connected to an electrical stimulator in the “off” position was inserted in the biceps side of the probe in the plane of the ultrasound beam. The needle tip was placed in the vicinity of the MCN.MeasurementsLocal anesthetic spread pattern was determined by ultrasound imaging. The lowest effective current intensity was registered. The average depth of the MCN was measured by ultrasound.Main ResultsIn all patients, the local anesthetic solution spread was uneven. In 32% of patients (15/47), motor response was still elicited with electrical stimulation intensity lower or equal to 0.3 mA. In 26% of patients (12/47), motor response disappeared with electrical stimulation intensity higher or equal to 0.6 mA. In 42% of patients (20/47), motor response disappeared at intensities between 0.3 mA and 0.5 mA.ConclusionsA high success rate of MCN anesthesia occurred with non-circumferential spread of local anesthetic solution. Electrical current intensity was not a reliable indicator of needle-to-target-nerve distance.