Distinct Membrane Effects of Spinal Nerve Ligation on Injured and Adjacent Dorsal Root Ganglion Neurons in Rats

Background: Painful peripheral nerve injury results in disordered sensory neuron function that contributes to the pathogenesis of neuropathic pain. However, the relative roles of neurons with transected axons versus intact adjacent neurons have not been resolved. An essential first step is identification of electrophysiologic changes in these two neuronal populations after partial nerve damage.

Methods: Twenty days after spinal nerve ligation (SNL), intracellular recordings were obtained from axotomized fifth lumbar (L5) dorsal root ganglion neurons and adjacent, intact L4 neurons, as well as from control neurons and others subjected to sham-SNL surgery.

Results: Pronounced electrophysiologic changes were seen only in L5 neurons after SNL. Both A[alpha]/[beta] and A[delta] neuron types showed increased action potential duration, decreased afterhyperpolarization amplitude and duration, and decreased current threshold for action potential initiation. A[alpha]/[beta] neurons showed resting membrane potential depolarization, and increased repetitive firing during sustained depolarization developed in A[delta] neurons. The afterhyperpolarization duration in neurons with C fibers shortened after axotomy. In contrast to the axotomized L5 neurons, neighboring L4 neurons showed no changes in action potential duration, afterhyperpolarization dimensions, or excitability after SNL. Depolarization rate (dV/dt) increased after SNL in L4 A[alpha]/[beta] and A[delta] neurons but decreased in L5 neurons. Time-dependent rectification during hyperpolarizing current injection (sag) was greater after SNL in A[alpha]/[beta] L4 neurons compared with L5. Sham-SNL surgery produced only a decreased input resistance in A[alpha]/[beta] neurons and a decreased conduction velocity in medium-sized cells. In the L5 ganglion after axotomy, a novel set of neurons, consisting of 24% of the myelinated population, exhibited long action potential durations despite myelinated neuron conduction velocities, particularly depolarized resting membrane potential, low depolarization rate, and absence of sag.     

Incidence of lower thoracic ligamentum flavum midline gaps

Background. Lower thoracic epidural anaesthesia and analgesia(EDA) has gained increasing importance in perioperative paintherapy. The loss-of-resistance technique used to identify theepidural space is thought to rely on the penetration of theligamentum flavum. Investigations at the cervical and lumbarregions have demonstrated that the ligamentum flavum frequentlyexhibits incomplete fusion at different vertebral levels. Therefore,the aim of this study was to directly investigate the incidenceof lower thoracic ligamentum flavum midline gaps in embalmedcadavers. Methods. Vertebral column specimens were obtained from 47 humancadavers. Ligamentum flavum midline gaps were recorded betweenthe vertebral levels T₆ and L₁. Results. The incidence of midline gaps/number of viable specimensat the following levels was: T_6–7: 2/45 (4.4%), T_7–8:1/47 (2.1%), T_8–9: 2/45 (4.4%), T_9–10: 7/39 (17.9%),T_10–11: 12/34 (35.2%), T_11–12: 10/35 (28.5%), T₁₂/L₁:6/38 (15.8%). Conclusions. In the present study we have determined the frequencyof lower thoracic ligamentum flavum midline gaps. Gaps are lessfrequent than at cervical levels, but more frequent than atlumbar levels. Peak incidence was found in the region betweenT₁₀ and T₁₂. Using a strict midline approach, one cannot thereforerely on the ligamentum flavum to impede entering the epiduralspace in all patients.      

Accuracy in estimating the correct intervertebral space level during lumbar, thoracic and cervical epidural anaesthesia

BACKGROUND: Even in the absence of factors concealing anatomical landmarks, high failure rates in correctly determining a given lumbar interspace have been reported. METHODS: Therefore, it was the aim of the present study to compare the assessed and factual level (determined by computed tomography) of epidural puncture in attending a regional anaesthesia cadaver workshop. Eighty-two anaesthetists performed 117 punctures. RESULTS: Vertebral interspaces between T8-L4 were correctly identified more often than those between C3-T5 (P < 0.05). Identification of an arbitrarily chosen vertebral interspace was excellent in both the cervical/high thoracic and thoracic/lumbar regions. CONCLUSION: As previously conjectured only for the lumbar region, we could confirm the tendency of anaesthetists to perform neuraxial puncture more cranially than expected also for the thoracic and cervical regions. The large majority of punctures (93.7%) was performed within one interspace of the predicted level.     

Unintentional arterial puncture during cephalic vein cannulation: case report and anatomical study

Background. The cephalic antebrachial vein is often used forvenous access. However, superficial radial arteries of the forearmare known and unintentional arterial puncture can result fromattempts to cannulate the lateral veins of the arm. Methods. Accidental puncture of a superficial radial arteryduring peripheral venous cannulation prompted us to study theanatomy of 26 specimens and to assess the relationship betweenthe radial artery and the cephalic vein in the forearm. Results. In two cases, we found accessory branches of the radialartery close to the cephalic forearm vein. Venous cannulationat the lateral wrist carries a small risk of arterial punctureif arterial anomalies are present. Conclusions. If venous cannulation is attempted at the radialside of the wrist, palpation for pulsation should reduce thedanger of arterial puncture. Br J Anaesth 2004: 92: 740–2     

Aspiration and the laryngeal mask airway: three cases and a review of the literature

The primary limitation of the laryngeal mask airway (LMA®)     

Magnesium sulfate diminishes the effects of amide local anesthetics in rat sciatic-nerve block

BACKGROUND AND OBJECTIVES: Magnesium sulfate (MgSO(4)) is well known as an antagonist of N-methyl-d-aspartate receptors and was used for intrathecal analgesia a century ago. However, the effects of MgSO(4) combined with local anesthetics (LAs) on peripheral nerves are unclear. We tested the hypothesis that MgSO(4) could be used as an adjuvant to prolong and intensify conduction block by amide-type LAs in a rat sciatic-nerve block model. Further, the mechanism of possible synergy between LAs and MgSO(4) was investigated in whole-cell mode patch-clamp experiments. METHODS: Sciatic nerves were exposed to 2%/73.9 mM lidocaine, 0.25%/7.7 mM bupivacaine, and 0.5%/15.4 mM ropivacaine, with or without addition of 1.25%, 2.5%, or 5% MgSO(4)/50.7 mM, and nerve block characteristics were assessed. To elucidate the LA-MgSO(4) interaction, voltage-dependent inactivation curves were determined in cultured rat GH(3) cells that expressed neuronal Na(+) channels. RESULTS: Unexpectedly, the addition of MgSO(4) overall significantly shortened the duration of block by lidocaine, bupivacaine, and ropivacaine. The steady-state inactivation of Na(+) channels in the presence of 300 muM lidocaine was almost unchanged by the addition of 10 mM MgSO(4), indicating that MgSO(4) does not affect the potency of lidocaine toward the inactivated Na(+) channel. CONCLUSIONS: MgSO(4) coadministered with amide-type LAs shortened the duration of sciatic-nerve block in rats. Therefore, it does not seem to be useful as an adjuvant for peripheral-nerve block. The mechanism of this observed antagonism is unclear but appears to be independent of the action of LAs and MgSO(4) at the LA receptor within the Na(+) channel.