Articles: nerve-block.
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Anesthesia and analgesia · Dec 1992
Subblocking concentrations of local anesthetics: effects on impulse generation and conduction in single myelinated sciatic nerve axons in frog.
Phenomena seen in axons exposed to subblocking doses serve as the basis for interpreting clinical and behavioral observations during onset and recovery of peripheral nerve block. To delineate the changes in excitability and in impulse conduction caused by subblocking concentrations of local anesthetics (LAs) in myelinated peripheral nerve fibers, LAs were applied to excised frog sciatic nerves while impulse conduction was monitored in single axons. For concentrations ranging from 0.01 to 1.2 times the LA concentration needed to block impulse conduction, three measures of susceptibility to LA were made to quantify the action of the drugs on "resting" fibers (firing rates < or = 0.5 Hz): the increase in the threshold for electrical activation of impulses, the increase in conduction latency reflecting the slowing of impulse conduction in the region exposed to LA, and the "critical blocking concentration" of LA just sufficient to prevent impulse conduction in the recorded fiber. ⋯ Such "use-dependent" increases in threshold and decreases in conduction velocity showed kinetics that were agent specific, lasting 1 s after a burst of impulses for lidocaine and lasting > 10 s for bupivacaine. At low concentrations, within the range of nontoxic plasma concentrations after systemic administration, the predominant actions of LAs on conducting fibers were transient decreases in excitability and conduction velocity in combination with a reduction of intrinsic oscillatory aftereffects of impulse discharge. These effects may degrade decoding of information in discharge patterns without actually blocking conduction of infrequent impulses, suggesting how functional blockade of coordinated movement and perception may occur even without complete blockade of impulse conduction.
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The caudal limit of the thoracic paravertebral space was examined in 13 human cadavers. The origin of the psoas major muscle was found to completely seal off the paravertebral space below the level of the 12th thoracic vertebra. The possibility of caudal spread of a thoracic paravertebral block below this level through the paravertebral space would appear unlikely.
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Anesthesia and analgesia · Dec 1992
A rat sciatic nerve model for independent assessment of sensory and motor block induced by local anesthetics.
The purpose of this study was to develop a reliable model to independently quantify motor and sensory block produced by local anesthetics. The sciatic nerve was blocked in 52 rats by injecting 0.2 mL of 0.125%, 0.25%, 0.5%, or 0.75% bupivacaine (n = 13 for each concentration). Accurate needle placement was achieved using a nerve stimulator at 0.2 mA and 1 Hz. ⋯ The intensity of both motor and sensory block measured at 30-min intervals was plotted against time until full recovery to obtain the area under the curve. Intergroup comparisons using analysis of variance showed increasing area under the curve with increasing concentrations of bupivacaine for motor blocks (P < 0.05 for all intergroup comparisons except 0.5% vs 0.75%) and sensory blocks (P < 0.05 for all intergroup comparisons). Normal saline or sham nerve stimulation did not result in any motor or sensory block.
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Randomized Controlled Trial Comparative Study Clinical Trial
[Premedication in retrobulbar anesthesia. A blood gas analysis comparison of sublingual flunitrazepam and intravenous midazolam].
Benzodiazepines for sedation may decrease the PaO2, the arterial O2 saturation (SaO2), and the CO2 response more in the elderly than in the young. The purpose of this study was to assess changes in blood gases due to i.v. midazolam or sublingual flunitrazepam given as premedication in elderly patients for unilateral cataract surgery. METHODS. ⋯ The results of the study show the potential hazards of i.v. midazolam in the elderly. If sedation is required for cataract surgery under local anaesthesia, we recommend sublingual flunitrazepam or the use of benzodiazepines with lower hypnogenic effects in the elderly. A thorough preoperative discussion of anaesthesia and the operation might be an adequate substitute for any premedication in high-risk patients; the best blood gas analysis results were obtained in the control group.
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Although the use of peripheral nerve stimulators and unsheathed needles in performing nerve blocks has been previously described, there has been a growing emphasis on the use of specific, expensive equipment to ensure success. The availability of equipment designed for use in peripheral nerve blocks, insulated needles and nerve stimulators, will help promote the use of these techniques, but the expense of purchasing this specialized equipment may contribute to a decline in performance of certain types of blocks. This article reviews the use of standard equipment, available in most anesthesia departments, for performing peripheral nerve blocks successfully.