Anesthesiology
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The differential susceptibility of large and small axons to lidocaine was studied on units in the rabbit vagus nerve. The results classified the units into three groups: 1) myelinated, conduction velocity 37.5-5 m/s, which were blocked by lidocaine 0.4-0.8 mM; 2) slow, unmyelinated axons, conduction velocity 1.2-0.5 m/s, and these axons were not blocked by 0.2, 0.4, or 0.6 mM lidocaine but usually were blocked by 0.8 mM lidocaine; and 3) Axons of intermediate conduction velocity, between 1.2 and 4 m/s. The last group of axons was the most sensitive: some were blocked by as little as 0.2 mM lidocaine. No size-related trend was detected within the groups.
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The brachial plexus sheath was examined in cadavers by using a combination of anatomic dissection, histologic preparations, and x-rays made after injection of x-ray contrast media, and in surgical patients by using computed tomography (CT) dye studies. The connective tissue forming the sheath was organized more densely proximally near its origin and became loosely organized distally as it ended by joining the medial intermuscular septum of the arm. The connective tissue forming the sheath extends inward, forming septa between components of the plexus. ⋯ They serve functionally to limit the circumferential spread of injected solutions of local anesthetics. These studies also indicate that injected anesthetic solutions spread easily in a longitudinal manner up and down the nerve and remain compartmentalized. The data presented here provide a rational explanation for the not uncommon occurrence of a profound block of rapid onset in one nerve, yet partial or absent block in other nerves, following any of the techniques of brachial plexus anesthesia.
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Differential block of peripheral nerve fibers was attempted in vitro by a new approach based on inhibiting the membrane pump with ouabain. Sequential concentration dependent extinction of the components of the compound action potential was obtained: C extinguished first, A delta next, A beta last. The sequence conforms to expectations based on axonal size. Because pain is mediated by C and A delta fibers, and block of these groups by ouabain was not reversed readily, further investigation of the practicability of the new approach seems warranted.