Anesthesiology
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The study of compound action potentials has not succeeded in determining exact limits to differential block of nerve fibers by local anesthetics. Further observations on individual neurons therefore were undertaken. Rabbit vagus nerve and ganglion were superfused in vitro at 37 degrees C, pH 7.4. ⋯ The differences were significant by Scheffé's multiple comparisons test (P less than 0.01). Although the myelinated axons were blocked by a lower average concentration of lidocaine than unmyelinated axons, they manifested significantly more slowing of conduction before block (P less than 0.001). No relation between blocking concentration or latency increase and conduction velocity (fiber size) was evident within any fiber group.
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To determine the effect of the commonly used volatile anesthetics on a vecuronium-induced neuromuscular blockade, the authors studied 54 patients anesthetized with 1.2 MAC or 2.2 MAC enflurane, isoflurane, or halothane (MAC value includes contribution from 60% nitrous oxide). During 1.2 MAC enflurane, isoflurane, and halothane, the ED50S (the doses depressing twitch tension 50%) for vecuronium were 12.8, 14.7, and 16.9 micrograms/kg, respectively. During 2.2 MAC enflurane, isoflurane, and halothane, the ED50S for vecuronium were 6.3, 9.8, and 13.8 micrograms/kg, respectively (P less than 0.05). ⋯ The duration of a 50% block from injection to 90% recovery was the same for each group (mean 20 +/- 4 min), except for the group given 2.2 MAC enflurane (46.5 min) (P less than 0.05). The authors conclude that enflurane is the most potent volatile anesthetic, followed by isoflurane and then halothane, in augmenting a vecuronium-induced neuromuscular blockade. Increasing the concentration of volatile anesthetic has less effect on a neuromuscular blockade produced by vecuronium than on one produced by other nondepolarizing relaxants (e.g., pancuronium and d-tubucurarine).