Anesthesiology
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This study examines in vitro the contractures induced by halothane and succinylcholine in skeletal muscle taken as biopsy specimens from 42 patients referred to the authors' laboratory for diagnosis of malignant hyperthermia (MH) susceptibility. In addition, possible differences between the response of preparations from these same patients with and without a history of masseter muscle rigidity following succinylcholine (SCh) administration were determined to investigate the in vitro relationship of masseter muscle rigidity to MH. Halothane 3%-induced contractures in preparations from MH susceptibles were similar, whether the group had a history of masseter muscle rigidity (1.15 +/- 0.18 g; n = 10) or not (1.02 +/- 0.21 g; n = 14). ⋯ In contrast, halothane added in the presence of SCh did not induce contractures to the same extent in preparations from MH-negative patients without a history of masseter muscle rigidity. This is the first reported in vitro method by which to examine the clinically troublesome interaction between SCh and halothane. This approach also may prove to be important in further investigations of the relationship between masseter muscle rigidity and MH.
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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.