Anesthesiology
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The authors obtained boiling point-composition data and vapor pressure-composition data for the halothane-enflurane system at 20 degrees C and 25 degrees C. This was used to demonstrate the existence of an azeotropic mixture of halothane and enflurane and to predict the output of an enflurane vaporizer contaminated with different amounts of halothane and a halothane vaporizer contaminated with different amounts of enflurane. ⋯ It was concluded that when halothane and enflurane vaporizers are mounted in series, the halothane should be downstream. It is explained why the halothane-enflurane azeotrope is unlikely to be useful clinically.
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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.
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The relative central nervous system and cardiovascular toxicity of bupivacaine was compared in pregnant and nonpregnant ewes during continuous infusion of bupivacaine into the jugular vein at the rate of 0.5 mg X kg-1 X min-1. In all animals, identical symptoms of toxicity occurred in the following order: convulsions, hypotension, respiratory arrest, and circulatory collapse. The dose of bupivacaine required to produce central nervous system (CNS) toxicity in the pregnant ewe tended to be lower than in the nonpregnant animal, although the difference was not statistically significant (P less than 0.1). ⋯ Similarly, bupivacaine blood concentrations at the onset of respiratory arrest and circulatory collapse were lower in the pregnant group, being 5.2 +/- 0.7 micrograms/ml and 5.5 +/- 0.8 micrograms/ml, respectively, versus 7.5 +/- 1.0 microgram/ml and 8.0 +/- 0.9 micrograms/ml, respectively, in the nonpregnant group (P less than 0.05). The concentration of bupivacaine in the brain of pregnant ewes at the time of cardiovascular collapse was significantly lower (P less than 0.01) than in the nonpregnant group (7.5 +/- 1.5 vs. 16.3 +/- 1.7 micrograms/g). The myocardial tissue concentration of bupivacaine also tended to be lower in the pregnant group, although the differences were not statistically significant (P less than 0.1).(ABSTRACT TRUNCATED AT 250 WORDS)