Anesthesiology
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Recent disputes about the relevance of membrane expansion to the mechanism of anesthesia indicate that there is confusion about the concept of membrane expansion and stabilization. One theory suggests that the membrane is expanded when its size is increased by the size of the incorporated anesthetic molecules, whereas another theory contends that extra space must be created over the size of the incorporated anesthetic molecules in order for the membrane to be considered as expanded. This article is intended to clarify the discrepancies between these concepts. ⋯ The physical meaning of the pressure reversal of anesthesia is described, and the absolute necessity of the presence of excess volume for pressure to antagonize anesthesia is discussed. Excess volume expansion per se may not be the cause of anesthesia, but the mechanism by which the excess volume is created must be the key event that induces anesthesia. The mean excess volume hypothesis postulates that the size of the membrane is irrelevant to anesthesia.(ABSTRACT TRUNCATED AT 250 WORDS)
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In anesthetized cats ventilated with oxygen, 0.5 ml of the inert gas sulfur hexafluoride (SF6) was substituted for vitreous. When the ventilating gas was changed to nitrous oxide (N2O) 66%, balance oxygen, intraocular pressure increased from 14.4 to 30.3 mmHg in 19.5 min. ⋯ This intraocular pressure change secondary to gas volume alteration may adversely affect therapeutic outcome of ophthalmic surgery. Accordingly, N2O should be avoided in patients during and following intravitreal injection of SF6 for up to 10 days.
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We tested the hypothesis that different anesthetic techniques for elective cesarean section would be reflected in the pattern of breathing and its control after birth. The pattern of breathing, including tidal volume, total breath duration (TTOT), minute ventilation, inspiratory (TI) and expiratory times, TI/TTOT ratio, and mouth occlusion pressure, was measured in 27 infants delivered by elective cesarean section during maternal epidural (lidocaine-carbon dioxide-epinephrine, n = 19) or general anesthesia (66% oxygen in N2O and 0.5% halothane, n = 8) at 10, 60, and 90 min and 3-5 days of age. ⋯ In general, at any given age the values of the respiratory parameters measured and their variability were similar between the two groups of infants. These findings indicate that the pattern of breathing after birth is not different following epidural or general anesthesia, and on the basis of our measurements, both epidural or general anesthesia appeared equally suitable for elective cesarean section.
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In a previous study, the authors found that infants, in the first 6 months of life, required the highest minimum alveolar concentration (MAC) of any age group (1.09% halothane). Because only two neonates (0-31 days of age) were included in the original study and because profound depression of blood pressure and heart rate have been reported in neonates, the authors determined 1) whether the MAC of halothane in neonates (n = 12) differs from that in infants (1-6 months of age) (n = 12) and 2) whether the blood pressure and heart rate responses in neonates differ from those in infants at approximately 1 MAC. The authors found that the MAC of halothane in neonates, 0.87% +/- 0.03 SEM, was significantly lower (P less than 0.01) than that in infants, 1.20% +/- 0.06 SEM. ⋯ The authors conclude that the MAC of halothane in neonates is 25% less than that in infants and significantly less than was thought previously. The MAC in infants is the highest of any age group. The decrease in blood pressure and the incidence of hypotension in neonates are similar to those in infants at approximately 1 MAC of halothane.