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.