Is membrane expansion relevant to anesthesia? Mean excess volume

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 volume theories of anesthesia are reviewed critically. The volume change of the membrane, induced by the interaction of anesthetics, is not a simple summation of membrane volume and anesthetic volume. There are a number of factors that affect the volume when anesthetic molecules interact with the membrane in water. The theories that envision membrane expansion as the increase of volume by the size of anesthetic molecules assume that there is no interaction between membrane and anesthetic molecules (if there is interaction, there is excess volume change) and are incompatible with the pressure reversal of anesthesia. 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)