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)