Theories of anesthesia
Please note that we are speaking of theories (in the plural!). This simply reflects the fact that a single theory could not possibly explain the phenomenon of induced coma: there are simply too many different substances that can render a person reversibly unconscious. In some instances, we can imagine a mecha-nism, for example, lack of oxygen will stop the functioning of cells dependent on oxygen. But then think of a knock on the head, very high or low blood sugar, alcohol, sleeping pills, noble gases (xenon), inorganic gases (nitrous oxide), ace-tone, organic solvents such as chloroform, carbon tetrachloride, trichlorethy-lene, ethylene, diethyl ether, and a slew of halogenated compounds, not to mention narcotics, benzodiazepines, barbiturates, steroids, phenols, etc. To complicate matters, one fluorinated hydrocarbon, hexafluorodiethyl ether, is a convulsant (in the past used instead of electroconvulsant therapy in the treatment of depression) while several of its close relatives (isoflurane, enflu-rane, desflurane, sevoflurane) are in common use as anesthetics. Isoflurane and enflurane are isomers, both being di-fluromethyl-triflurochloroethyl-ether (see formulae below) but, despite their close relationship, only enflurane can some-times elicit minor convulsive motions. The same yin–yang kinship exists among barbiturates, which can be turned into convulsants with a chemist’s sleight of hand.
Investigators of the mechanism by which drugs produce reversible coma have focused on the cell membrane where lipids and proteins can be affected by sub-stances that alter the wonderful order of these complex structures. Some theorists stress the lipid solubility (think grease stain removers) of anesthetics, others their ability to insinuate themselves in the intricate ways proteins coil up and trap water.
Some anesthetics may work by changing the membrane, expanding it, altering its fluidity, and by one or the other or a combination of effects chan-ging cell membrane function and responses to transmitter substances. Many of the organic intravenous drugs, such as propofol, barbiturates, and the benzodia-zepines appear to increase the inhibitory action of GABA receptors, while opiates have their Greek alphabet of receptors (see below). From all of this, it must be apparent that we really do not understand all that much about reversibly induced coma or indeed about consciousness itself.