Alterations in Membrane Dimension

Lipid monolayers adsorb inhaled agents, resulting in an increase in the lateral pressure of the monolayer that parallels the potency of the anesthetic. This finding is consistent with the notion that anesthetics may expand membranes and exert pressure on the ionic channels needed for impulse transmission—a variation of the volume expansion theory of anesthesia—and thereby inhibit the opening or accelerate the closure of ionic channels. However, precise volume measurements show only a small (0.1%) expansion when suspensions of membranes in aqueous media are combined with inhaled agents at concentrations near 1.0 MAC.^[122] Most studies indicate little or no effect of clinical concentrations of inhaled agents on membrane thickness.^[117]

According to the critical volume hypothesis, an increase in pressure or a decrease in temperature should reverse anesthesia because these physical changes should compress membranes. However, a decrease in body temperature consistently decreases MAC (see Fig. 4-3 ). This apparent contradiction is partly resolved by the increased partitioning of anesthetics into membranes at lower temperatures. Moreover, the expansion of membranes by anesthetics and an increase in temperature may not be equivalent. For example, anesthetics may expand membranes without changing thickness.^[117]