Intertissue Diffusion

Carpenter and colleagues^{[27] [28]} measured the washin and washout of isoflurane, enflurane, halothane, and methoxyflurane given simultaneously for fixed periods to healthy young humans. Washout was examined for several days after discontinuation of anesthetic administration. Analysis suggested that a model with five compartments best explained the resulting data for all the anesthetics (i.e., the model was independent of solubility and anesthetic metabolism). The half-times of four of these compartments were consistent with the model described earlier in this chapter. Four of these compartments could be related to washin and washout of the lungs, the VRG, the MG, and the FG.

An additional important compartment was more difficult to explain. This compartment had a half-time of roughly 300 minutes, which is between those for muscle and for fat. Uptake by highly perfused fat, such as that found in bone marrow, could explain part of this additional compartment, which accounted for almost one third of the anesthetic taken up. However, blood flow to marrow is too small to account for most of the uptake by this compartment. Carpenter and associates ^{[27] [28]} speculated that this uptake resulted from diffusion of anesthetic from lean tissue to an adjacent thin layer of fat tissue: from the heart to pericardial fat, from the kidney to perirenal fat, from the intestine to mesenteric and omental fat, and from the dermis to subcutaneous fat. Yasuda and associates^{[11] [12]} confirmed these findings, extending them to desflurane and sevoflurane.